Kaplan MCAT General Chemistry Review

Note that the exponents in the rate law are not equal to the stoichiometric coefficients, unless the reaction actually occurs via a single-step mechanism. Also, note that product concentrations never appear in a rate law. Don’t fall into the common trap of confusing the rate law with an equilibrium expression!

The second trap to be wary of is mistaking the equilibrium aspect of the law of mass action for the kinetic aspect. The equations for the two aspects do look similar, and if you’re not alert, you may mistake one for the other or use one when you should be using the other. The expression for equilibrium includes the concentrations of all the species in the reaction, both reactants and products. The expression for chemical kinetics, the rate law expression, includes only the reactants. K_eq tells you where the reaction’s equilibrium position lies. The rate tells you how quickly the reaction will get there (that is, reach equilibrium).

The third trap we need to warn you about is regarding the rate constant, k. Technically speaking, it’s not a constant, because its particular value for any specific chemical reaction will depend on the activation energy for that reaction and the temperature at which the reaction takes place. However, for a specific reaction at a specific temperature, the rate coefficient is constant. For a reversible reaction, the K_eq is equal to the ratio of the rate constant, k, for the forward reaction, divided by the rate constant, k_-1, for the reverse reaction (see Chapter 6, Thermochemistry).

The fourth and final trap we need to warn you about is that the notion and principles of equilibrium apply to the system only at the end of the reaction; that is, the system has reached equilibrium. The reaction rate, while it theoretically can be measured at any time, is usually measured at or near the beginning of the reaction to minimize the effects of the reverse reaction.

Experimental Determination of Rate Law

We’ve stated this a few times now, but it bears repeating: The values of k, x, and y in the rate law equation (rate = k[A]^x[B]^y) must usually be determined experimentally for a given reaction at a given temperature. Although rate laws can be quite complex and the orders of the reaction difficult to discern, the MCAT limits its coverage of this topic to fairly straightforward reaction mechanisms, experimental data, and rate laws.

The first step in determining a rate law for a specific reaction is to write out the generic rate law on the scratch material provided for you at the testing center. Then look for the necessary data. Typically, you’ll recognize a chart that includes initial concentrations of the reactants and the initial rates of product formation as a function of the reactant concentrations. Usually, the data for three or four trials are included in the chart.

Once you’ve located the data, the next step is to identify a pair of trials in which the concentration of one of the reactants is changed while the concentration of all other reactants remains constant. Under these conditions, any change in rate of product formation (if there is any) from the one trial to the other is due solely to the change in concentration of one reactant. Let’s imagine that compound A’s concentration is constant, while the concentration of B has been doubled. If the rate of the formation of product C has subsequently quadrupled, then you can say to yourself (using your in-the-head-voice, because nobody wants to hear you talk to yourself on Test Day!), “Doubling the concentration of B has resulted in a quadrupling of the production rate of C, so to determine the order of the reaction, y, with respect to reactant B, I need to calculate the power by which the number 2 must be raised to equal 4. Because 2^y = 4, y = 2.” (Kaplan wants you to think exactly this way. When you read this sentence, exactly as we’ve written it, you will be analyzing the data in the correct manner and working through the correct process for generating the correct rate law.)

MCAT Expertise

Traditionally, the MCAT has loved rate problems. With practice, you’ll be able to do these quickly in your head with minimal paper-and-pencil calculations. Remember to look for pairs of reaction trials in which the concentration of only one species changes while the other(s) remain constant.