MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Compounds and Stoichiometry
Balancing Chemical Equations

LEARNING GOALS

After Chapter 4.4, you will be able to:

· Balance a chemical equation

Because chemical equations express how much and what types of reactants must be used to obtain a given quantity of product, it is of utmost importance that the reaction be balanced so as to reflect the laws of conservation of mass and charge. The mass of the reactants consumed must equal the mass of products generated. More specifically, one must ensure that the number of atoms of each element on the reactant side equals the number of atoms of that element on the product side. Stoichiometric coefficients, which are the numbers placed in front of each compound, are used to indicate the relative number of moles of a given species involved in the reaction. For example, the balanced equation expressing the combustion of nonane is:

C₉H₂₀ (g) + 14 O₂ (g) → 9 CO₂ (g) + 10 H₂O (l)

The coefficients indicate that one mole of C₉H₂₀ gas must be reacted with fourteen moles of O₂ gas to produce nine moles of carbon dioxide and ten moles of water. In general, stoichiometric coefficients are given as whole numbers.

MCAT Expertise

It is unlikely that you will come across a question that explicitly asks you to balance an equation. However, you will need to recognize unbalanced reactions and quickly add the necessary coefficients. To balance a reaction, look at the number of atoms of each element and the charge on both sides (especially for oxidation—reduction reactions).

The steps taken to balance a chemical reaction are necessary to ensure that calculations regarding the reaction are performed correctly. Let’s review the steps involved in balancing a chemical equation, using an example.

Example:

Balance the following reaction:

C₄H₁₀ (l) + O₂ (g) → CO₂ (g) + H₂O (l)

Method One:

First, balance the carbons (4 on reactant side) in the products. Carbons are a good choice to start with because they appear only once on both sides of the reaction:

C₄H₁₀ (l) + O₂ (g) → 4 CO₂ (g) + H₂O (l)

Then, balance the hydrogens (10 on reactant side) in the products. Again, hydrogens appear only once on each side, making them a good choice to work on next:

C₄H₁₀ (l) + O₂ (g) → 4 CO₂ (g) + 5 H₂O (l)

Next, balance the oxygens (now 13 on product side) in the reactants. Note that oxygens appear in multiple reactants and products, making them the most complex atom to balance and therefore the best to leave until the end:

Finally, produce a whole number ratio. In this case, double each coefficient.

2 C₄H₁₀ (l) + 13 O₂ (g) → 8 CO₂ (g) + 10 H₂O (l)

Finally, check that all of the elements and the total charges are balanced correctly. If there is a difference in total charge between the reactants and products, then the charge will also have to be balanced. (Instructions for balancing charge in oxidation—reduction reactions are found in Chapter 11 of MCAT General Chemistry Review.)

Method Two:

First, if in doubt, take a guess. Assume there are 4 of the first reactant and balance the carbons appropriately.

4 C₄H₁₀ (l) + O₂ (g) → 16 CO₂ (g) + H₂O (l)

Second, balance the hydrogens (40 on reactant side) in the products.

4 C₄H₁₀ (l) + O₂ (g) → 16 CO₂ (g) + 20 H₂O (l)

Third, balance the oxygens (now 52 on product side) in the reactants.

4 C₄H₁₀ (l) + 26 O₂ (g) → 16 CO₂ (g) + 20 H₂O (l)

Fourth, produce the simplest whole number ratio through the greatest common factor. In this case, divide each side by 2.

2 C₄H₁₀ (l) + 13 O₂ (g) → 8 CO₂ (g) + 10 H₂O (l)

Finally, check that all of the elements and the total charges are balanced correctly. Notice that both methods produce a multiple of our final answer. These ratios are both correct, but in terms of the stoichiometry one performs on the MCAT, the simpler the numbers are, the easier calculations will become.

Key Concept

When balancing equations, focus on the least represented elements first and work your way to the most represented element of the reaction (usually oxygen or hydrogen). If you’re stuck, take a guess for the coefficient of the first reactant and balance the remainder appropriately.

MCAT Concept Check 4.4:

Before you move on, assess your understanding of the material with this question.

1. Balance the following reactions:

o ___ Fe + ___ Cl₂ → ___ FeCl₃

o ___ Zn + ___ HCl → ___ ZnCl₂ + ___ H₂

o ___ C₅H₁₂ + ___ O₂ → ___ CO₂ + ___ H₂O

o ___ Pb(NO₃)₂ + ___ AlCl₃ → ___ PbCl₂ + ___ Al(NO₃)₃