Kaplan MCAT General Chemistry Review

CaCl₂ (aq) + 2AgNO₃ (aq) Ca(NO₃)₂ (aq) + 2AgCl (s)

NEUTRALIZATION REACTIONS

Neutralization reactions are a specific type of double displacement in which an acid reacts with a base to produce a salt. For example, hydrochloric acid and sodium, hydroxide will react to form sodium chloride and water.

HCl (aq) + NaOH NaCl (aq) + H₂O

Key Concept

Decomposition reactions generally have more products than reactants.

C A + B

Bridge

Acids and bases (which we will see in Chapter 10) combine in neutralization reactions to produce salts and water.

Net Ionic Equations

Just as in our discussion of equivalents and normality, in which we admitted that we really care about the acid and the base functionalities themselves rather than the compounds that are donating the hydrogen protons and hydroxide ions, here we have another opportunity to confess our little secrets. Come on, say it with us—you’ll feel better getting this off your chest: When it comes to many reactions, there are certain species that we find boring and want to ignore. For example, in many reactions, such as displacements, the ionic constituents of the compounds are in solution, so we can write the chemical reaction in ionic form. In the previous example involving the reaction between zinc and copper (II) sulfate, the ionic equation would be

Zn (s) + Cu²⁺ (aq) + SO₄^2- (aq) Cu (s) + Zn²⁺ (aq) + SO₄^2- (aq)

You’ll notice that the SO₄^2- (aq) is just hanging, not really doing anything. It’s not taking part in the overall reaction but simply remains in the solution unchanged. We call such species spectator ions. They’re like boring people who go to parties and just stand around, taking up space. Because the SO₄^2- ion isn’t doing anything of interest, we can ignore it and write a net ionic reaction showing only the species that actually participate in the reaction:

Zn (s) + Cu²⁺ (aq) Cu (s) + Zn²⁺ (aq)

Net ionic equations list only the cool people at the party who are actually doing fun stuff. They are important for demonstrating the actual reaction that occurs during a displacement reaction.

Balancing Equations

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. Look at the

1) charge on each side; and

2) number of atoms of each element.

Balancing a checkbook is a dying art. In this age of automated and computer banking, it’s easy enough to monitor the account balance without taking the time to record by hand each and every transaction. Nevertheless, the art of balancing chemical equations is one in which you must be skilled, and you ought to expect the MCAT to test your understanding of the steps involved. Because chemical equations express how much and what type 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, you must ensure that the number of atoms on the reactant side equals the number of atoms on the product side. Stoichiometric coefficients, which are placed in front of the compound, are used to indicate the number of moles of a given species involved in the reaction. For example, the balanced equation expressing the formation of water is

2 H₂ (g) + O₂ (g) 2 H₂O (l)

The coefficients indicate that two moles of H₂ gas must be reacted with one mole of O₂ gas to produce two moles of water. In general, stoichiometric coefficients are given as whole numbers.

The steps you take to balance a chemical reaction are necessary to ensure you have the correct recipe. You wouldn’t want to bake a cake using a recipe that didn’t properly balance the amounts of flour, eggs, sugar, and butter, and you wouldn’t want to conduct an experiment or chemical process without the balanced equation.

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).

Let’s review the steps involved in balancing a chemical equation, using an example.

Example: Balance the following reaction.

C₄H₁₀() + O₂(g) CO₂(g) + H₂O()

Solution: First, balance the carbons in the reactants and products.

C₄H₁₀ + O₂ 4 CO₂ + H₂O

Second, balance the hydrogens in the reactants and products.

C₄H₁₀ + O₂ 4 CO₂ + 5 H₂O

Third, balance the oxygens in the reactants and products.

2 C₄H₁₀ + 13 O₂ 8 CO₂ + 10 H₂O