Kaplan MCAT General Chemistry Review

The rate at which a reaction takes place may also be affected by the medium in which it takes place. Just as children playing tag would prefer to play on a grassy field, but another group of children wanting to get a game of ice hockey going would be looking for an ice rink, some molecules are more likely to react with each other in aqueous environments, while others are more likely to react in a nonaqueous solvent, such as DMSO (dimethylsulfoxide) or ethanol. Furthermore, the physical state of the medium (liquid, solid, or gas) can also have a significant effect. Generally, polar solvents are preferred because their molecular dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, which permits the reaction to occur faster.

Catalysts

Catalysts are substances that increase reaction rate without themselves being consumed in the reaction. Catalysts interact with the reactants, either by adsorption or through the formation of intermediates, and stabilize them so as to reduce the energy of activation necessary for the reaction to proceed. While many catalysts, including all enzymes, chemically interact with the reactants, upon formation of the products, they return to their original chemical state. They may increase the frequency of collisions between the reactants; change the relative orientation of the reactants, making a higher percentage of the collisions effective; donate electron density to the reactants; or reduce intramolecular bonding within reactant molecules. In homogeneous catalysis, the catalyst is in the same phase (solid, liquid, gas) as the reactants. In heterogeneous catalysis, the catalyst is in a distinct phase. Figure 5.3 compares the energy profiles of catalyzed and uncatalyzed reactions.

Figure 5.3

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Enzymes are essential to most biological processes.

If you look closely at the energy profiles in the figure, you’ll notice that the only effect of the catalyst is the decrease in the energies of activation, E_a, for both the forward and reverse reactions. The presence of the catalyst has no impact on the potential energies of the reactants or the products or the difference between them. This means that catalysts change only the rate of reactions, and in fact, they change the forward rate and the reverse rate by the same factor. Consequently, they have no impact whatsoever on the equilibrium position or the measure of K_eq. Of course, you have to remember that as useful as catalysts are in biological and nonbiological systems, catalysts are not miracle workers: They will not transform a nonspontaneous reaction into a spontaneous one; they only make spontaneous reactions go more quickly toward equilibrium.

Equilibrium

We’ve been dancing around this term for the past couple of pages now. We warned you not to confuse the chemical equilibrium expression for the rate expression. We stressed that catalysts make reactions go faster toward their equilibrium position but that they can’t actually change the equilibrium position or alter the value of K_eq. Well, now we’re really going to get into it—and you’d better start paying attention, because the principles and concepts that are the focus of the rest of this chapter will direct our discussion in the upcoming chapters about some of the most important general chemistry topics for the MCAT: solutions, acids and bases, and redox reactions.

DYNAMIC EQUILIBRIUM OF REVERSIBLE CHEMICAL REACTIONS

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Equilibrium, like biological homeostasis, is a dynamic process that seeks to find balance in all systems. We can use this concept to our advantage on the MCAT in all four of the basic sciences. Equilibria are dynamic, meaning that they do undergo change but their net change will be zero.