5.3 Quantitative Relationships Based on Chemical Equations

A balanced chemical equation not only describes some of the chemical properties of substances—by showing us what substances react with what other substances to make what products—but also shows numerical relationships between the reactants and the products. The study of these numerical relationships is called stoichiometry. The stoichiometry of chemical equations revolves around the coefficients in the balanced chemical equation because these coefficients determine the molecular ratio in which reactants react and products are made.

Note

The word stoichiometry is pronounced “stow-eh-key-OM-et-tree.” It is of mixed Greek and English origins, meaning roughly “measure of an element.”

Looking Closer: Stoichiometry in Cooking

Let us consider a stoichiometry analogy from the kitchen. A recipe that makes 1 dozen biscuits needs 2 cups of flour, 1 egg, 4 tablespoons of shortening, 1 teaspoon of salt, 1 teaspoon of baking soda, and 1 cup of milk. If we were to write this as a chemical equation, we would write

2 c flour + 1 egg + 4 tbsp shortening + 1 tsp salt + 1 tsp baking soda + 1 c milk → 12 biscuits

(Unlike true chemical reactions, this one has all 1 coefficients written explicitly—partly because of the many different units here.) This equation gives us ratios of how much of what reactants are needed to make how much of what product. Two cups of flour, when combined with the proper amounts of the other ingredients, will yield 12 biscuits. One teaspoon of baking soda (when also combined with the right amounts of the other ingredients) will make 12 biscuits. One egg must be combined with 1 cup of milk to yield the product food. Other relationships can also be expressed.

We can use the ratios we derive from the equation for predictive purposes. For instance, if we have 4 cups of flour, how many biscuits can we make if we have enough of the other ingredients? It should be apparent that we can make a double recipe of 24 biscuits.

But how would we find this answer formally, that is, mathematically? We would set up a conversion factor, much like we did in Chapter 1 “Chemistry, Matter, and Measurement”. Because 2 cups of flour make 12 biscuits, we can set up an equivalency ratio:

[latex]\frac{\text{12 biscuits}}{\text{2 c flour}}[/latex]

We then can use this ratio in a formal conversion of flour to biscuits:

[latex]\text{4c flour}\times\frac{\text{12 biscuits}}{\text{2c flour}}=\text{24 biscuits}[/latex]

Similarly, by constructing similar ratios, we can determine how many biscuits we can make from any amount of ingredient.

When you are doubling or halving a recipe, you are doing a type of stoichiometry. Applying these ideas to chemical reactions should not be difficult if you use recipes when you cook.

Consider the following balanced chemical equation:

2C2H2 + 5O2 → 4CO2 + 2H2O

The coefficients on the chemical formulas give the ratios in which the reactants combine and the products form. Thus, we can make the following statements and construct the following ratios:

Statement from the Balanced Chemical Reaction Ratio Inverse Ratio
two C2H2 molecules react with five O2 molecules [latex]\frac{2C_{2}H_{2}}{5O_{2}}[/latex] [latex]\frac{5O_{2}}{2C_{2}H_{2}}[/latex]
two C2H2 molecules react to make four CO2 molecules [latex]\frac{2C_{2}H_{2}}{4CO_{2}}[/latex] [latex]\frac{4CO_{2}}{2C_{2}H_{2}}[/latex]
five O2 molecules react to make two H2O molecules [latex]\frac{5O_{2}}{2H_{2}O}[/latex] [latex]\frac{2H_{2}O}{5O_{2}}[/latex]
four CO2 molecules are made at the same time as two H2O molecules [latex]\frac{2H_{2}O}{4CO_{2}}[/latex] [latex]\frac{4CO_{2}}{2H_{2}O}[/latex]

Other relationships are possible; in fact, 12 different conversion factors can be constructed from this balanced chemical equation.  Notice that the numbers in the rations have not been reduced to lowest terms in order to make it clear that these numbers are the coefficients from the balanced equation. In each ratio, the unit is assumed to be molecules because that is how we are interpreting the chemical equation.  In future sections, the unit will be moles.

