Chemistry: A Self-Teaching Guide - Post R., Snyder C., Houk C.C. 2020

Acids and Bases

Chapters 11 and 12 gave you an indication that all acids have certain properties in common and all bases have certain properties in common. The major common property is that acids react with bases (and vice versa) to produce salts. For example, if solutions of HCl (an acid) and KOH (a base) are mixed, the following reaction occurs.

Such a reaction gives a solution that no longer has the acidic or basic properties that were evident before mixing, provided the correct volumes and concentrations were used.

What then is an acid? What is a base? There are three definitions that have been developed through the years. Each has its own particular usefulness, depending upon the nature of the reactants and the conditions of the reaction. In this chapter we will discuss each of the definitions and their particular usefulness.

Our discussion of acids and bases will touch on several other important concepts, including reactions of salts with water, another concentration term specially developed for acid—base solutions, and the importance of acid—base chemistry to physiological and industrial processes.

OBJECTIVES

After completing this chapter, you will be able to

· recognize and apply or illustrate the following: Arrhenius, Brønsted—Lowry, and Lewis acids and bases, neutralization, hydrolysis, pH, buffer solution, titration, conjugate acid or base, amphiprotic, indicator, hydronium ion, and hydrated;

· write a chemical equation for a neutralization reaction between any acid and base;

· predict whether a solution of a given salt will be acidic, basic, or neutral;

· calculate the pH of a solution when given:

1. the degree of ionization of a weak acid or base and vice versa,

2. Ka or Kb of the acid or base and vice versa,

3. the concentration of a solution of a strong acid or base;

· solve titration problems.

ARRHENIUS ACIDS AND BASES

There are several chemical theories of acids and bases. The most familiar is that of Arrhenius. According to the Arrhenius definition, an acid is any substance that produces or increases the H⁺ ion concentration in an aqueous solution (remember, water is the solvent). Both HCl and HC₂H₃O₂ can increase the H⁺ ion concentration of water. Do HCl and HC₂H₃O₂ qualify as acids according to Arrhenius? __________

Answer: yes (because they increase the H⁺ concentration of water)

An Arrhenius base is any substance that produces or increases the concentration of OH⁻ ion in aqueous solutions. All of the substances that have been named acids and bases so far in this book qualify as Arrhenius acids and bases.

NaOH is a strong electrolyte and NH₄OH is a weak electrolyte. In pure water, they both increase the OH⁻ concentration. Do NaOH and NH₄OH qualify as Arrhenius bases? __________

Answer: yes (because they both increase the OH⁻ ion concentration in aqueous solution)

A neutralization reaction takes place between an acid and a base in aqueous solution if the moles of H⁺ ion equal the moles of OH⁻ ion. The general equation (net ionic) for such a neutralization is as follows.

Complete and balance the following neutralization reaction, which takes place in aqueous solution. (You may complete the reaction in molecular form.)

Answer: BaCl₂ + 2H₂O

Here is another typical neutralization reaction.

In each of these neutralization reactions, the product that all these equations have in common is _________.

Answer: H₂O

In any neutralization reaction, regardless of the acid—base definition we are using, one of the products is the same as the solvent. In a neutralization reaction taking place between an Arrhenius acid and an Arrhenius base, one product will always be ____________.

Answer: H₂O

If the concentration of an acid solution is unknown, a basic solution of known concentration can be added slowly in measured amounts until the solution is neutralized. The concentration of the acid can then be determined. It is also possible to do the reverse, adding an acid solution to a base. This process of adding a basic solution to an acid or vice versa for neutralization is called titration. An indicator is used in the solution being titrated. The indicator shows one color at a specific level of hydrogen ion concentration and another color at another level of hydrogen ion concentration.

The concentration of a basic solution can be determined by a process called ___________.

Answer: titration

The hydrogen ions of an acid react with the hydroxide ions of a base during neutralization to produce water.

The ions other than H⁺ and OH⁻ that make up an acid or a base are spectator ions that do not enter the reaction.

1. Complete and balance the following neutralization reaction.

2. The complete ionic equation for this reaction is __________.

3. The spectator ions are __________ and __________.

Answer:

1. CaCl₂ + 2H₂O

2. 2H⁺ + 2Cl⁻ + Ca²⁺ + 2OH⁻ → Ca²⁺ + 2Cl⁻ + 2H₂O

3. Ca²⁺; 2Cl⁻

The net ionic equation for a neutralization reaction is simply:

Answer: H₂O

In a neutralization reaction, one H⁺ ion reacts with one OH⁻ ion to form one molecule of H₂O. (The ratio of H⁺ to OH⁻ is 1 to 1.)

Molarity is a measure of concentration. Molarity equals moles of solute per liter of solution. Remember that M indicates moles/liter (this was introduced in Chapter 11). A 1 liter solution of 1 M H⁺ ions could be expected to completely neutralize 1 liter of 1 M OH⁻ ions. (One mole of H⁺ ions completely neutralizes 1 mol of OH⁻ ions.) It would take how many liter(s) of 0.5 M H⁺ ion solution to neutralize 2 liters of 0.5 M OH⁻ ion solution? __________

Answer: 2 liters (Neutralization of H⁺ and OH⁻ takes an equal number of moles of each. Two liters of 0.5 M H⁺ solution represents 1 mol of H⁺ ions (Chapter 11). Two liters of 0.5 M OH⁻ solution represents 1 mol of OH⁻ ions.)

The neutralization of H⁺ and OH⁻ to form H₂O is possible when an equal number of moles of each type of ion are present. A 1 liter solution of 0.1 M H⁺ ions represents 0.1 mol of H⁺ ions and can be neutralized (through titration) by any of the following:

1 liter of 0.1 M OH⁻ ion solution, which is 0.1 mol of OH⁻ ions, or 100 milliliters of 1.0 M OH⁻ solution (also 0.1 mol of OH⁻ ions), or 200 milliliters of 0.5 M OH⁻ solution (also 0.1 mol of OH⁻ ions), or 500 milliliters of ____________ M OH⁻ ion solution (also 0.1 mol of OH⁻ ions).

Answer:

How many liter(s) of 0.4 M H⁺ solution are needed to neutralize 3 liters of 0.2 M OH⁻ solution? __________

Answer:

(As you probably guessed, “soln” is the abbreviation for “solution.”)

How many milliliters of 0.6 M H⁺ solution are needed to titrate (neutralize) 250 milliliters of 1.8 M OH⁻ solution? __________

Answer:

Let's use H₂SO₄ to titrate an OH⁻ solution. H₂SO₄ dissociates to form two H⁺ ions.

Therefore, a 1 M solution of H₂SO₄ will dissociate to form a 2 M H⁺ solution. (Ignore the ion since it is a spectator ion.)

How many liters of 1 M H₂SO₄ are needed to neutralize 1 liter of 1 M OH⁻ solution? __________

Answer:

How many milliliters of 0.1 M KOH are needed to titrate (neutralize) 25 milliliters of 0.2 M H₃PO₄ solution?

