A-Level Chemistry: Acids, Bases, pH and Buffers
Overview: This part of A-level physical chemistry focuses on acid-base theory, pH calculations, weak acid equilibria, the ionic product of water, titration curves, indicators, and buffer solutions.
1. Core definitions and acid-base theory
- Brønsted–Lowry acids are proton donors.
- Brønsted–Lowry bases are proton acceptors.
- Students should understand conjugate acid-base pairs.
- Some specifications also include monobasic, dibasic, and tribasic acids.
2. pH and hydrogen ion concentration
- Definition of pH as a measure of hydrogen ion concentration.
- Use of the expressions
pH = -log[H+] and [H+] = 10^-pH.
- Converting between pH values and hydrogen ion concentration.
3. Strong and weak acids and bases
- Difference between strong and weak acids in terms of degree of dissociation.
- Comparison of pH values for strong and weak acids of the same concentration.
- Effects of dilution on strong and weak acids may also be considered.
4. Weak acid equilibria: Ka and pKa
- Definition and use of the acid dissociation constant, Ka.
- Relationship between Ka and pKa.
- Calculating pH of weak acid solutions.
- Recognising assumptions and approximations used in Ka calculations.
5. Ionic product of water: Kw
- Definition and use of the ionic product of water, Kw.
- Calculating the pH of strong bases using
Kw.
- Understanding the equilibrium between
H+ and OH- in water.
6. Titration curves and indicators
- Acid-base titrations and how pH changes during the reaction.
- Sketching and interpreting pH curves for strong/weak acid-base combinations.
- Selecting suitable indicators from titration curves.
7. Buffers
- Meaning of a buffer solution.
- How buffers resist pH change when small amounts of acid or alkali are added.
- Formation of acidic buffers from a weak acid and its salt.
- Calculation of buffer pH and preparation of buffers of a chosen pH.
8. Practical skills
- Using a pH meter accurately.
- Carrying out acid-base titrations.
- Measuring pH for strong and weak acids and bases.
- Investigating pH changes during titration.
- Determining Ka from experimental data.
9. Typical exam skills
- Defining acid-base terms precisely.
- Writing equilibrium expressions.
- Performing calculations involving
pH, Ka, pKa, and Kw.
- Explaining buffer action.
- Interpreting titration curves and choosing indicators.