Kaplan MCAT General Chemistry Review

Le Châtelier’s principle tells us that changing the temperature of a system will also cause the system to react in a particular way so as to “return” to its equilibrium state, but we have to be careful here, because unlike the effect of changing concentrations or pressures, the result of changing temperature is not simply a change in the reaction quotient, Q_c or Q_p, but a change in K_eq. The change in temperature doesn’t cause the concentrations or partial pressures of the reactants and products to change immediately, so the Q immediately after the temperature change is the same before the temperature change. Before the temperature change, the system was at equilibrium, and Q was equal to K_eq; now after the temperature change, K_eq is a different value (because it depends on temperature), so Q ≠ K_eq. The system has to move in whichever direction allows it to reach its new equilibrium state at the new temperature. That direction is determined by the enthalpy of the reaction (see Chapter 6, Thermochemistry). You can think of heat as a reactant if the reaction is endothermic (+H) and as a product if the reaction is exothermic (-H). Thinking about heat as a reactant or product allows you to apply the principle that we discussed in regards to concentration changes to temperature changes. For example, consider the following exothermic reaction:

A B + heat (-H)

If we placed this system in an ice bath, its temperature would decrease, driving the reaction to the right to replace the heat lost. Conversely, if the system were placed in a boiling water bath, the reaction would shift to the left due to the increased “concentration” of heat. All of this is to say that on Test Day, when you are asked to predict the direction a reaction would go in response to a change in temperature, you must look for the enthalpy change for the reaction, which will be given somewhere in the passage, figure, or question stem.

Key Concept

The reaction

A (aq) + 2 B (g) C (g) + heat

Will shift to the right if... Will shift to the left if...•A or B added•C added•C removed•A or B removed•pressure increased or volume reduced•volume increased or pressure reduced•temperature reduced•temperature increased

Conclusion

We’ve discussed some very important concepts and principles in this chapter related to the studies of reaction rates and chemical equilibria. We began with a consideration of chemical reactions and the mechanisms that illustrate the possible individual steps necessary to transform reactants into products. We demonstrated the way to derive a reaction’s rate law through the analysis of experimental data, and we looked at the factors that can affect the rates of chemical reactions. The second part of this chapter focused on the law of mass action and the significance of the equilibrium state of a chemical reaction. With our understanding of the significance of K_eq and Q, we are able to predict the direction that a reaction will go in response to various stresses—concentration, pressure, or temperature changes—that might be applied to a system.

You’ve worked through another chapter and reviewed some very important topics for Test Day. We cannot stress enough how much all of your hard work and dedication will pay off in points on the MCAT. There’s more to learn and review, but you have already made great progress and will continue to do so. Take pride in the work you are doing, and have confidence in yourself and in your preparation with Kaplan.

CONCEPTS TO REMEMBER

Reaction mechanisms propose a series of steps, the sum of which gives the overall reaction that explains the chemical processes in the transformation of reactants into products. The slowest step in a reaction mechanism is the rate-determining step, and it limits the maximum rate at which the reaction can proceed.

Reaction rates can be measured in terms of the rate of disappearance of reactant or the appearance of product, as measured by changes in their respective concentrations.

The generic rate law is rate = k[A]^x[B]^y, and a particular reaction’s actual rate law usually must be determined by analyzing experimental rate data that relate concentrations of reactants to rates of product formation.

For a reaction to occur, molecules of reactants must collide with each other at the proper angle and with an amount of kinetic energy at least as great as the energy maximum of the transition state, known as the energy of activation, E_a.

Reaction rates can be increased by increasing reactant concentrations (except for zero-order reactions), increasing the temperature, changing the medium, or adding a catalyst.