18.1 Rates of Reaction

• The rate of a chemical reaction is usually expressed as the change in the amount of a reactant per unit time.
• Factors that influence the rate of a chemical reaction are temperature, concentration, particle size, and the use of a catalyst.

18.2 Reversible Reactions and Equilibrium

• Chemical equilibrium is a state in which the forward and reverse reactions take place at the same rate. At chemical equilibrium, no net change occurs in the system.
• Stresses that can alter the equilibrium position of a reaction are changes in the concentrations of reactants or products, changes in temperature, and changes in pressure.
• A K_eq value greater than 1 means that products are favored over reactants; a K_eq less than 1 means that reactants are favored over products.

18.3 Solubility Equilibrium

• The smaller the numerical value of the solubility product constant, the lower the solubility of the compound.
• A precipitate will form if the product of the concentrations of two ions in a mixture is greater than the K_sp of the compound formed from the ions.

18.4 Entropy and Free Energy

• Spontaneous reactions produce substantial amounts of product and release free energy.
• An increase in entropy favors the spontaneous reaction; a decrease favors the nonspontaneous reaction.
• The size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous.
• The numerical value of ΔG is negative in spontaneous processes because the system loses free energy.

18.5 The Progress of Chemical Reactions

• The value of the specific rate constant, k, in the rate equation is large if products form quickly from reactants.
• Graphs of the progress of a reaction show peaks that correspond to the energies of activated complexes and valleys that correspond to the energies of intermediates and products.