KINETICS AND THERMODYNAMICS - CHEMICAL REACTIONS - The Handy Chemistry Answer Book (2014)

The Handy Chemistry Answer Book (2014)

CHEMICAL REACTIONS

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KINETICS AND THERMODYNAMICS

What is a chemical reaction?

A chemical reaction is any process that involves the transformation of one or more molecules into one or more molecules—most of the time the reactant(s) and product(s) will be different molecules. A chemical reaction almost always involves the breaking and/or formation of new chemical bonds.

How do we write an equation for a chemical reaction?

Chemists often write “equations” to describe chemical reactions. It is conventional to list the initial species, or reactants, on the left side of the equation, followed by an arrow, and the final species, or products, on the right side of the equation. The equation below shows the reaction of methane and oxygen to produce water and carbon dioxide.

CH4 + 2 O2 Image CO2 + 2H2O

The arrow to the right indicates that the reactant species are converted to the product species during the course of the reaction. In some cases, reactions are reversible, which will be represented by two arrows, one pointing in either direction (Image). Be careful, because in chemistry Image is not the same as Image!

What is the yield for a reaction?

The yield of a chemical reaction is the amount of product that is produced (an example might be two grams). It is often of greater interest to consider the percent yield, which describes the amount of product formed relative to the maximum amount of product one could have expected based on the quantities of reactants used. The percent yield provides a measure of how efficient a process is for producing the target product.

What does selectivity mean for chemical reactions?

Selectivity can have several meanings for chemical reactions, but there are two main categories: either a reaction will occur selectively with a particular chemical species or at a particular location in a molecule to avoid unwanted side-reactions, or a reaction will produce a particular product selectively.

How do modern chemists characterize the products of chemical reactions?

Chemists need to characterize the products of their chemical reactions so that they can be sure of the structure and composition of the molecule(s) they have made. One common way is by measuring the melting point of a solid substance. This doesn’t provide specific information about the arrangement of chemical bonds in a molecule, though, so more advanced techniques are required to completely characterize a molecule. These techniques often involve using electromagnetic radiation to probe the energy levels in the molecules (see “Physical and Theoretical Chemistry” for more on these topics). Knowledge of what energies/wavelengths of light the molecule can absorb can be related directly to structural features of the molecule.

Are chemists still looking for new reactions?

Yes. Chemistry has hundreds of years of knowledge to build on, but it is in no way a complete field. New ways of making existing molecules, and making chemical structures that are completely new to our planet, are absolutely goals of modern chemistry. Developing new chemical reactions, and understanding old ones, are topics of eternal interest to chemists.

What is the law of conservation of matter?

The law of conservation of matter states that matter cannot be created or destroyed. This is relevant to chemical reactions because it tells us that we must have just as many atoms of each element at the beginning of a reaction as we do at the end of a reaction. In the example above, this is reflected by the fact that we use two molecules of oxygen, or four oxygen atoms from our reactants, to produce one molecule of CO2 and two molecules of H2O for a total of four oxygen atoms in our products.

What is the stoichiometry of a reaction?

The stoichiometry of a reaction is closely tied to the idea of the conservation of matter; it tells us the ratio in which the molecules react. Again using the example at the beginning of this chapter, the reaction stoichiometry 1:2:1:2 describes the ratio in which methane and oxygen react to form carbon dioxide and water.

Why do some chemical reactions cause a color change?

The colors we see all have to do with what wavelengths of light something absorbs or reflects. For a chemical reaction to cause a change in the color of something, all that has to happen is that the products of the reaction absorb and reflect different wavelengths of light than the reactants do. We’ll discuss how light interacts with molecules in more detail in “Physical and Theoretical Chemistry.”

What are a couple of examples of familiar chemical reactions?

Fire is one example of a chemical reaction that everyone has seen take place. Fire involves a combustion reaction, which is any reaction where a hydrocarbon reacts with oxygen to form carbon dioxide and water. Another example is when your car accumulates rust. This reaction involves oxidation of the iron in the metal. Lots of complicated chemical reactions are taking place all the time in our bodies too. Every movement you make, for example, involves many chemical reactions taking place in your muscles and nerves.

What is enthalpy?

Enthalpy is a measure of the energy that something contains, and it’s defined as the total heat content of a system. In terms of chemical reactions, we are most often interested in the change in enthalpy (denoted H) associated with a reaction. The H for a reaction is defined as the enthalpy of the products minus the enthalpy of the reactants, and this is typically measured via changes in temperature that take place during the reaction.

What is a calorie?

A calorie is a unit of heat energy defined as the amount of energy it takes to raise one gram of water by one degree Celsius. Calories are also often used to describe the energy content of foods. When used for foods, a “calorie” actually refers to 1,000 calories, or a kilocalorie of energy (which can be rather confusing).

What is a bond enthalpy?

Bond enthalpy refers to the amount of energy it takes to break a chemical bond. This tells us how favorable a chemical bond is relative to the separation of the two fragments on either side of the bond.

What is a heat of formation?

The standard heat of formation for a substance is the change in enthalpy associated with its formation of one mole (see “History of Chemistry”) of a substance from its elements with the constituent elements in their standard states (see “Analytical Chemistry”).

What is Gibbs free energy?

Gibbs free energy is a quantity that describes the amount of useful work (see “Physical and Theoretical Chemistry”) that can be obtained from a system at a constant temperature and pressure. In the context of chemical reactions, changes in Gibbs free energy will typically dictate whether or not a reaction is favorable.

What makes a reaction happen spontaneously?

