Kaplan MCAT General Chemistry Review

Once a reaction begins, however, the standard state conditions (i.e., 1 M solutions) no longer hold. The value of the equilibrium constant must be replaced with another number that is reflective of where the reaction is in its path toward equilibrium. To determine the free energy change for a reaction that is in progress, we relate G_rxn (not G°_rxn) to the reaction quotient Q. We reviewed Q in Chapter 5, Chemical Kinetics and Equilibrium. As a reminder,

Key Concept

Note that the right side of this equation is the same as that for K_eq. The use of the Q rather than K indicates that the system is not at equilibrium.

And the relation between G_rxn and Q is given as follows:

G_rxn = G°_rxn + RT lnQ

where R is the gas constant and T is the temperature in Kelvin. This can be simplified to

This reaction is very useful for quick qualitative assessments of Test Day problems in which you are asked to determine the spontaneity of a reaction at some point in the reaction process. By calculating the value of the reaction quotient and then comparing that value to the known equilibrium constant for the reaction, you will be able to predict whether the free energy change for the reaction is positive (giving a nonspontaneous reaction) or negative (giving a spontaneous reaction). That is, if the ratio of Q/K_eq is less than one (Q < K_eq) , then the natural log will be negative and the free energy change will be negative, so the reaction will spontaneously proceed forward until equilibrium is reached. If the ratio of Q/K_eq is greater than one (Q > K_eq) , then the natural log will be positive, and the free energy change will be positive. In that case, the reaction will spontaneously move in the reverse direction until equilibrium is reached. Of course, if the ratio is equal to one, then that means that the reaction quotient is equal to the equilibrium constant; the reaction is at equilibrium, and by definition, the free energy change is zero (the natural log of 1 is 0).

Conclusion

We began our discussion of thermochemistry with a review of different ways in which we characterize systems (isolated, closed, etc.) and processes (adiabatic, isothermal, etc.). We then further classified systems according to their state functions: system properties such as volume, pressure, temperature, enthalpy, entropy, and Gibbs free energy that describe the equilibrium state. We defined enthalpy as the heat content of the system and the change in enthalpy as the change in heat content of the system from one equilibrium state to another. Enthalpy is characterized as the energy found in the intermolecular interactions and in the bonds of the compounds in the system. We explored the various ways Hess’s law can be applied to calculate the total enthalpy change for a series of reactions. Moving on to entropy, we described this property as a measure of the degree to which energy in a system becomes spread out through a process. There is danger in thinking too literally about entropy as “disorder,” as a system’s entropy may be increasing even if there is no observable change in the system’s macroscopic disorder (e.g., ice warming from -10°C to -5°C). Gibbs free energy combines enthalpy and entropy considerations, and the change in Gibbs function determines whether a process will be spontaneous or nonspontaneous. When the change in Gibbs function is negative, the process is spontaneous, but when the change in Gibbs function is positive, the process is nonspontaneous.

Great job! This is a difficult material, but your focus and attention to these topics will pay off in points on Test Day. The changes in energy that accompany chemical processes are tested heavily on the MCAT.

CONCEPTS TO REMEMBER

Systems can be characterized as isolated (no heat or matter exchange), closed (no matter exchange), or open (both heat and matter exchange possible).

Some processes are identified by some constant property of the system: isothermal (constant internal energy/temperature), adiabatic (no heat exchange), and isobaric (constant pressure).

State functions are physical properties of a system that describe the equilibrium state; as such, changes in state functions are pathway-independent. Some examples of state functions are temperature, volume, pressure, internal energy, enthalpy, entropy, and Gibbs free energy.

Standard state of a substance is the most stable form (phase) of the substance under standard state conditions (298 K and 1 atm). Standard enthalpy, standard entropy, and standard free energy changes are measured under standard state conditions.

Heat and temperature are not the same thing. Temperature is a scale related to the average kinetic energy of the molecules in a substance. Heat is the transfer of energy that results from two objects at different temperatures being put in thermal contact with each other. Heat energy transferred from one substance to another is measured by the methods of calorimetry.