IX. Chemical Equilibrium

Key focus of this chapter: equilibrium constant

This chapter focuses on equilibrium constant and gives concise summaries of the important things about Le Chatelier’s principle, acid and base equilibrium, pH, and solubility product constant in more detail.

A. Reversible and irreversible reactions

Reversible reaction
Irreversible reaction

B. Equilibrium state

: a state of balance in a reversible reaction, which is reached when the forward and reverse reactions have the same reaction rate (velocity) and the concentrations of reactant and product are no longer changing.

Fig. 1 Equilibrium state of reaction rate and concentration over time

Does not stop forward and reverse reaction (same rate between forward and reverse reaction).
Not the same concentration between the reactants and the products.

C. The equilibrium constant, K

1. K_eq

Dependent on temperature but independent of the concentrations or catalysis
Does not apply to solid and liquids in the equilibrium equation

2. K_cand K_p

Fig. 1 Factors that affect K_cand K_p, reaction rate, and equilibrium

D. Le Chatelier’s principle

: a system stressed by external conditions is relieved by a shift in the equilibrium.

Changes of conditions	Direction of reactions
Concentration
Pressure (volume) & Mole
Temperature & ΔH

Qs/ Which of the following shifts the equilibrium in the forward direction in the reaction shown?

A. Adding WO₃

B. Adding H₂

C. Removing W

D. Removing H₂O

E. Both B and D

ANS: E Adding WO₃ or removing W does not affect the direction of equilibrium, because they are solid. When adding H₂ or removing H₂O, the equilibrium moves in a forward direction to relieve the stress of the different concentrations between the reactants and products.

Qs/ Which of the following is the reverse direction of the equilibrium from the reaction shown?

A. Increasing pressure

B. Decreasing pressure

C. Increasing volume

D. Removing Y

E. A and C

ANS: A When the pressure is increased, the equilibrium moves from big moles of the product (2Y + 2Z) to small moles of the reactant (3X).

Qs/ Which of the following shifts the equilibrium in the forward direction in the reaction shown?

A. Adding X

B. Removing Y

C. Decreasing pressure

D. Increasing temperature

E. All are correct

ANS: E When the reaction is endothermic (ΔH >0), the direction of the equilibrium is shifted forward by an increase in temperature.

E. Acid and base equilibrium

1. Acid and base theory

	Acid	Base

2. Acid and base strength

a. Acid strength

b. Acids and bases in a solution

Transferring H⁺ from stronger acid to stronger base

Strong acids	• 100% dissociation of H
Weak acids	• Partial dissociation of H

Strong acids and strong bases
Strong acids and weak bases Weak acids and strong bases Weak acids and weak bases

	with halogens
	with soluble compound anions

	with 1A group
	with 2A group

3. Equilibrium constant, K_a, K_b, and K_w

a. K_a (acid-dissociation constant) and K_b (base-dissociation constant)

: expression of equilibrium reactions for acid and base dissociations.

The equilibrium constants K_a and K_bdepend on temperature, but are independent of the concentration of acids and bases

Stronger acids and bases have higher values of K_a and K_b

b. K_w (ion-product constant for water)

: Eexpression of dissociation of water

Equilibrium constants	Equilibrium Equations

Qs/ Calculate the value of K_a when 0.002M of HOBr is 0.1% dissociated in a solution.

Qs/ K_a of HNO₂ is 4.5 × 10^-4. Calculate the value of K_b.

F. pH

Qs/ Calculate the pH of a solution containing [H⁺] = 0.000035 M.

Qs/ 0.005M CH₃COOH is dissociated in a solution. When the K_a of CH₃COOH is 1.8×10^-5, calculate the pH of the solution.

G. Solubility product constant, K_sp

: expression of equilibrium reaction for a soluble ionic compound in a solution

Qs/ What is the expression for K_sp of Ca₃(PO₄)₂ in a solution?

Qs/ Calculate K_sp for a saturation of BaF₂when the concentration of Ba²⁺ is 2.0 × 10^-5 M.