Answer:

Titration is a very useful laboratory technique. You will more than likely have to use it to determine the concentration of an unknown acid or base. You may also be required to calculate the formula weight or molar concentration of an acid or a base using titration data. With your understanding of moles and molarity, you should be able to calculate them if required to do so.

We now introduce a new concentration term that relates specifically to H⁺ and OH⁻ concentrations in aqueous solutions. It is used to indicate the degree of acidity or alkalinity (or basicity) of an aqueous solution.

Acidity of an aqueous solution is measured by the hydrogen ion concentration of the solution. If you remember the ion product constant for pure water at 25°C, write it below. If not, use the following information to calculate it. Pure water has a H⁺ concentration of 1 × 10⁻⁷ M (at 25°C). The OH⁻ concentration of pure water is also 1 × 10⁻⁷ M (at 25°C). The ion product constant for pure water at 25°C is __________.

Answer: Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ M²

The Kw for water is usually listed as 1 × 10⁻¹⁴ with no reference to units or temperature. The temperature of 25°C and moles²/liter² (which is the same as M²) are both assumed.

If the H⁺ concentration is changed to 1 × 10⁻³ M for an aqueous solution, what is the OH⁻ concentration? (Remember that Kw is a constant.) __________

Answer:

If [OH⁻] is equal to 1 × 10⁻⁹ M, then the [H⁺] must be equal to ________ M.

Answer:

If the hydrogen ion concentration is known, then the hydroxide ion concentration is easily determined, and vice versa. A solution with a hydrogen ion concentration of greater than 1 × 10⁻⁷ M is acidic. A solution with a hydrogen ion concentration of less than 1 × 10⁻⁷ M is basic.

Is a solution with a hydroxide ion concentration [OH⁻] of 1 × 10⁻¹⁰ M acidic or basic? (Find the [H⁺] first.) __________

Answer:

Since 1 × 10⁻⁴ M is greater than 1 × 10⁻⁷ M, the solution is definitely acidic.

pH AND pOH

Instead of writing the hydrogen ion concentration to indicate the acidity (or alkalinity) of a solution, a simple number indicating the pH of a solution is often used. The pH, sometimes called the “hydrogen ion exponent,” is defined mathematically as the negative logarithm to the base 10 of the hydrogen ion concentration. For the following H⁺ ion concentrations, compare the power of 10 exponent with the corresponding value of pH.

Answer: 6

If the hydrogen ion concentration is 1 × 10^{—(exponent)}, then the pH is equal to the exponent. Or, expressed another way:

If a solution has a pH of 3, what is the hydrogen ion concentration? _____

Answer: [H⁺] = 1 × 10⁻³ M

What is the pH of pure neutral water? __________

Answer:

A solution is acidic if [H⁺] is greater than 1 × 10⁻⁷ M. A solution is alkaline if [H⁺] is less than 1 × 10⁻⁷ M. Is a solution with a pH of 6 acidic or alkaline? __________

Answer: acidic (The [H⁺] = 1 × 10⁻⁶, which is greater than 1 × 10⁻⁷.)

To determine the pH of a solution if the [H⁺] is something other than 1 × 10^{—(exponent)} requires use of common logarithms (log to the base 10). Some instructors may arrange problems so that all [H⁺] will be 1 × 10^{—(exponent)} to simplify calculations. More often than not, [H⁺] is something other than 1 × 10^{—(exponent)}. We have chosen such examples and included a log table for your use in the Appendix. You may also use your scientific calculator or the handy calculators available on the Internet by way of a web browser and search engines. In this book, the answers to problems involving logarithms will assume you understand and know how to use logarithms.

To determine the pH of a hydrogen ion concentration of n × 10^{—(exponent)}, where n is a number other than 1, we must determine the logarithm of the number n.

Determine the pH of a solution if [H⁺] = 3 × 10⁻⁵ (to the nearest hundredth). Use the log table in the Appendix.

pH = __________

Answer:

Determine the pH of a solution with [H⁺] = 6.4 × 10⁻⁴ (to the nearest hundredth).

pH = __________

Answer: The log of 6.4 is 0.81 (see table).

If [H⁺] = n × 10^{—(exponent)}, then

pH = exponent − log n = 4 − log  6.4 = 4 − 0.81 = 3.19.

Determine the pH of a 0.002 M aqueous solution of HCl (to the nearest hundredth). (Determine the [H⁺] first. The acid HCl is a strong electrolyte that dissociates completely.)

pH = __________

Answer:

Conversely, we can determine [H⁺] if the pH is known. A pH of 8.58 indicates a hydrogen ion concentration of 1 × 10^—(8.58) M. Such a number is not usable as is; it merely indicates an actual answer of between 1 × 10⁻⁹ and 1 × 10⁻⁸ M. We must convert it to a usable answer.

The table of logs reads only positive logs. Therefore, we must change the negative antilog, antilog —8.58, to an expression of the next largest negative integer (−9) and find the antilog of the remainder (−8.58 = −9 + 0.42).

The antilog of 0.42 is 2.6 (from the table). The antilog of −9 is 10⁻⁹. Then [H⁺] = (antilog 0.42) × (antilog −9) = 2.6 × 10⁻⁹ M.

Determine the [H⁺] for a solution with pH of 4.72 (to the nearest tenth).

[H⁺] = ______________

Answer:

Determine the [H⁺] of a solution if the pH is 10.8 (to the nearest tenth).

[H⁺] = ______________

Answer:

Just as the pH is the negative log of [H⁺], we can evaluate pOH, which is the negative log of [OH⁻]. Remember that for pure neutral water, [H⁺] = 1 × 10⁻⁷ and [OH⁻] = 1 × 10⁻⁷.

1. What is the pH for pure water? __________

2. What is the pOH for pure water? __________

3. For pure water, pH + pOH = __________.

Answer: (a) 7; (b) 7; (c) 14

The pH and pOH of any aqueous solution add up to 14. If either the pH or the pOH is known for a solution, the other can quickly be determined. If the pOH is equal to 3, what is the pH? __________

Answer:

Now that you can determine pH, what is its significance? Many chemical and physiological processes are pH dependent, that is, the pH of the reaction system determines whether or not the reaction will take place. The pH of your blood is normally around 7.6 (slightly basic). If it varies much, either higher or lower, you get sick. Your stomach contains HCl as part of your gastric juices and is normally acidic. What then is an “acid stomach”? The pH of the contents of your stomach has decreased (greater H⁺ concentration), creating a greater than normal acidity. Electrolyte balance inside and outside the cells of our body is also pH dependent. To maintain the proper balance requires a carefully regulated system wherein the pH of the system remains virtually constant within very specific limits. We now discuss this system that is so prevalent in our bodies and that is extensively used in commercial processes to maintain a constant pH.

BUFFER SOLUTIONS

When chemists wish to keep the pH of a solution fairly constant even if some small amount of strong acid or base is added, they will use a buffer solution. A buffer solution involves a chemical equilibrium between either a weak acid and its salt or a weak base and its salt, and shows the common ion effect.

