BackGeneral Chemistry I – Chemical Equilibrium and Acid-Base Concepts: Study Notes
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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:
Interpreting Kc: A large Kc (>1) indicates products are favored; a small Kc (<1) indicates reactants are favored.
Example: For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), the equilibrium expression is:
Calculating Equilibrium Concentrations
Given initial concentrations and Kc, set up an ICE (Initial, Change, Equilibrium) table to solve for unknowns.
Example: If 0.150 M SO2 and 0.0750 M O2 are mixed with Kc = 429, solve for [SO3] at equilibrium using the equilibrium expression.
Le Châtelier’s Principle
Le Châtelier’s Principle predicts how a system at equilibrium responds to disturbances (changes in concentration, temperature, or pressure).
Adding Reactant/Product: Shifts equilibrium to consume the added substance.
Removing Reactant/Product: Shifts equilibrium to replace the removed substance.
Changing Temperature: For endothermic reactions, increasing temperature shifts equilibrium right; for exothermic, left.
Example: For 2HI(g) ⇌ H2(g) + I2(g), ΔH = +130 kJ, increasing temperature shifts equilibrium right (toward products).
Visualizing Equilibrium with Particle Diagrams
Equilibrium can be represented with diagrams showing relative numbers of reactant and product particles.
At equilibrium, the ratio of products to reactants matches the value of Kc.
Acids, Bases, and pH
pH and pOH Calculations
The pH scale measures the acidity of a solution. pOH is related to the concentration of hydroxide ions.
pH:
pOH:
Relationship: at 25°C
Ion Product of Water: at 25°C
Example: If [H+] = 1.5 × 10−5 M, then [OH−] = M
Acid and Base Strength
Strong Acids/Bases: Completely ionize in solution (e.g., HCl, NaOH).
Weak Acids/Bases: Partially ionize; characterized by equilibrium constants Ka (acid) and Kb (base).
Example: For HF (hydrofluoric acid), the acid dissociation expression is:
Comparing Acid Strength
Acid strength is compared using Ka values; higher Ka means a stronger acid.
Example: Solution A (0.10 M HNO2, Ka = 4.5 × 10−4) vs. Solution B (0.10 M H3Y, Ka = 9.0 × 10−8): Solution A is more acidic.
Conjugate Acid-Base Pairs
Acids donate protons (H+), bases accept protons.
Each acid has a conjugate base, and each base has a conjugate acid.
Example: For HCO3−/CO32−, HCO3− is the acid, CO32− is the conjugate base.
Sample Table: Equilibrium Constants and Reaction Direction
K Value | Relative Magnitude | Reaction Direction Favored |
|---|---|---|
1.0 × 10−10 | Very small | Reactants |
1.0 × 10−3 | Small | Reactants |
1.0 × 103 | Large | Products |
1.0 × 1010 | Very large | Products |
Summary of Key Equations
Equilibrium constant: (coefficients become exponents)
pH:
pOH:
Ion product:
Acid dissociation:
