Acids and bases (A-level only) Revision Notes

Acids and Bases Revision Note

Key Definitions

• Brønsted-Lowry Acids: Substances that donate protons (H⁺ ions).
• Brønsted-Lowry Bases: Substances that accept protons.

Conjugate Acid-Base Pairs

• A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton.

Proton Transfer Equations

• Writing equations for proton transfer is essential for understanding acid-base reactions. For example, in the reaction of hydrochloric acid (HCl) with water:

HCl + H₂O ⇌ Cl⁻ + H₃O⁺

Acid Strength vs. Concentration

• Acid strength refers to the degree of ionization in solution, while concentration refers to the amount of acid present in a given volume of solution.

pH Calculations

• Calculating pH: pH = -log[H⁺]
• Calculating [H⁺] from pH: [H⁺] = 10^(-pH)
• Using Kw: Kw = [H⁺][OH⁻] = 1.0 x 10⁻¹⁴ at 25°C.

Weak Acids and Ka

• The acid dissociation constant (Ka) measures the strength of a weak acid. The expression for a weak acid HA dissociating in water is:

Ka = [H⁺][A⁻] / [HA]

pH of Weak Acids

• To calculate the pH of a weak acid, use the formula:

pH = 0.5(pKa - log[C])

where C is the concentration of the acid.

Acid-Base Titrations

• Understand how to perform calculations using titration data, including determining the equivalence point and calculating concentrations.

pH Curves

• Sketch and explain pH curves for titrations involving strong and weak acids/bases, noting the shape and key points such as the equivalence point.

Indicators

• Select suitable indicators based on the pH range of the titration curve.

Buffer Solutions

• Buffers resist changes in pH upon the addition of small amounts of acid or base. Understand how acidic and basic buffers work and how to calculate their pH.