Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Chemical Quantities
Information Given by Chemical Equations

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A scientist in a laboratory using a pipette to measure quantities of a liquid.

Suppose you work for a consumer advocate organization and you want to test a company’s advertising claims about the effectiveness of its antacid. The company claims that its product neutralizes times as much stomach acid per tablet as its nearest competitor. How would you test the validity of this claim?

Or suppose that after graduation you go to work for a chemical company that makes methanol (methyl alcohol), a substance used as a starting material for the manufacture of products such as antifreeze and aviation fuels. You are working with an experienced chemist who is trying to improve the company’s process for making methanol from the reaction of gaseous hydrogen with carbon monoxide gas. The first day on the job, you are instructed to order enough hydrogen and carbon monoxide to produce kg of methanol in a test run. How would you determine how much carbon monoxide and hydrogen you should order?

After you study this chapter, you will be able to answer these questions.

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Methanol is a starting material for some jet fuels.

Information Given by Chemical Equations

Objective

· To understand the molecular and mass information given in a balanced equation.

Reactions are what chemistry is really all about. Recall from Chapter 6 that chemical changes are actually rearrangements of atom groupings that can be described by chemical equations. These chemical equations tell us the identities (formulas) of the reactants and products and also show how much of each reactant and product participates in the reaction. The numbers (coefficients) in the balanced chemical equation enable us to determine just how much product we can get from a given quantity of reactants. It is important to recognize that the coefficients in a balanced equation give us the relative numbers of molecules. That is, we are interested in the ratio of the coefficients, not individual coefficients.

To illustrate this idea, consider a nonchemical analogy. Suppose you are in charge of making deli sandwiches at a fast-food restaurant. A particular type of sandwich requires slices of bread, slices of meat, and slice of cheese. We can represent making the sandwich with the following equation:

Your boss sends you to the store to get enough ingredients to make sandwiches. How do you figure out how much of each ingredient to buy? Because you need enough to make sandwiches, you multiply the preceding equation by .

That is

Notice that the numbers correspond to the ratio , which represents the coefficients in the “balanced equation” of making a sandwich. If you were asked to make any number of sandwiches, it would be easy to use the original sandwich equation to determine how much of each ingredient you need.

The equation for a chemical reaction gives you the same type of information. It indicates the relative numbers of reactant and product molecules involved in the reaction. Using the equation permits us to determine the amounts of reactants needed to give a certain amount of product or to predict how much product we can make from a given quantity of reactants.

To illustrate how this idea works with a chemistry example, consider the reaction between gaseous carbon monoxide and hydrogen to produce liquid methanol, . The reactants and products are

Because atoms are just rearranged (not created or destroyed) in a chemical reaction, we must always balance a chemical equation. That is, we must choose coefficients that give the same number of each type of atom on both sides. Using the smallest set of integers that satisfies this condition gives the balanced equation

Check Reactants: , , ; Products: , ,

Again, the coefficients in a balanced equation give the relative numbers of molecules. That is, we could multiply this balanced equation by any number and still have a balanced equation. For example, we could multiply by :

to obtain

This is still a balanced equation (check to be sure). Because represents a dozen, we could even describe the reaction in terms of dozens:

We could also multiply the original equation by a very large number, such as :

which leads to the equation

Just as is called a dozen, chemists call a mole (abbreviated mol). Our equation, then, can be written in terms of moles:

Various ways of interpreting this balanced chemical equation are given in Table 9.1.

Table 9.1. Information Conveyed by the Balanced Equation for the Production of Methanol

Example 9.1. Relating Moles to Molecules in Chemical Equations

Propane, , is a fuel commonly used for cooking on gas grills and for heating in rural areas where natural gas is unavailable. Propane reacts with oxygen gas to produce heat and the products carbon dioxide and water. This combustion reaction is represented by the unbalanced equation

Give the balanced equation for this reaction, and state the meaning of the equation in terms of numbers of molecules and moles of molecules.

Solution

Using the techniques explained in Chapter 6, we can balance the equation.

Check

This equation can be interpreted in terms of molecules as follows:

· molecule of reacts with molecules of to give

· molecules of plus molecules of

or as follows in terms of moles (of molecules):

· mole of reacts with moles of to give moles of plus moles of