General Chemistry I – Chemical Equilibrium and Acid-Base Concepts (Unit 4 Study Guide)

Chemical Equilibrium

Equilibrium Concepts

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. The equilibrium constant, K, quantifies the ratio of product to reactant concentrations at equilibrium.

• Equilibrium Constant Expression (Kc): For a reaction aA + bB ⇌ cC + dD, the equilibrium constant is given by:

$K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

• Homogeneous Equilibria: All reactants and products are in the same phase (usually aqueous or gaseous).

• Heterogeneous Equilibria: Reactants and products are in different phases; pure solids and liquids are omitted from the equilibrium expression.

Calculating Equilibrium Concentrations

• Given initial concentrations and the equilibrium constant, use an ICE (Initial, Change, Equilibrium) table to solve for unknown concentrations.

• Example: For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, with initial concentrations and $K_c$ provided, set up the equilibrium expression and solve for the unknown.

Interpreting Equilibrium Constants

• Large K: Product-favored equilibrium (more products at equilibrium).

• Small K: Reactant-favored equilibrium (more reactants at equilibrium).

• K ≈ 1: Significant amounts of both reactants and products at equilibrium.

Visualizing Equilibrium with Particle Diagrams

• Diagrams can represent the relative amounts of reactants and products at equilibrium.

• At equilibrium, the ratio of products to reactants matches the value of $K_c$.

Acid-Base Equilibria

pH, pOH, and Ion Product of Water

The pH and pOH scales measure the acidity and basicity of solutions. The ion product of water ($K_w$) at 25°C is $1.0 \times 10^{-14}$.

• $pH = -\log[H^+]$

• $pOH = -\log[OH^-]$

• $pH + pOH = 14$ (at 25°C)

• $K_w = [H^+][OH^-] = 1.0 \times 10^{-14}$

Acid and Base Strength

• Strong acids/bases: Completely ionize in solution.

• Weak acids/bases: Partially ionize; characterized by their dissociation constants ($K_a$ for acids, $K_b$ for bases).

• Comparing Acid Strength: Larger $K_a$ means a stronger acid; smaller $pK_a$ also indicates a stronger acid.

Conjugate Acid-Base Pairs

• Acids donate protons to form their conjugate bases; bases accept protons to form their conjugate acids.

• Example: $NH_3$ (base) + $H_2O$ (acid) ⇌ $NH_4^+$ (conjugate acid) + $OH^-$ (conjugate base)

Acid Dissociation Expression

• For a weak acid HA dissociating in water: $HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$

• The acid dissociation constant is:

$K_a = \frac{[H_3O^+][A^-]}{[HA]}$

Le Châtelier’s Principle

Shifting Equilibrium

Le Châtelier’s Principle states that if a system at equilibrium is disturbed, it will shift to counteract the disturbance and restore equilibrium.

• Adding Reactant/Product: System shifts to consume the added substance.

• Removing Reactant/Product: System shifts to replace the removed substance.

• Changing Temperature: For endothermic reactions, increasing temperature shifts equilibrium right; for exothermic, shifts left.

Sample Table: Comparison of Acid Strength

Solution	Concentration (M)	$K_a$	Acid Strength
HNO2	0.10	$4.5 \times 10^{-4}$	Weaker acid
H2S	0.10	$9.0 \times 10^{-8}$	Weaker acid (than HNO2)

Key Equations and Constants

• Equilibrium constant: $K_c = \frac{[\text{products}]}{[\text{reactants}]}$

• Ion product of water: $K_w = [H^+][OH^-] = 1.0 \times 10^{-14}$ at 25°C

• pH and pOH: $pH = -\log[H^+]$, $pOH = -\log[OH^-]$, $pH + pOH = 14$

• Acid dissociation: $K_a = \frac{[H_3O^+][A^-]}{[HA]}$

Example Problems

• Calculating Equilibrium Concentration: Given initial concentrations and $K_c$, use an ICE table to solve for equilibrium values.

• Determining pH: If $[H^+] = 1.0 \times 10^{-5}$ M, then $pH = 5.0$.

• Identifying Conjugate Bases: The conjugate base of $HSO_4^-$ is $SO_4^{2-}$.

• Le Châtelier’s Principle: Adding a catalyst does not shift equilibrium; changing concentration or temperature does.

Additional info: These notes cover core concepts from General Chemistry I, Unit 4, focusing on equilibrium and acid-base chemistry, as reflected in the sample questions provided.