A typical buffer solution is one made up of acetic acid (HC₂H₃O₂), which dissociates to a small degree into H⁺ and ions, and sodium acetate, a salt of acetic acid that dissociates completely into Na⁺ and ions. Which ion is common to acetic acid and sodium acetate? __________

Answer: , the acetate ion

A buffer solution can consist of a weak acid and its salt or a weak base and its salt, depending upon the desired pH of the buffer solution. A buffer solution with a pH in the acidic range (1 — 7) can be made from a solution of a weak acid and its salt. A buffer solution with a pH in the basic range (7—14) can be made from a solution of a weak base and its salt. HC₂H₃O₂ and its salt NaC₂H₃O₂ are useful for making a buffer solution with a pH in the _________ range.

Answer: acidic (HC₂H₃O₂ is a weak acid.)

The key to understanding the action of a buffer solution is to remember that a weak acid (or weak base) is only dissociated to a very small degree. Most of the HC₂H₃O₂ is still in molecular form when in aqueous solution. The salt, in contrast, is completely dissociated. All of the NaC₂H₃O₂ becomes Na⁺ and .

If equal molar quantities of NaC₂H₃O₂ and HC₂H₃O₂ are mixed in an aqueous buffer solution, almost all of the acetate ion comes from (HC₂H₃O₂, NaC₂H₃O₂) __________.

Answer: NaC₂H₃O₂

If just acetic acid is mixed in water, the result is a large amount of molecular HC₂H₃O₂ and only a little H⁺ and a little .

If NaC₂H₃O₂ is mixed in water, the result is a large amount of Na⁺ ion and a large amount of ion and no NaC₂H₃O₂.

If equal molar amounts of HC₂H₃O₂ and NaC₂H₃O₂ were added together in the same aqueous solution, the result would be:

1. (a lot, a little) _____________ HC₂H₃O₂

2. (a lot, a little) _____________ H⁺

3. (a lot, a little) _____________

Answer: (a) a lot; (b) a little; (c) a lot

The resulting combination of both the weak acid and its fully dissociated salt results in a buffer solution with a large proportion of weak acid (HC₂H₃O₂) in molecular form, a large proportion of the common ion from the salt , and a small proportion of H⁺ ion. We can ignore the large proportion of the positive ion from the salt (Na⁺). The addition of extra from the salt shifts the equilibrium to increase the HC₂H₃O₂ and decrease the H⁺, according to Le Chatelier's Principle.

According to Le Chatelier's Principle, what would happen to this new equilibrium if some additional H⁺ were added to the solution? The concentration of HC₂H₃O₂ would (increase, decrease) _________. The concentration of would (increase, decrease) __________.

Answer: Adding some H⁺ would shift the equilibrium to increase the concentration of HC₂H₃O₂ and, in the process, some of the would be used up so that its concentration would decrease. The equilibrium would shift to the left.

In this buffer system some additional hydrogen ion is added to the solution. The additional hydrogen ion, instead of remaining in the solution to increase the hydrogen ion concentration and decrease the pH, will combine with the acetate ion to form HC₂H₃O₂. In this buffer system, extra H⁺ ion added to the solution shifts the equilibrium so that more (HC₂H₃O₂, ) __________ is produced and some of the (HC₂H₃O₂, ) __________ is used up.

Answer: HC₂H₃O₂;

If some additional OH⁻ ion is added to the solution, a neutralization reaction would immediately occur.

Some of the H⁺ ion from the equilibrium would be used up. The result is a shift in the equilibrium so that more H⁺ is produced. The reaction would shift to the right temporarily until the extra OH⁻ ion added had combined with the H⁺ ion. At that point, the equilibrium is again restored.

Adding extra OH⁻ to the solution causes the equilibrium to shift in order to produce more H⁺ ion. In order to produce more H⁺ ion, the equilibrium shifts so that some HC₂H₃O₂ is (produced, used up) __________ and some is (produced, used up) __________.

Answer: used up; produced

This chemical equilibrium represents a buffer system.

1. The addition of a little H⁺ from a strong acid such as HCl would shift the equilibrium so that the concentration of HC₂H₃O₂ is (increased, decreased) _________ and the concentration of is (increased, decreased) _________.

2. The addition of a little OH⁻ from a strong base such as NaOH has the opposite effect. It will remove some of the H⁺ and shift the equilibrium so that the concentration of HC₂H₃O₂ is (increased, decreased) _________ and the concentration of the is (increased, decreased) __________.

Answer: (a) increased, decreased; (b) decreased, increased

In a buffer system such as acetic acid and the acetate ion from sodium acetate, the pH remains relatively constant because extra hydrogen ion (H⁺) undergoes a chemical reaction rather than remaining in the solution. Extra hydroxide ion (OH^—) is neutralized by additional hydrogen ion produced by the acetic acid as needed. The extra OH⁻ ion also does not remain in solution since it is neutralized. An aqueous buffer system will continue to absorb most of the extra H⁺ or OH⁻ until one of the equilibrants is depleted.

This buffer system will continue to absorb additional H⁺ ion until the (HC₂H₃O₂, ) __________ is used up.

Answer:

A buffer solution useful for maintaining a pH of greater than 7 is made up of a weak base and a salt of the base. Such a buffer system involves an aqueous solution of ammonia (NH₃ · H₂O) and its salt (NH₄Cl).

A buffer solution may be made from equal molar amounts of NH₃ · H₂O and NH₄Cl. Would the resulting solution contain a lot or a little of the following substances? (Ignore the Cl⁻ ion.)

1. NH₃ · H₂O ____________

2. __________

3. OH⁻ __________

Answer: (a) a lot; (b) a lot; (c) a little

The resulting buffer solution equilibrium can be represented as follows if the Cl⁻ ion is ignored.

According to Le Chatelier's Principle, the addition of a small quantity of extra OH⁻ ion from a strong base would shift the equilibrium so that the concentration of NH₃ · H₂O would (increase, decrease) __________ and the concentration of would (increase, decrease) __________.

Answer: increase; decrease

The addition of more H⁺ from a strong acid would cause an immediate neutralization reaction (H⁺ + OH⁻ → H₂O). The neutralization reaction would remove OH⁻ ions from the buffer equilibrium system. To compensate for a loss of OH^— ions (according to Le Chatelier's Principle), the equilibrium will shift so that the concentration of NH₃ · H₂O would (increase, decrease) __________ and the concentration of would (increase, decrease) __________.

Answer: decrease; increase

Both the acidic and the basic buffer systems react with small additional quantities of H⁺ and OH⁻ to shift a buffer equilibrium. Instead of remaining in solution, the extra H⁺ or OH⁻ ions serve to shift the equilibrium and result in an increase (or decrease) of the weak acid or base and a decrease (or increase) in the salt of the acid or base. The final result is a relatively small change in the pH rather than a large change. Buffering action will continue until so much H⁺ or OH⁻ is added that one of the equilibrants is mostly used up.

