Chapter 19Chemical Thermodynamics

A.P. Chemistry

Intro• 19.1 Spontaneous Processes• Spontaneous processes are defined, and several examples given. Previously, we have seen that very exothermic reactions tend to be spontaneous, but

there are many examples of spontaneous endothermic processes. The focus of this chapter is to bring together thermodynamic concepts for the purpose of reliably predicting the spontaneity of reactions and processes.

• 19.2 Entropy and the Second Law of Thermodynamics• The concept of disorder, first presented in connection with solution formation, is given a thermodynamic name, entropy. Entropy, symbolized S, is

presented as a state function. You will see that processes leading to an increase in entropy tend to be spontaneous. Generalizations are made about the types of processes that lead to an increase in the entropy of a system. A movie illustrates the increasing disorder associated with solution formation.

• 19.3 The Molecular Interpretation of Entropy• Molecules can store energy in three forms of motion: translational, vibrational, and rotational. Each mode of motion is referred to as a degree of

freedom. You will see that the more degrees of freedom a molecule has, the greater its entropy.• 19.4 Calculation of Entropy Changes• Here you will learn to calculate the entropy change, S° , associated with a chemical reaction. You will see that thermodynamic tables list absolute

entropies, in contrast to enthalpies that can only be tabulated as changes in enthalpy.• 19.5 Gibbs Free Energy• Bringing together the concepts of enthalpy and entropy, both of which can serve as reasonably good predictors of spontaneity, we introduce the

concept of free energy. For a process at constant pressure and constant temperature, a negative value for G° means that a reaction is spontaneous as written, under standard conditions.

• 19.6 Free Energy and Temperature• G° for a reaction changes with temperature. In this section you will learn how to determine G° at temperatures other than 25°C by assuming that H°

and S° remain constant with changing temperature. You will see a demonstration of the formation of water from the combustion of hydrogen gas. An animation illustrates the decomposition reaction with which automobile air bags are inflated. A simulation allows you to explore the relationship between G° and temperature.

• 19.7 Free Energy and the Equilibrium Constant• In this section you will see that G° tells us essentially the same thing as K, namely whether an equilibrium lies to the right or to the left. And you will

see that G gives us essentially the same information as Q, namely, in which direction the reaction has to proceed to achieve equilibrium.

19.1 Spontaneous Processes• A spontaneous process is one that occurs without outside

intervention• A spontaneous process is not reversible. Chemical reactions that

are highly exothermic, where the internal energy of the system is much lower after the reaction has occurred, tend to be spontaneous.

• Only for a reversible process is it possible to return a system to its original state with no net change in either system or surroundings.

• Finally, it is important to understand that because a reaction or process is spontaneous does not mean that it is fast. Thermodynamics tells us nothing about rates of reaction

19.2 Entropy and the Second Law of Thermodynamics

• The second law of thermodynamics states that for any reversible process, the entropy change of the universe is zero, and that for any irreversible process, the entropy change of the universe is positive.

• In thermodynamic terms randomness is referred to as entropy and given the symbol S. The more random a system, the greater its entropy. Like internal energy (E) and enthalpy (H), entropy is a state function. The change in entropy, S, depends only on the initial and final states and not on the path between them

Entropy

Sample Entropy Calculation

Entropy solution

19.3 The Molecular Interpretation of Entropy

• Entropy is a measure of the randomness of a system. A molecule's degrees of freedom are related to the number of different types of motion it can exhibit. The three types of motion associated with molecules are translational motion, vibrational motion, and rotational motion. Translational motion is the movement of all of the atoms of a molecule simultaneously in the same direction. Gas molecules have more translational motion than liquid molecules, which have more translational motion than solid molecules. Vibrational motion is the simultaneous motion of atoms within a molecule, in different directions. Rotational motion is the spinning of a molecule.

Relative Entropy changes

19.4 Calculation of Entropy Changes

Note that• Unlike enthalpies of formation, the standard molar

entropies of elements are not zero.• The standard molar entropies of gases are greater

than those of liquids and solids• The standard molar entropies generally increase with

increasing molar mass of the substance.• The standard molar entropies generally increase with

the number of atoms in the formula of the substance.

19.5 Gibbs Free Energy

• In order to predict whether or not a process is spontaneous, we can combine the concepts of enthalpy and entropy to introduce a new state function: Gibbs free energy, G (also called, simply, free energy).

• If G is negative, the reaction is spontaneous as written.

• If G is zero, the reaction is at equilibrium. Neither the forward nor reverse process is spontaneous.

• If G is positive, the reaction is nonspontaneous as written. (The reverse process is spontaneous.)

19.6 Free Energy and Temperature

At 298 K

At any temperature

19.7 Free Energy and the Equilibrium Constant

• The gauge of spontaneity under standard conditions is G° . The gauge of spontaneity under nonstandard conditions is G.

• At equilibrium, G = 0 and Q = K. This lets us derive a relationship between G° and the equilibrium constant, K.