MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Equilibrium
Concept Summary

Equilibrium

· Reversible reactions eventually reach a state in which energy is minimized and entropy is maximized.

o Chemical equilibria are dynamic—the reactions are still occurring, just at a constant rate.

o The concentrations of reactants and products remain constant because the rate of the forward reaction equals the rate of the reverse reaction.

· The law of mass action gives the expression for the equilibrium constant, K_eq. The reaction quotient, Q, has the same form but can be calculated at any concentrations of reactants and products.

o Q is a calculated value that relates the reactant and product concentrations at any given time during a reaction.

o K_eq is the ratio of products to reactants at equilibrium, with each species raised to its stoichiometric coefficient. K_eq for a reaction is constant at a constant temperature.

o Pure solids and liquids do not appear in the law of mass action; only gases and aqueous species do.

· Comparison of Q to K_eq provides information about where the reaction is with respect to its equilibrium state.

o If Q < K_eq, ΔG < 0, and the reaction proceeds in the forward direction.

o If Q = K_eq, ΔG = 0, and the reaction is in dynamic equilibrium.

o If Q > K_eq, ΔG > 0, and the reaction proceeds in the reverse direction.

· Equilibrium calculations are broadly applicable to many areas of chemistry but are often formulaic in their application. The magnitude of K_eq determines the balance of a reaction and whether the amount that has reacted can be treated as negligible when compared to other concentrations.

o If K_eq > 1, the products are present in greater concentration at equilibrium.

o If K_eq ≈ 1, products and reactants are both present at equilibrium at reasonably similar levels.

o If K_eq < 1, the reactants are present in greater concentration at equilibrium.

o If K_eq <<< 1, the amount of reactants that have been converted to products can be considered negligible in comparison to the initial concentration of reactants.

Le Châtelier’s Principle

· Le Châtelier’s principle states that when a chemical system experiences a stress, it will react so as to restore equilibrium.

· There are three main types of stresses applied to a system: changes in concentration, pressure and volume, and temperature.

o Increasing the concentration of reactants or decreasing the concentration of products will shift the reaction to the right. Increasing the concentration of products or decreasing the concentration of reactants will shift the reaction to the left.

o Increasing pressure on a gaseous system (decreasing its volume) will shift the reaction toward the side with fewer moles of gas. Decreasing pressure on a gaseous system (increasing its volume) will shift the reaction toward the side with more moles of gas.

o Increasing the temperature of an endothermic reaction or decreasing the temperature of an exothermic reaction will shift the reaction to the right. Decreasing the temperature of an endothermic reaction or increasing the temperature of an exothermic reaction will shift the reaction to the left.

Kinetic and Thermodynamic Control

· Reactions may have both kinetic and thermodynamic products that can be regulated by temperature and the presence of a catalyst.

o Kinetic products are higher in free energy than thermodynamic products and can form at lower temperatures. These are sometimes termed “fast” products because they can form more quickly under such conditions.

o Thermodynamic products are lower in free energy than kinetic products and are therefore more stable. Despite proceeding more slowly than the kinetic pathway, the thermodynamic pathway is more spontaneous (more negative ΔG).