A buffer solution is useful for maintaining a relatively constant pH if small quantities of __________ or __________ are added to the solution.

Answer: H⁺; OH⁻

Here are other important equilibria involved in physiological processes.

We now examine the situation where just adding a salt to water may change the pH of the water.

HYDROLYSIS OF SALTS

A salt is a product of the neutralization of an acid and a base. Common table salt (NaCl) is a product of the neutralization reaction involving HCI acid and Na OH base.

A salt can be a product of (a) a strong acid and a strong base, (b) a strong acid and a weak base, (c) a weak acid and a strong base, or (d) a weak acid and a weak base. HCl is a strong acid (completely dissociated). NaOH is a strong base (completely dissociated). NaCl is a product of a __________ acid and a __________ base.

Answer: strong; strong

By the Arrhenius definition of acids and bases, an acid and a base react to yield water and a salt.

· NaOH is a strong (completely dissociated) base.

· KOH is a strong base.

· H₂SO₄ is a strong acid.

· HCl is a strong acid.

· HC₂H₃O₂ is a weak (partially dissociated) acid.

· H₂CO₃ is a weak acid.

· NH₄OH is a weak base. [Also written NH₃·H₂O or NH₃(aq).]

NaCl is a product of a strong acid and a strong base.

1. K₂CO₃ is a product of a __________ acid and a __________ base.

2. NH₄Cl is a product of a __________ acid and a __________ base.

3. Na₂SO₄ is a product of a __________ acid and a __________ base.

4. NH₄C₂H₃O₂ is a product of a __________ acid and a __________ base.

Answer:

1. weak; strong (H₂CO₃, KOH)

2. strong; weak (HCI, NH₄OH)

3. strong; strong (H₂SO₄, NaOH)

4. weak; weak (HC₂H₃O₂, NH₄OH)

The general formula for the neutralization of Arrhenius acids and bases is:

The reverse reaction is possible, especially since weak acids or bases usually involve reversible equilibrium reactions. When a salt is mixed with water, it dissociates. The resulting ions may react with water in a reaction known as hydrolysis.

Pure water is ionized (dissociated) to a small degree. The ions of some salts may react with the ions of pure water. What ions result from the ionization of pure water? __________

Answer: H⁺ and OH⁻

The ions of pure water could possibly react with the ions of the various salts. Table salt (NaCl) is produced by reaction of a strong acid (HCl) and a strong base (NaOH). If NaCl is mixed with water, the equation could be as follows.

Both the acid (HCl) and the base (NaOH) are completely dissociated and can be shown as ions. NaCl is also completely dissociated.

Eliminate the spectator ions in the above reaction and write the net ionic equation. ________________

Answer: H₂O → H⁺ + OH⁻

The net ionic equation of the reaction of NaCl with water is simply H₂O → H⁺ + OH⁻, the ionization of water. The addition of a salt from a strong acid and a strong base has no effect on the H⁺ concentration or the OH⁻ concentration. The aqueous solution of such a salt remains neutral.

The salt Na₂SO₄ is the product of H₂SO₄ (a strong acid) and NaOH (a strong base). Would the addition of Na₂SO₄ to pure neutral water have any effect on the H⁺ concentration or the OH⁻ concentration? __________

Answer: no (because it is a salt of a strong acid and a strong base)

NH₄Cl is a salt of a strong acid and a weak base. The hydrolysis reaction is:

The salt (NH₄Cl) dissociates completely and so does the strong acid (HCl). The H₂O and the weak base (NH₄OH) remain largely in molecular form. The complete ionic equation results in:

Eliminate the spectator ions in this equation. The net ionic equation is __________________.

Answer:

The hydrolysis of a salt from a strong acid and a weak base produces a solution that is acidic. The net ionic equation of the hydrolysis of such a salt shows the production of additional H⁺ ions. The concentration of H⁺ ions is therefore increased and the solution becomes acidic.

The pH of neutral water is 7. The pH of a solution containing a salt from a strong acid and a weak base is (greater than, less than) __________ 7.

Answer: less than

NaC₂H₃O₂ is the salt of a weak acid and a strong base. The hydrolysis reaction is:

The salt (NaC₂H₃O₂) dissociates completely. The strong base (NaOH) also dissociates completely. The weak acid (HC₂H₃O₂) and H₂O remain largely in molecular form. The complete ionic equation becomes:

The net ionic equation (eliminate spectator ions) is ______________.

Answer:

The hydrolysis of a salt from a weak acid and a strong base results in a solution that is basic. The net ionic equation of the hydrolysis of such a salt shows the production of OH⁻ ions. The concentration of OH⁻ ions is increased, and therefore the solution becomes basic.

The pH of neutral water is 7. The pH of a solution containing a salt from a weak acid and a strong base is (greater than, less than) __________ 7.

Answer: greater than

The hydrolysis of a salt from:

· a strong acid and a weak base yields an acidic solution.

· a weak acid and a strong base yields a basic solution.

If the acid is stronger than the base, the salt solution will be acidic. If the base is stronger than the acid, the salt solution will be basic. If the acid and the base are both strong, the solution will be neutral.

NaNO₃ is the salt of HNO₃ (a strong acid) and NaOH (a strong base). Which will an aqueous solution of NaNO₃ be? (acidic, basic, or neutral) __________

Answer: neutral

The only combination not yet covered is the salt of a weak acid and a weak base. In the case of a weak acid or a weak base, both may be weak but one may be slightly stronger than the other. Strength of a weak acid is measured by its Ka (ionization constant of a weak acid). The strength of a weak base is measured by its Kb (ionization constant of a weak base).

Imagine that a salt is produced from a weak acid and a weak base. The weak acid, however, is stronger than the weak base (Ka is larger than Kb). Based upon what you have already learned from the hydrolysis of a salt from a strong acid and a weak base, what would you expect from the hydrolysis of this salt? _______________________________________

Answer: It would be slightly acidic, because the weak acid is stronger than the weak base.

Another salt of a weak acid and a weak base undergoes hydrolysis.

· Ka for the weak acid is 2.0 × 10⁻⁶

· Kb for the weak base is 2.0 × 10⁻⁴

The weak base is slightly (weaker, stronger) __________ than the weak acid.

Answer: stronger (Kb is larger than Ka, therefore the weak base is somewhat stronger than the weak acid.)

A salt is produced from a weak acid and a weak base.

· Ka for the weak acid is 2.0 × 10⁻⁶

· Kb for the weak base is 2.0 × 10⁻⁴

Hydrolysis of the salt produces an aqueous solution that is slightly (acidic, basic) ____________.

Answer: basic

NH₄C₂H₃O₂ is a salt of a weak acid (HC₂H₃O₂) and a weak base (NH₄OH).

