Comprehensive Guide to AP Chemistry Unit 7: Equilibrium

Dynamic Equilibrium

A state achieved when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products.

Reversible Reactions

Reactions where reactants can be converted into products and vice versa, indicated by a double arrow.

Haber Process

The industrial process for synthesizing ammonia, represented by the equation: N2(g) + 3H2(g) ⇌ 2NH3(g).

Law of Mass Action

The principle stating that at equilibrium, the ratio of concentrations of products to reactants remains constant.

Equilibrium Constant (K)

A numerical value that expresses the ratio of concentrations of products to reactants at equilibrium.

Pure Solids and Liquids in K

Pure solids (s) and liquids (l) are omitted from the equilibrium expression because their concentrations do not change significantly.

Kc

The equilibrium constant for reactions expressed in terms of molar concentrations.

Kp

The equilibrium constant for reactions expressed in terms of partial pressures.

Ksp

The solubility product constant, which describes the equilibrium between a solid and its ions in solution.

Reaction Quotient (Q)

An expression similar to K but calculated using concentrations at any given moment, not just at equilibrium.

Q < K

Indicates that the reaction will shift to the right, favoring the formation of products.

Q > K

Indicates that the reaction will shift to the left, favoring the formation of reactants.

Q = K

Indicates that the system is at equilibrium.

Manipulating K

Operations that can change the equilibrium constant K, such as reversing reactions or multiplying coefficients.

RICE Table

A method used to organize data for equilibrium calculations: Reaction, Initial, Change, and Equilibrium.

Le Châtelier’s Principle

If a change or stress is applied to a system at equilibrium, the system will adjust to counteract that change.

Change in Concentration Effect on Equilibrium

Adding a reactant shifts the equilibrium to the right, while adding a product shifts it to the left.

Change in Pressure Effect on Equilibrium

Increasing pressure shifts the equilibrium toward the side with fewer moles of gas.

Effect of Temperature on Equilibrium

Increasing temperature shifts the equilibrium left for exothermic reactions and right for endothermic reactions.

Common Ion Effect

The decrease in solubility of a salt due to the presence of a common ion in solution.

pH Effect on Solubility

Solubility of salts can increase in acidic conditions if the anion is a weak base.

Precipitation Conditions with Q and Ksp

If Q > Ksp, a precipitate will form; if Q < Ksp, no precipitation occurs.

Equilibrium Expression Rules

Only include concentrations of gases (g) and aqueous solutions (aq) in the equilibrium constant expression.

Temperature's Effect on K

Temperature is the only factor that changes the value of the equilibrium constant K.

Using Molar Concentrations

Always convert to molarity (M) when calculating Q or K for equilibrium expressions.

Reciprocal of K

If a reaction is reversed, the new equilibrium constant is the reciprocal of the original constant.

Exponents in K Calculations

Coefficients in the balanced equation become exponents in the equilibrium expression.

Adding Reactions for K

When adding reactions, multiply their respective K values to find the new K.

$Kp$ to $Kc$ Conversion

The formula Kp = Kc(RT)^Δn relates equilibrium constants for gases in terms of partial pressure and concentration.

Interpretation of K Magnitudes

K >> 1 indicates product favorability, K << 1 indicates reactant favorability, K ≈ 1 indicates significant amounts of both.

Dilution Effect on Equilibrium

Diluting a solution shifts equilibrium toward the side with more aqueous particles.

Effect of Inert Gas Changes

Adding inert gases at constant volume does not change partial pressures of reactants or shift equilibrium.

Precipitation Reaction Shift

The formation of a precipitate occurs when Q exceeds Ksp in a saturated solution.

Concentration vs. Time Graphs at Equilibrium

Graphs flatten as concentrations remain constant, indicating a system at equilibrium.

Rate vs. Time Graphs at Equilibrium

The forward reaction rate decreases while the reverse reaction rate increases until they meet.

Molar Solubility (s)

The number of moles of solute that can dissolve per liter to reach saturation.

Stoichiometric Ratios in RICE

Changes in concentration for RICE tables must reflect the stoichiometry of the reaction.

Hess's Law and K

Conditions derived from Hess’s Law can be applied to equilibrium constants with different mathematical operations.

Equilibrium Achieved

Achieved when the rates of forward and reverse reactions are equal, not when amounts of reactants and products are equal.

Water's Role in K

Water is excluded from equilibrium expressions; its concentration remains constant.

Increases in Moles

Reactions shift toward sides with fewer moles to counterbalance direct pressure changes.

Temperature and K Adjustments

Temperature adjustments cause shifts in equilibrium positions without altering reaction scores.

Understanding Q and K Relationship

Knowledge of Q and K assists in predicting shifts towards equilibrium under certain conditions.