Kaplan MCAT General Chemistry Review

This chapter focuses on two primary topics: chemical kinetics and chemical equilibrium. As the term suggests, chemical kinetics is the study of reaction rates, the effects of reaction conditions on these rates, and the mechanisms implied by such observations. Chemical equilibrium is a dynamic state of a chemical reaction at which the concentrations of reactants and products stabilize over time in a low-energy configuration. Pay particular attention to the concepts of chemical equilibrium, as we will return to them in our review of solutions, acid-base, and redox chemistry.

Chemical Kinetics

Reactions can be spontaneous or nonspontaneous; the change in Gibbs free energy determines whether or not a reaction will occur, by itself, without outside assistance (see Chapter 6, Thermochemistry). However, even if a reaction is spontaneous, this does not necessarily mean that it will run quickly. In fact, nearly every reaction that our very lives depend upon, while perhaps spontaneous, proceeds so slowly that without the aid of enzymes and other catalysts, we might not ever actually “see” the reaction occur over the course of an average human lifetime. In biology, we discuss the function of enzymes, which selectively enhance the rate of certain reactions (by a factor of 10⁶ to 10¹⁴) over other thermodynamically feasible reaction pathways, thereby determining the course of cellular metabolism, the collection of all chemical reactions in a living cell. For now, however, let us review the topics of reaction mechanisms, rates, rate laws, and the factors that affect them.

REACTION MECHANISMS

Very rarely is the balanced reaction equation, with which we work to calculate limiting reactants and yields, an accurate representation of the actual steps involved in the chemical process from reactants to products. Many reactions proceed by more than one step, the series of which is known as the mechanism of a reaction and the sum of which gives the overall reaction (the one that you, typically, are asked to balance). When you know the accepted mechanism of a reaction, this helps you explain the reaction’s rate, position of equilibrium, and thermodynamic characteristics (see Chapter 6). Consider this generic reaction:

Overall reaction: A₂ + 2B 2AB

Bridge

Mechanisms are proposed pathways for a reaction that must coincide with rate data information from experimental observation. We will be studying mechanisms more in Organic Chemistry.

On its own, this equation seems to imply a mechanism in which two molecules of B collide with one molecule of A₂ to form two molecules of AB. Suppose instead, however, that the reaction actually takes place in two steps:

Step 1: A₂ + B A₂B (slow)

Step 2: A₂B + B 2AB (fast)

You’ll note that the two steps, taken together, give the overall (net) reaction. The molecule A₂B, which does not appear in the overall reaction, is called an intermediate. Reaction intermediates are often difficult to detect, because they may be consumed almost immediately after they are formed, but a proposed mechanism that includes intermediates can be supported through kinetic experiments. One of the most important points for you to remember is that the slowest step in any proposed mechanism is called the rate-determining step, because it acts like a kinetic “bottleneck,” preventing the overall reaction from proceeding any faster than the slowest step. It holds up the entire process in much the same way that the overall rate of an assembly line production can only be as fast as the slowest step or slowest person (who will probably soon find himself out of a job).

Reaction Rates

Reactions, unfortunately, do not come with handy built-in speedometers. We can’t just look at a dial or gauge and read the reaction rate. It takes a little more effort than that. To determine the rate at which a reaction proceeds, we must take measurements of concentrations of reactants and products and note their change over time.

DEFINITION OF RATE