· Ka for HC₂H₃O₂ is 1.8 × 10⁻⁵

· Kb for NH₄OH is 1.8 × 10⁻⁵

In this case, the Ka and the Kb are equal. The weak acid and the weak base are equal in strength. You learned earlier that a solution of a salt of a strong acid (completely dissociated) and a strong base (completely dissociated) would be neutral because the base and acid were equal in strength. If NH₄C₂H₃O₂ undergoes hydrolysis, would you expect the aqueous solution to be slightly acidic, slightly basic, or neutral? _____________

Answer: neutral (Since neither the acid nor the base is stronger than the other, the salt solution will be neutral.)

NH₄CN is a salt of a weak acid (HCN) and a weak base (NH₃(aq)).

The aqueous solution resulting from the hydrolysis of NH₄CN will be (acidic, basic, neutral) __________.

Answer: basic

The aqueous solution of a salt derived from:

1. a strong acid and strong base is ___________.

2. a strong acid and weak base is __________.

3. a weak acid and strong base is __________.

4. a weak acid and a weak base (Ka is larger than Kb) is __________.

5. a weak acid and a weak base (Kb is larger than Ka) is __________.

6. a weak acid and a weak base (Ka equals Kb) is __________.

Answer: (a) neutral; (b) acidic; (c) basic; (d) acidic; (e) basic; (f) neutral

The Arrhenius Theory is the most extensively used theory in beginning chemistry classes today. However, many chemists have used solvents other than water in their research. They noticed that strong Arrhenius acids and bases that were 100% ionized in water and therefore considered to be of the same strength did not all ionize to the same extent in other solvents. The strength of the acid or base depended upon the solvent used. The Arrhenius acid—base definitions limit acids to the presence of H⁺ in solution and bases to the presence of OH⁻ in solution. A broader definition is required when using solvents other than water. We now discuss the theory that was developed for use in other solvents (nonaqueous solvents).

BR—NSTED—LOWRY ACIDS AND BASES: LEVELING EFFECT OF THE SOLVENT

HCl, HNO₃, and H₂SO₄ are all strong acids. They each dissociate completely in an aqueous solution. In aqueous solution, all of these acids are equal in strength because they all dissociate completely.

Perchloric acid (HClO₄) also dissociates completely in aqueous solution. Since acid strength is based upon the extent to which an acid dissociates, HClO₄ is (equal to, stronger than, weaker than) __________ HCl, HNO₃, and H₂SO₄ in aqueous solutions.

Answer: equal to

For simplicity, the dissociation of HCl in water is written as follows.

All strong acids form a positive ion and a negative ion. In reality, however, the H⁺ ion from an acid combines with H₂O to form H₃O⁺, which is known as the hydronium ion.

1. What is the positive ion from all strong acids? __________

2. In aqueous solutions, this ion combines with H₂O to form what ion?

Answer: (a) H⁺ or hydrogen ion; (b) H₃O⁺ or hydronium ion

The polar nature of H₂O molecules causes the H⁺ ion to combine with H₂O and form the H₃O⁺ ion. An ion surrounded by H₂O molecules is said to be hydrated. The aqueous solution of any strong acid is actually composed of H₃O⁺ ions and a negative ion.

What ion is common to all of these strong acids? __________

Answer: H₃O⁺ or hydronium ion

Because they dissociate completely, all strong acids form aqueous solutions that are composed of H₃O⁺ ions and a negative ion. The negative ion has no effect on acid strength. The strongest acid that can exist in an aqueous solution is the hydronium ion (H₃O⁺). All strong acids that dissociate completely in H₂O are of equal strength because all form the hydronium ion (H₃O⁺). Even though HClO₄ may be stronger than HNO₃ in some other solvent, they are of equal strength in H₂O solution because both dissociate completely to form hydronium ions. In aqueous solution, no acid can be stronger than what ion? __________

Answer: hydronium ion, H₃O⁺

In some solvent other than water, HClO₄ may be stronger than HNO₃. In aqueous solution, both are of equal strength because both form H₃O⁺ equally well. All acids that dissociate completely in H₂O form H₃O⁺ equally well. The strength of acid is limited by the solvent because H₃O⁺ is the actual acid in aqueous solutions. This equalization of acid strength depending upon the solvent is known as the leveling effect of the solvent. Because the H⁺ ion is hydrated when introduced in H₂O solvent, all acids are leveled to the strength of the H₃O⁺ ion. The leveling effect of the solvent holds true also for bases. The strongest base that can exist in aqueous solution is the OH⁻ ion.

The hydroxide ion (OH⁻) can be described as a water molecule (H₂O) without a hydrogen ion (H⁺). The hydronium ion (H₃O⁺) can be described as a water molecule (H₂O) with a hydrogen ion (H⁺).

1. When the hydronium ion and the hydroxide ion combine, what is formed? (H₃O⁺ + OH⁻→?) __________

2. The OH⁻ is the strongest base that can exist in what kind of solution? __________

3. All strong bases are leveled to the strength of what ion in aqueous solutions? __________

Answer: (a) H₂O; (b) aqueous or H₂O; (c) OH⁻ or hydroxide ion

Acids that dissociate completely in water may dissociate only partially in other solvents. A process similar to hydration occurs in such solvents. For solvents other than water this process is called solvation. Acids or bases in solvents other than water are no longer under the Arrhenius definition of acids and bases. Two scientists named Brønsted and Lowry formed a wider definition of acids and bases that includes solvents other than water.

The word “hydration” and the Arrhenius definition of an acid or base are both limited to what kind of solutions? __________

Answer: aqueous (H₂O or water)

Brønsted and Lowry defined an acid as a substance capable of donating a proton (proton donor) and a base as a substance capable of accepting a proton (proton acceptor). The proton is exactly the same as the H⁺ ion in Arrhenius acid—base equations. (The H⁺ ion is viewed as the hydrogen atom without the electron. The only subatomic particle left after removal of the electron is the proton.)

According to Brønsted—Lowry and the equation above, HCN is acid because it can donate __________.

Answer: H⁺ (a proton)

Identifying the proton donor (acid) is not very difficult. Brønsted—Lowry bases (proton acceptors), however, include a variety of substances and ions in addition to the OH⁻ ion. In the following equation, the cyanide ion (CN⁻) can act as a proton acceptor in the reverse reaction.

The CN⁻ ion is therefore the Brønsted—Lowry base in the equation. Using Brønsted—Lowry definitions, identify the acid, base, and proton in the equation below.

Answer:

The Brønsted—Lowry acid (HCN) and the Brønsted—Lowry base (CN⁻) are called a conjugate acid—base pair because the only difference between them is the presence or absence of a proton. HCN and CN⁻ are exactly the same with the exception of the proton (H⁺).

The following substances are Brønsted—Lowry acids or bases. Only one conjugate pair is present, however.

The conjugate acid—base pair is _________ and __________.

Answer: ; NH₃ ( is the same as NH₃ plus a proton.)

and NH₃ are a conjugate acid—base pair.

