Kaplan MCAT General Chemistry Review

When a system experiences a change in one or more of its properties (such as concentration of reactant or product, temperature, or pressure), we say that it undergoes a process. While processes, by definition, are associated with changes of state of systems, some processes are uniquely identified by some property that is constant throughout the process. For example, isothermal processes occur when the system’s temperature is constant. Constant temperature implies that the total internal energy of the system is constant throughout the process. Adiabatic processes occur when no heat is exchanged between the system and the environment; thus, the heat content of the system is constant throughout the process. Finally, isobaric processes occur when the pressure of the system is constant. Isothermal and isobaric processes are common, because it is usually easy to control temperature and pressure.

Processes themselves can be also classified as spontaneous or nonspontaneous. A spontaneous process is one that can occur by itself without having to be driven by energy from an outside source. Calculating the change in the Gibbs free energy (G) for a process, such as a chemical reaction, allows us to predict whether the process will be spontaneous or nonspontaneous. The same quantities that are used to calculate the change in the Gibbs free energy, H and S, can also tell us whether or not the process will be temperature-dependent; that is, spontaneous at some temperatures and nonspontaneous at others (see the section on Gibbs free energy).

Spontaneous reactions, as you’ll recall from our discussion of chemical kinetics and equilibrium in Chapter 5, will not necessarily happen quickly and may not go to completion. Many spontaneous reactions have very high activation energies and, therefore, rarely actually take place. For example, when was the last time you saw a match ignite itself spontaneously? Or for that matter, when was the last time you or a loved one spontaneously combusted? However, provide an amount of thermal energy (generated by the friction associated with “striking the match”) that equals or exceeds the energy of activation, and the match will light and burn spontaneously. Combustion, the combination of the chemical components of the match with molecular oxygen in the air, will not need any additional external energy input in order to proceed once the energy of activation has been supplied. Some spontaneous reactions proceed very slowly. The role of enzymes, biological catalysts, is to selectively enhance the rate of certain spontaneous but slow chemical reactions so that the biologically necessary products can be formed at a rate sufficient for sustaining life. As we learned in Chapter 5, some reactions do not go to completion but settle into a low-energy state called equilibrium. Spontaneous reactions may go to completion, but many simply reach equilibrium with dynamically stable concentrations of reactants and products.

States and State Functions

Every year in January, the president of the United States speaks to the nation in a State of the Union address. In the speech, the president gives an assessment of the parameters by which we can measure the relative well-being of the nation and its citizens. There is usually discussion of war or peace; economic indicators; domestic programs; and other measurements of the social, political, and economic status of the country. These indicators help the people understand where they “are” as a nation in comparison to where they were the year prior, but less so how they got to where they are now.

Similarly, the state of a system is described by certain macroscopic properties of the system. These properties, or state functions, describe the system in an equilibrium state. They cannot describe the process of the system; that is, how the system got to its current equilibrium. They are useful only for comparing one equilibrium state to another. The pathway taken from one equilibrium state to another is described quantitatively by the process functions, the most important of which are mechanical work (W) and heat (Q).

The state functions include temperature (T), pressure (P), volume (V), density (), internal energy (E or U), enthalpy (H), entropy (S), and Gibbs free energy (G). When the state of a system changes from one equilibrium to another, one or more of these state functions will change. In addition, while state functions are independent of the path (process) taken, they are not necessarily independent of one another. For example, Gibbs free energy is related to enthalpy, entropy, and temperature, as you will see.