Any of these fractions can be used as a conversion factor to relate an amount of one substance to an amount of another substance. For example, suppose we want to know how many CO2 molecules are formed when 26 molecules of C2H2 are reacted. As usual with a conversion problem, we start with the amount we are given—26C2H2—and multiply it by a conversion factor that cancels out our original unit and introduces the unit we are converting to—in this case, CO2. That conversion factor is [latex]\frac{4CO_{2}}{2C_{2}H_{2}}[/latex], which is composed of terms that come directly from the balanced chemical equation. Thus, we have

  [latex]26C_2H_2(\frac{4CO_{2}}{2C_{2}H_{2}})[/latex]

The C2H2 labels cancel leaving

[latex]26\left ( \frac{4CO_{2}}{2} \right )[/latex]

Thus, 52 molecules of CO2 are formed.  Notice that the word molecules was assumed but not written in the original labels.  Stoichiometry is most valuable when it is scaled up from molecules, which we cannot see or weigh, to moles.  Moles will be discussed in Chapter 6, at which time the equation coefficients will be interpreted as moles of the different substances, rather than molecules.

This application of stoichiometry is extremely powerful in its predictive ability, as long as we begin with a balanced chemical equation. Without a balanced chemical equation, the predictions made by simple stoichiometric calculations will be incorrect.

Example 2

Start with this balanced chemical equation.

KMnO4 + 8HCl + 5FeCl2 → 5 FeCl3 + MnCl2 + 4H2O + KCl

  1. Verify that the equation is indeed balanced.
  2. Give 2 ratios that give the relationship between HCl and FeCl3.

Skill-Building Exercise

Start with this balanced chemical equation.

3 CH2=CH2(g) + 2 KMnO4 (aq) + 4 H2O (l) → 3 CH2OHCH2OH (aq) + 2 MnO2 (s) + 2 KOH (aq)

  1. Verify that the equation is balanced.

  2. Give 2 ratios that give the relationship between KMnO4 and CH2=CH2. (A total of 30 relationships can be constructed from this chemical equation. Can you find the other 28?)

Concept Review Exercises

  1. Explain how stoichiometric ratios are constructed from a chemical equation.

  2. Why is it necessary for a chemical equation to be balanced before it can be used to construct conversion factors?

Key Takeaway

  • A balanced chemical equation gives the ratios in which molecules of substances react and are produced in a chemical reaction.

Exercises

  1. Balance this equation and write every stoichiometric ratio you can from it.

    NH4NO3 → N2O + H2O

  2. Balance this equation and write every stoichiometric ratio you can from it.

    N2 + H2 → NH3

  3. Balance this equation and write every stoichiometric ratio you can from it.

    Fe2O3 + C → Fe + CO2

  4. Balance this equation and write every stoichiometric ratio you can from it.

    Fe2O3 + CO → Fe + CO2

  5. Balance this equation and determine how many molecules of CO2 are formed if 15 molecules of C6H6 are reacted.

    C6H6 + O2 → CO2 + H2O

  6. Balance this equation and determine how many molecules of Ag2CO3(s) are produced if 20 molecules of Na2CO3 are reacted.

    Na2CO3(aq) + AgNO3(aq) → NaNO3(aq) + Ag2CO3(s)

  7. Copper metal reacts with nitric acid according to this equation:

    3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(ℓ)

    1. Verify that this equation is balanced.
    2. How many Cu atoms will react if 488 molecules of aqueous HNO3 are reacted?

  8. Gold metal reacts with a combination of nitric acid and hydrochloric acid according to this equation:

    Au(s) + 3HNO3(aq) + 4HCl(aq) → HAuCl4(aq) + 3NO2(g) + 3H2O(ℓ)

    1. Verify that this equation is balanced.
    2. How many Au atoms react with 639 molecules of aqueous HNO3?

  9. Sulfur can be formed by reacting sulfur dioxide with hydrogen sulfide at high temperatures according to this equation:

    SO2(g) + 2H2S(g) → 3S(g) + 2H2O(g)

    1. Verify that this equation is balanced.
    2. How many S atoms will be formed from by reacting 1,078 molecules of H2S?

  10. Nitric acid is made by reacting nitrogen dioxide with water:

    3NO2(g) + H2O(ℓ) → 2HNO3(aq) + NO(g)

    1. Verify that this equation is balanced.
    2. How many molecules of NO will be formed by reacting 2,268 molecules of NO2?