1. Which is the acid (proton donor)? __________

2. Which is the base (proton acceptor)? __________

Answer: (a) ; (b) NH₃

Since the proton (H⁺) does not generally exist by itself, Brønsted—Lowry equations usually show two sets of conjugate acid—base pairs and no protons. A Brønsted—Lowry reaction generally shows an acid and a base as reactants and an acid and a base as products.

In the equation below, identify the acids and bases by writing the correct names in the blanks.

Answer:

In the equation in frame 69 identify the two conjugate acid—base pairs.

Answer:

HCN (acid); CN⁻ (base)

(acid); NH₃ (base)

(The only difference between the acid and base of a conjugate pair is the proton.)

Identify the acids and bases in the following Brønsted—Lowry equation. Draw a line joining the acid and base of each conjugate pair. For example:

Answer:

Identify the acids, bases, and conjugate pairs in the following reactions as you did in frame 71.

Answer:

In one reaction above, H₂O accepted a proton to become H₃O⁺. In another reaction, H₂O donated a proton to become OH⁻.

A substance that can act as both a Brønsted—Lowry acid and a base is said to be amphiprotic. In the above reactions, what substance is amphiprotic? __________

Answer: H₂O (It can either donate or accept a proton. Donating a proton is the definition for a Brønsted—Lowry acid. Accepting a proton is the definition for a Brønsted—Lowry base.)

Some pure solvents ionize slightly. For example, we have shown that water ionizes (H₂O ⇋ H⁺ + OH⁻). The ionization is more correctly written to include the hydronium (H₃O⁺) ion (H₂O + H₂O ⇋ H₃O⁺ + OH⁻). Whenever a pure solvent ionizes in this manner, the process is called self-ionization or auto-ionization. Identify the acids, bases, and conjugate pairs in the equation of H₂O ionization.

Answer:

(It doesn't matter which H₂O is labeled the acid or which is the base as long as the conjugate pairs are correct.)

We have seen that the strongest acid that can exist in an aqueous solution is the positive ion formed by the solvent plus a proton (H₂O + proton = H₃O⁺). The strongest base that can exist is the negative ion formed by the solvent minus a proton (H₂O − proton = OH⁻). Other solvents behave in a similar manner.

Liquid ammonia (NH₃) can be used as a solvent for Brønsted—Lowry acid base reactions. It self-ionizes to produce two ions.

1. Identify the strongest acid that can exist in a liquid ammonia solution. ________________

2. Identify the strongest base that can exist in a liquid ammonia solution. ________________

Answer: (a) (solvent + proton); (b) (solvent − proton)

In the self-ionization equation in frame 75, ammonia is acting as both an acid and a base. Since it acts as both a proton acceptor and a proton donor, ammonia can be called __________.

Answer: amphiprotic

Pure acetic acid (HC₂H₃O₂) can also be used as a solvent for Brønsted—Lowry acid—base reactions. The self-ionization equation for acetic acid is as follows.

1. What is the strongest acid that can exist when acetic “acid” is the solvent? _____________

2. What is the strongest base that can exist when acetic “acid” is the solvent? _____________

Answer: (a) ; (b)

In the Brønsted—Lowry concept of acids and bases, the stronger the acid, the weaker its conjugate base. Conversely, the stronger the base, the weaker its conjugate acid. In the following Brønsted—Lowry equation, the hydration of HCl produces the hydronium ion and the Cl⁻ ion. The reaction is virtually complete. A negligible amount of HCl remains in molecular form. Identify the acids, bases, and conjugate pairs in the reaction.

Answer:

In this reaction, HCl is a much stronger acid than H₃O⁺ since HCl donates virtually all of its protons to H₂O to form H₃O⁺. On the other hand, H₂O is a much stronger base than Cl⁻ since it accepts virtually all of the protons while leaving almost none for the Cl⁻ ion.

1. Is the weaker base (Cl⁻) the conjugate of the weaker acid or the stronger acid? __________

2. Is the weaker acid (H₃O⁺) the conjugate of the weaker base or the stronger base? __________

Answer: (a) stronger; (b) stronger

Every Brønsted—Lowry acid—base reaction favors the production of the weaker acid and the weaker base. A reaction with the stronger acid and base as reactants and the weaker acid and base as products will proceed favorably to produce a relatively large proportion of products.

The reverse reaction below is not favored and very little product will be formed.

The following reaction produced a large proportion of products with very little of the reactants left over. Identify the stronger acid and the stronger base.

1. The stronger acid is ___________.

2. The stronger base is ___________.

Answer: (a) HI; (b) NH₃ (The weaker acid and base are favored in a reaction. If a large proportion of products results from a reaction, then the reactants are stronger.)

Using solvents other than water, we can differentiate between strengths of those acids that are completely dissociated in water. The leveling effect of water is thus avoided. For example, HCl and HI are both strong acids (dissociate completely) in aqueous solution. However, in a solvent such as liquid ammonia, it has been determined that HI dissociates to a greater degree than HCl. HI is therefore stronger than HCl.

A table of some Brønsted—Lowry acids and bases appears above. The acids are listed in order of decreasing strength. Their conjugate bases are listed in order of increasing strength.

1. What is the strongest Brønsted—Lowry acid listed? ______________

2. The conjugate base of the strongest acid is the weakest base. What is the weakest base? __________

Answer: (a) perchloric acid (HCIO₄); (b) perchlorate ion

The above table of Brønsted—Lowry acid—base strengths can be used to determine whether a reaction will proceed to favor the formation of products. If the product acid and base are weaker than the reactant acid and base, the forward reaction will proceed favorably to form products. If the product acid and base are stronger than the reactant acid and base, the reaction will not proceed to favor the products.

Will the following Brønsted—Lowry reaction proceed and favor the formation of products? (Use the table of acid and base strengths to determine the relative strengths of the acids and bases in the reaction.) __________

Answer: Yes. HI is a stronger acid than is a stronger base than I⁻. Because the acid and base are the reactants, the reaction will proceed to favor the products.

Which does this Brønsted—Lowry reaction favor: the products or the reactants? _____________________

Answer: the products ( is stronger than NH₃. is stronger than )

Will the following Brønsted -Lowry reaction favor the reactants or the products? (Hint: First determine the acids and bases.) __________

Answer:

HCI is a stronger acid than C₂H₅OH. C₂H₅O⁻ is a stronger base than Cl⁻. Since the products are stronger than the reactants, the reactants are favored and only very slight amounts of products are formed.

In the following reaction, are the reactants or the products favored? __________

Answer: The products are favored. (HCI is a stronger acid than HCN. CN^— is a stronger base than Cl⁻. The weaker acid and base of the products are favored.)

The general term that covers all Brønsted—Lowry acid—base reactions is protolysis. All protolytic reactions favor the production of the (weaker, stronger) __________ acid and base.

Answer: weaker

Arrhenius defined an acid—base neutralization as a reaction in which H₂O is always a product. Brønsted—Lowry defines an acid—base neutralization as a reaction in which the solvent is always a product, and an acid—base reaction is referred to as protolysis.