A spontaneous chemical reaction is one for which the associated change in Gibbs free energy is negative. The fact that a reaction is spontaneous actually doesn’t tell us anything about how quickly the reaction takes place, though. A spontaneous reaction may happen very quickly or take thousands of years!

What is a unimolecular reaction?

A unimolecular reaction involves only a single reactant molecule undergoing a chemical reaction to form products. One possible outcome is that the bonds rearrange within a single molecule to form only one product molecule, while another possibility is that the reactant molecule will fragment, producing multiple product molecules.

What is a bimolecular reaction?

As you might be able to guess if you’ve read the previous question, a bimolecular reaction involves two reactant molecules undergoing a chemical reaction. They may form a single product molecule (if they combine) or multiple product molecules.

What is the equilibrium constant for a reaction?

Some reactions can go both in forward and reverse, while others can only go in one direction. For a reaction that can go both ways, the equilibrium constant describes the ratio of products to reactants. For the reaction:

A Image B

The equilibrium constant would be:

Keq = [B]/[A]

For the reaction:

A + B Image C

The equilibrium constant would be:

Keq = [C]/[A][B]

and for the reaction:

A + B Image C + D

The equilibrium constant would be:

Keq = [C][D]/[A][B]

Reactions with a large equilibrium constant (Keq > 1) favor formation of the products, while reactions with a small equilibrium constant (Keq < 1) favor formation of the reactants.

What is Le Chatelier’s principle?

Le Chatelier’s principle tells us how to predict the effect a change in conditions will have on a chemical equilibrium. It tells us that a system at equilibrium will shift to counteract changes that disturb the equilibrium. These could be changes in concentrations of chemical species, temperature, pressure, or other conditions. The most commonly discussed changes involve changes in concentration of chemical species, so we’ll just focus on those here. For this equilibrium:

A + B Image C + D

If we decrease the concentration of A, some C and D will react to replenish the A that is depleted, so the concentrations of C and D will decrease. As species A is replenished, more B will be created as well. So the net effect is that decreasing the concentration of A will also decrease the concentrations of C and D, and at the same time increase the concentration of B. More generally, decreasing the concentration of a reactant will cause the equilibrium to shift toward the reactants, increasing the concentrations of other reactants and decreasing the concentrations of products. The converse is also true: Decreasing the concentration of a product will cause the equilibrium to shift toward the products, increasing the concentrations of other products and decreasing the concentrations of reactants.

It is important to keep in mind that Le Chatelier’s principle only applies to reversible chemical processes (chemical equilibria), so everything we have said here does not apply to reactions that can only proceed in the forward direction.

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An example of a free energy diagram.

What is a free energy diagram for a chemical reaction?

A free energy diagram is probably easiest to understand by taking a look at one (see diagram) as we explain the key features.

The y-axis measures the relative free energy of the chemical species we’re dealing with, while the x-axis describes the reaction coordinate (it’s common that going left to right is forward progress in the reaction, but this isn’t necessarily the case 100% of the time). On the left we have our reactants. In general there may be any number of reactants, and here we’ve just denoted two species, A and B. The “hill” in the middle is the energetic barrier to the chemical reaction, and the quantity Ea denotes the height of this energy barrier. The quantity Ea is commonly referred to as the activation energy for the reaction. On the right-hand side of the diagram we have our products. Again, there can be any number of products, and here we’ve denoted them C and D. Finally we have the quantity G, which describes the change in Gibbs free energy associated with the reaction. The fact that the reactants are higher in free energy than the products tells us that this particular example is a spontaneous reaction. If the reactants were lower in free energy than the products, the reaction would not be spontaneous.

Can chemical reactions involve multiple steps?

Yes, and many do. While some chemical reactions may only involve a single step, others may involve ten or more elementary steps. Of course, chemists working in different subfields may have different definitions of what constitutes a step of a reaction, depending on what aspects of the reaction they focus on.

What is an example of a multistep chemical reaction?

Many reactions in biological chemistry (see also “Biochemistry”) are multistep chemical reactions. Glycolysis, which is the process of breaking down sugar to generate energy, for example, involves ten sequential steps. Each step is carried out by a special type of catalyst, called an enzyme. There are countless multistep processes in biological systems.

What is meant by “dynamic equilibrium”?

Equilibrium conditions in a reversible chemical reaction are described as a dynamic equilibrium. This means that even at equilibrium the reaction has not stopped, and the forward and reverse reactions are still taking place. The bulk concentrations of reactants and products don’t change, but this is just because the forward and reverse reactions are happening at equal rates. The reaction never stops, it just reaches equilibrium.

What is the rate-determining step of a reaction?

In a reaction with multiple steps, the rate-determining step is the slowest step. It’s the step that limits the rate of formation of the final products, usually because it has the highest activation energy.

What does it mean when we say a chemical reaction takes one minute?

When we say a reaction takes one minute, what we’re really saying is that a certain fraction (specifically that fraction is 1-1/e, or about 63%) of the reactant molecules have gone on to form products after one minute has passed (e refers to the irrational number e = 2.7182818…). If we say a reaction takes one year, or any other amount of time, we’re also referring to that same fraction of reactants becoming products. It’s not that 100% of the reactants have become products, but rather just a certain fraction.

Why are chemical reactions important in biological systems?

Chemical reactions make everything in your body work! This is true in all living things (plants, animals, insects, bacteria, etc.). Anytime you make even a slight movement, lots of chemical reactions have to take place. Digesting food, building up or breaking down fat, breathing, cell reproduction, or pretty much any other process that happens in your body involves lots of chemical reactions. Chemistry isn’t only important in the lab, it’s essential to everything related to life!