7: Equilibrium and Thermodynamics

• 7.1: The Concept of Dynamic Equilibrium
At equilibrium, the forward and reverse reactions of a system proceed at equal rates. Chemical equilibrium is a dynamic process consisting of forward and reverse reactions that proceed at equal rates. At equilibrium, the composition of the system no longer changes with time. The composition of an equilibrium mixture is independent of the direction from which equilibrium is approached.
• 7.2 The Equilibrium Constant
The law of mass action describes a system at equilibrium in terms of the molar concentrations of the products and the reactants.
• 7.3: Calculating the Equilibrium Constant From Measured Equilibrium Concentrations
Various methods can be used to solve the two fundamental types of equilibrium problems: (1) those in which we calculate the concentrations of reactants and products at equilibrium and (2) those in which we use the equilibrium constant and the initial concentrations of reactants to determine the composition of the equilibrium mixture. When an equilibrium constant is calculated from equilibrium concentrations, concentrations are plugged into the equilibrium constant expression.
• 7.4 Predicting the Direction of a Reaction
If a reaction is not at equilibrium, it is convenient to have a method for calculating in which direction a reaction will need to proceed in order to reach equilibrium, and by how much the reactant and product concentrations must change to reach equilibrium. Chemists calculate an instantaneous reaction quotient, Q, to determine the state the reaction at a given instant in time. By comparing the values of K and of Q, it is possible to determine what the reaction will do to reach equilibrium.
• 7.5$$:$$ Le Châtelier’s Principle: How a System at Equilibrium Responds to Disturbances
Systems at equilibrium can be disturbed by changes to temperature, concentration, and, in some cases, volume and pressure; volume and pressure changes will disturb equilibrium if the number of moles of gas is different on the reactant and product sides of the reaction. The system's response to these disturbances is described by Le Châtelier's principle: The system will respond in a way that counteracts the disturbance. Adding a catalyst affects the reaction rates but does not alter equilibrium.
• 7.6: The First Law of Thermodynamics
The first law of thermodynamics states that the energy of the universe is constant. The change in the internal energy of a system is the sum of the heat transferred and the work done. At constant pressure, heat flow (q) and internal energy (U) are related to the system’s enthalpy (H). The heat flow is equal to the change in the internal energy.
• 7.7: Enthalpy: The Heat Evolved in a Chemical Reaction at Constant Pressure
Enthalpy is a state function used to measure heat transferred from a system to its surroundings or vice versa at constant pressure. Only the change in enthalpy (ΔH) can be measured. A negative ΔH means that heat flows from a system to its surroundings; a positive ΔH means that heat flows into a system from its surroundings. Calorimetry measures enthalpy changes, where the amount of heat released or absorbed equals the mass of the system, the change in temperature, and the specific heat capacity.
• 7.8 Quantifying Heat
Heat is the amount of energy that is transferred from one system to its surroundings because of a temperature difference. All forms of energy can be interconverted. Three things can change the energy of an object: the transfer of heat, work performed on or by an object, or some combination of heat and work.
• 7.9: Entropy and the Second Law of Thermodynamics
Entropy (S) is a state function whose value increases with an increase in the number of available microstates. For a given system, the greater the number of microstates, the higher the entropy. During a likely process, the entropy of the universe increases.
• 7.10: Gibbs Free Energy
By combining the entropy, enthalpy, and temperature of a system, we obtain the Gibbs free energy (G). We can use the change in standard Gibbs free energy to predict whether a reaction is product-favored or reactant-favored at equilibrium.  We can use the change in free energy,  ΔG, to determine the instantaneous status of a reaction,  whether it is currently at equilibrium, must proceed in the reverse direction to reach equilibrium,  or must proceed in the forward direction to reach equilibrium.
• 7.11 Gibbs Free Energy and Equilibrium
We can use the sign of ΔG° to determine if a reaction is product-favored or reactant-favored at equilibrium. We can use the sign of ΔG to determine in which direction a reaction must proceed to reach equilibrium. We can use the equilibrium constant K at one temperature and ΔH° to estimate the equilibrium constant for a reaction at another temperature.