The Brønsted—Lowry definition broadens our concepts of acids and bases by permitting more compounds to be considered as acids or bases that are not under the Arrhenius definition. The Brønsted—Lowry definitions are particularly useful to organic chemists, who deal quite often with nonaqueous systems. However, the Brønsted—Lowry idea still requires the presence of a proton that may be transferred from an acid to a base. This is still somewhat limiting in scope, so a third concept involving something that is present in all substances, electrons, has been developed. We now discuss that third concept.

THE LEWIS ACID—BASE CONCEPT

The Arrhenius concept of acids and bases covers aqueous solutions of acids and bases. The Brønsted—Lowry concept of acids and bases covers acids and bases in other solvents as well as water. Some reactions have the characteristics of an acid—base reaction but do not undergo proton exchange.

An “acid” that does not have a proton to donate would not fit the Brønsted—Lowry concept. For all practical purposes, a proton is the same as what ion? __________

Answer: H⁺

A scientist by the name of Lewis developed an acid—base theory to include those reactions that seem to behave like acid—base reactions but do not involve proton (H⁺) exchange. According to Lewis, an acid is any substance that can accept a pair of electrons to form a coordinate covalent bond. A base, on the other hand, is any substance that is capable of donating a pair of electrons to form a coordinate covalent bond. To avoid confusion between the Brønsted—Lowry concept and the Lewis concept, remember that the electron and proton are oppositely charged.

1. A Brønsted—Lowry acid is a proton (acceptor, donor) __________

2. A Lewis acid is an electron pair (acceptor, donor) __________

3. A proton has what charge? __________

4. An electron has what charge? __________

Answer: (a) donor; (b) acceptor; (c) +; (d) −

Brønsted—Lowry protolysis normally deals with a single proton. Lewis acids and bases deal with a pair of electrons in coordinate covalent bonds. A Lewis base can be an electron-rich negative ion, such as OH⁻, , and , or a molecule that has one or more pairs of electrons that are not already part of a covalent bond. Examples include H₂O and NH₃. Lewis acids include positive ions, such as H⁺, , and Ag⁺, as well as molecules that have room for an additional pair of electrons. Examples include BF₃, SO₃, and CO₂.

is an ion with a pair of available electrons. This ion is probably a Lewis (acid, base) __________.

Answer: base (A Lewis base is an electron pair donor.)

The reactions of Lewis acids and bases are best depicted by dot symbols that are appropriately known as Lewis symbols. Dot symbols were encountered in Chapter 3. Remember that dots are used to symbolize electrons.

The H⁺ ion is a Lewis acid since it can accept a pair of electrons to share in a coordinate covalent bond. The NH₃ molecule is a Lewis base since it has an unshared pair of electrons.

Look at this Lewis reaction depicted by Lewis symbols. In a coordinate covalent bond, the electron pair forming the bond comes from one substance. In this reaction, both electrons come from what substance? __________

Answer: NH₃ (Since H⁺ has no electrons at all, both bonding electrons must come from the NH₃ molecule.)

The hydrolysis of HCl to form the hydronium ion (H₃O⁺) can also be interpreted through the Lewis concept.

Remember that HCl dissociates completely in aqueous solutions and that the Cl⁻ ion is a spectator ion.

1. Which reactant is the Lewis acid? __________

2. Which reactant is the Lewis base? __________

Answer: (a) HCI (Actually only the H⁺ ion. It accepts a pair of electrons.); (b) H₂O (It shares a pair of its free electrons to form a coordinate covalent bond.)

One example of a Lewis acid—base reaction has been encountered previously as an example of a coordinate covalent bond. That is the reaction between BF₃ and NH₃ to form NH₃BF₃.

1. The two electrons that form the coordinate covalent bond come from (NH₃, BF₃) __________.

2. The Lewis acid in this example is __________.

3. The Lewis base is __________.

Answer: (a) NH₃; (b) BF₃ (electron pair acceptor); (c) NH₃ (electron pair donor)

You have just learned three ways chemists use to describe acids, bases, and neutralization reactions while also learning about pH, buffer solutions, and titration. If you take more chemistry courses you will encounter these subjects again. This understanding will provide a firm foundation for further study. In any case, this general knowledge will help you understand the behavior of some common substances and their uses.

SELF-TEST

This self-test is designed to show how well you have mastered this chapter's objectives. Correct answers and review instructions follow the test.

1. One concept of acidity holds that hydrogen is not required to be present in an acid. This concept was proposed by:

1. Brønsted—Lowry

2. Arrhenius

3. Lewis

4. Lavoisier

2. According to Brønsted—Lowry concept, only one of the following statements is true. Which one is it?

1. A base is a neutron acceptor.

2. H₃O⁺ and OH⁻ are a conjugate acid—base pair.

3. An acid is a proton donor.

4. The stronger the acid, the stronger is its conjugate base.

3. The strongest acid that can exist in substantial amounts in aqueous solution is:

1. HClO₄

2. H₂O

3. H₃O⁺

4. OH⁻

5. O²⁻

6. NH₃

4. Which of the following is (are) true in the reaction H₃N + BF₃ → H₃NBF₃?

1. H₃N is a Brønsted—Lowry acid.

2. H₃N is a Lewis acid.

3. BF₃ is a Brønsted—Lowry acid.

4. BF₃ is a Lewis acid.

5. What kind of a solution will the salt of a strong acid and a weak base give when dissolved in H₂O? (acidic, basic, neutral) __________

6. How many milliliters of 0.60 M HNO₃ are required to neutralize 40 milliliters of 1.20 M Ca(OH)₂? _______________________________________________

7. Of the acids H₂SO₄, HCI, HNO₃, HCIO₄, all have the same strength in aqueous solutions. This is an example of what is called the __________.

8. What is the pH of a solution that is 1.00 × 10⁻⁷ M HCI? __________

9. What is the pH of a solution that is 2.25 × 10⁻² M HBr?

10. What is the H⁺ concentration of a solution whose pH is 4.0? _____________.

11. What is the H⁺ concentration of a solution whose pH is 10.5?

12. According to Lewis theory, any negative ion is a __________.

13. Would a solution of NaC₂H₃O₂ be acidic, basic, or neutral? __________ Why? ________________________________________________

14. Use the table on page 344 to predict if the formation of the products is favored. Explain why or why not. __________

1. Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O __________

2. C₂H₅OH + H₂O → C₂H₅O⁻ + H₃O⁺ __________

3. + H₂O → NH₃ + H₃O⁺ __________

15. Listed below are some statements. Indicate whether they are true or false.

1. A buffer solution is prepared by mixing solutions of a strong acid and a salt of that acid.

2. A solution whose H⁺ concentration is 1 × 10⁻⁴ is acidic.

3. NH₄OH is a typical weak base.

4. pH = −log(OH⁻).

5. The strongest base that can exist in substantial concentrations in a solvent that can act as an acid is the conjugate base of the solvent acid.

ANSWERS

Compare your answers to the self-test with those given below. If you answer all questions correctly, congratulations! If you miss any, review the frames indicated in parentheses following the answers. If you miss several questions, you should probably reread the chapter carefully.

1. (c) (frames 87, 88)

2. (c) (frame 65)

3. (c) (frame 62)

4. (d) (frame 88)

5. acidic (frame 58)

6. Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

(frames 3, 9—14)

7. leveling effect (All strong acids are “leveled” to the strength of the H₃O⁺ ion in aqueous solution.) (frame 63)

8.

(frames 19—25)

9.

10. [H⁺] = antilog(−pH) = antilog(−4) = 1 × 10⁻⁴ M (frames 26, 27)

11.

12. base (frames 87—92)

13. basic, because it is the salt of a weak acid and a strong base (frames 46—58)

14.

1. Yes. A weaker conjugate acid and base formed.

2. No. A stronger conjugate acid and base formed.

3. No. A stronger conjugate acid and base formed so the reverse reaction is favored. (frames 80—86)

15.

1. F (frames 30—35)

2. T (frame 18)

3. T (frame 39, 40)

4. F (frames 19—23)

5. T (frame 63)

SUCROSE AND COCAINE - CRITICAL THINKING

The next chapter deals with organic compounds with carbon as the core atom. But based upon what you have learned thus far, we pose a question for you with a real-life situation involving two very different compounds whose molecular formulas are somewhat similar: sucrose and cocaine.

Sucrose is the chemical name for table sugar, and it has the molecular formula C₁₂H₂₂O₁₁. This compound can be purchased in any grocery store, and it is used to give a sweet taste to foods. People around the world consume sugar. However, there are warnings about consuming too much of it. Excessive sugar consumption is linked to diabetes, obesity, and coronary heart disease. Sugar can make up a large portion of a person's regular food intake.

In 2014 the National Institute for Health reported that Americans consume about 15% of their daily calories come from added sugars (∼22 teaspoons of sugar per day). A number of published sources on nutrition have claimed that sucrose is a compound that should be consumed in moderation.

As odd as it may sound, sucrose has been compared to another compound that also contains carbon, hydrogen, and oxygen: cocaine (C₁₇H₂₁O₄N).

According to the National Institute on Drug Abuse, cocaine is a powerfully addictive stimulant. It can be used as a local anesthetic for some surgeries, but its recreational use is illegal. This compound is highly dangerous. The United States Drug Enforcement Agency classifies cocaine as a Schedule 1 drug, which means it has a high potential for abuse with little to no medical use in the United States.

Why has sucrose been compared to cocaine? That is a good question that you, the reader, should try to answer. An argument has come from the fact that both compounds contain carbon, hydrogen, and oxygen. But is this argument valid? Consider the following statement found on a number of websites and attributed to psychiatrist David Reuben, the author of Everything You Always Wanted to Know About Nutrition.

[W]hite refined sugar is not a food. It is a pure chemical extracted from plant sources, purer in fact than cocaine, which it resembles in many ways. Its true name is sucrose and its chemical formula is C₁₂H₂₂O₁₁. It has 12 carbon atoms, 22 hydrogen atoms, 11 oxygen atoms, and absolutely nothing else to offer. …The chemical formula for cocaine is C₁₇H₂₁NO₄. Sugar's formula again is C₁₂H₂₂O₁₁. For all practical purposes, the difference is that sugar is missing the “N”, or nitrogen atom.

While researchers have noted similarities in the cravings for sugar and drugs such as cocaine in animal studies, does the argument that just because the chemical formulas seem to be similar, is that sufficient to equate the two? Is it enough to compare two compounds because they have similar elements in their molecular formula? Consider dimethyl ether and ethanol, two compounds you will learn about in the next chapter. Both compounds are made up of carbon, hydrogen, and oxygen.

A number of published sources on nutrition have claimed that we can make a chemical reactivity comparison between dimethyl ether and ethanol. However, these two compounds have very different chemical and physical properties, even though they are made up of the same types of elements.

Ethanol is a liquid at room temperature. It is also a common organic solvent and is used in making a variety of organic compounds. It is flammable and has a boiling point of 78.4 °C and a density of 0.789 g/cm³. Dimethyl ether is a gas at room temperature and has a much lower boiling point of −24 °C. Its density is similar to that of ethanol, just a bit lower (0.735 g/cm³). Dimethyl ether is used in aerosols as a propellant in a variety of fuel applications.

Dimethyl ether and ethanol not only possess carbon, hydrogen, and oxygen; both compounds also have identical molecular formulas, C₂H₆O. Since dimethyl ether and ethanol have the same molecular formula, both have the identical elemental compositions. Yet even with identical molecular formulas and the same elemental composition, dimethyl ether and ethanol do not have the same physical and chemical properties. Something else must account for these differences.

Dimethyl ether and ethanol (C₂H₆O; molecular mass = 46.07 g/mol)

Dimethyl ether and ethanol, 52.14%C, 13.13% H, 34.73% O

Before we answer the question of why dimethyl ether and ethanol react so differently, let's return to sucrose and cocaine. Sucrose and cocaine also both are made from carbon, hydrogen, and oxygen, but their similarities stop there. Sucrose and cocaine's elemental compositions are not identical even though both compounds are composed of the same elements, with the exception of cocaine having nitrogen.

Sucrose (C₁₂H₂₂O₁₁, molecular mass = 342.29 g/mol)

Sucrose: 42.11%C, 6.48% H, 51.42% O

Cocaine (C₁₇H₂₁O₄N, molecular mass = 303.35 g/mol)

Cocaine: 67.31% C, 6.98% H, 21.10% O

As you can probably tell, merely having similar elements (C, H, and O) is not enough to validate the argument that two compounds must behave roughly the same way. What else is going on then? More important than possessing similar elements is the way in which these atoms are connected together. In other words, it is the structure of these compounds that makes all the difference.

In chemistry and biology, structure determines function. As you can see from the following figure, even though sucrose and cocaine have similar types of atoms, they have two very different structures. These two different structures will therefore have two different functions. Not only that, both will have different chemical and physical properties. Cocaine has a melting point of 98°C; sucrose does not even have a melting point. Instead it decomposes at 186°C. Other physical differences include different boiling points, different densities, and different solubilities in water.

Structures of (A) sucrose, (B) cocaine, (C) dimethyl ether, and (D) ethanol.

As for chemical reactivity, both obviously behave very differently. Even dimethyl ether and ethanol, which are more similar elementally than sucrose and cocaine (same elements, same molecular formula, same elemental composition), have different physical and chemical properties because these compounds have different structures.

As you can see, elemental similarity does not account for much more than that. Even compounds with similar elemental composition may not have similar physical and chemical properties. Rather, it is how the atoms are arranged in the structure of the compound that determines the chemical's function. So we ask you, is it good chemistry to compare sucrose and cocaine based on them both having carbon, hydrogen, and oxygen in their molecular formulas?