Acids and bases (A-level only) — AQA A-Level Chemistry Revision

    This topic covers the Brønsted-Lowry theory of acids and bases, focusing on proton transfer in aqueous solutions. It includes the quantitative treatment of

    Topic Synopsis

    This topic covers the Brønsted-Lowry theory of acids and bases, focusing on proton transfer in aqueous solutions. It includes the quantitative treatment of pH, the ionic product of water (Kw), the dissociation constant (Ka) for weak acids, and the application of pH curves and buffer solutions.

    Key Concepts & Core Principles

    Exam Tips & Revision Strategies

    Common Misconceptions & Mistakes to Avoid

    Examiner Marking Points

    Acids and bases (A-level only)

    AQA
    A-Level

    This topic covers the Brønsted-Lowry theory of acids and bases, focusing on proton transfer in aqueous solutions. It includes the quantitative treatment of pH, the ionic product of water (Kw), the dissociation constant (Ka) for weak acids, and the application of pH curves and buffer solutions.

    0
    Objectives
    5
    Exam Tips
    6
    Pitfalls
    0
    Key Terms
    11
    Mark Points

    Topic Overview

    Acids and bases are fundamental to chemistry, governing reactions from industrial processes to biological systems. At A-level, you move beyond simple definitions to explore the Brønsted-Lowry theory, where acids are proton donors and bases are proton acceptors. This topic introduces the pH scale, strong and weak acids, and the concept of acid dissociation constants (Ka) to quantify acid strength. You'll also study buffer solutions, which resist pH changes, and titrations with pH curves, linking theory to practical analysis.

    Understanding acids and bases is crucial for topics like equilibria, thermodynamics, and organic chemistry. For example, buffer systems maintain blood pH in biology, while acid-base reactions are key in synthesis and analysis. Mastery of this topic allows you to predict reaction outcomes, calculate pH, and design experiments, forming a cornerstone of your chemical knowledge for exams and beyond.

    Key Concepts

    Core ideas you must understand for this topic

    • →Brønsted-Lowry theory: acids donate H⁺, bases accept H⁺; conjugate acid-base pairs differ by one proton.
    • →pH = -log[H⁺]; strong acids fully dissociate (e.g., HCl → H⁺ + Cl⁻), weak acids partially dissociate (e.g., CH₃COOH ⇌ H⁺ + CH₃COO⁻).
    • →Acid dissociation constant Ka = [H⁺][A⁻]/[HA]; pKa = -log Ka; smaller Ka means weaker acid.
    • →Buffer solutions: mixtures of weak acid and its conjugate base (or weak base and conjugate acid) that resist pH change; calculate pH using Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).
    • →pH curves for titrations: strong acid-strong base (vertical at pH 7), weak acid-strong base (vertical above pH 7), strong acid-weak base (vertical below pH 7); choose indicator with pKa near equivalence point pH.

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Definition of Brønsted-Lowry acid as a proton donor and base as a proton acceptor.
    • Calculation of pH using pH = -log10[H+].
    • Calculation of [H+] from pH using [H+] = 10^-pH.
    • Use of Kw = [H+][OH-] to calculate pH of strong bases.
    • Construction of Ka expressions for weak acids.
    • Calculation of pH of weak acids using Ka and concentration.
    • Conversion between Ka and pKa.
    • Interpretation of pH curves for strong/weak acid-base titrations.

    Marking Points

    Key points examiners look for in your answers

    • Definition of Brønsted-Lowry acid as a proton donor and base as a proton acceptor.
    • Calculation of pH using pH = -log10[H+].
    • Calculation of [H+] from pH using [H+] = 10^-pH.
    • Use of Kw = [H+][OH-] to calculate pH of strong bases.
    • Construction of Ka expressions for weak acids.
    • Calculation of pH of weak acids using Ka and concentration.
    • Conversion between Ka and pKa.
    • Interpretation of pH curves for strong/weak acid-base titrations.
    • Selection of appropriate indicators based on pH range.
    • Qualitative explanation of acidic and basic buffer action.
    • Calculation of pH of acidic buffer solutions.

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always show working for logarithmic calculations to ensure partial credit.
    • 💡Remember that Kw is temperature-dependent; check if the question specifies 298K.
    • 💡When sketching pH curves, ensure the starting pH and equivalence point are realistic for the acid/base strength.
    • 💡Use the approximation [H+]^2 = Ka * [HA] for weak acids only when appropriate.
    • 💡Clearly distinguish between the roles of the acid and the conjugate base in buffer systems.
    • 💡Always write equilibrium expressions correctly: for HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA]. Include square brackets and units (mol dm⁻³).
    • 💡When calculating pH of weak acids, use the approximation [H⁺] = √(Ka × [HA]) only if Ka is small and [HA] >> [H⁺]. Check that [H⁺] < 5% of [HA] to validate.
    • 💡For buffer calculations, identify the weak acid and its conjugate base. Use the Henderson-Hasselbalch equation only if the concentrations are equilibrium concentrations (often approximated as initial concentrations for buffers).

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the dissociation of strong acids/bases with weak acids/bases.
    • Incorrectly assuming [H+] = [acid] for weak acids.
    • Failing to account for temperature changes when using Kw.
    • Misinterpreting the pH scale as linear rather than logarithmic.
    • Incorrectly identifying the equivalence point on a pH curve.
    • Forgetting to include the salt concentration in buffer calculations.
    • Misconception: Strong acids have high pH. Correction: Strong acids have low pH (e.g., pH 1 for 0.1 M HCl). pH decreases as [H⁺] increases.
    • Misconception: Weak acids are dilute. Correction: Weakness refers to extent of dissociation, not concentration. A concentrated weak acid (e.g., 1 M ethanoic acid) still has a higher pH than a strong acid of same concentration.
    • Misconception: Buffers maintain constant pH. Correction: Buffers resist pH change but do not prevent it; they minimise changes upon addition of small amounts of acid or base.

    Frequently Asked Questions

    Common questions students ask about this topic

    Before You Start

    Prior knowledge that will help with this topic

    • •Equilibrium: understanding dynamic equilibrium and Le Chatelier's principle.
    • •Moles and concentrations: ability to calculate moles, volumes, and concentrations in mol dm⁻³.
    • •Ionic equations: writing and balancing equations for dissociation and neutralisation.

    Likely Command Words

    How questions on this topic are typically asked

    Define
    Calculate
    Explain
    Sketch
    Outline
    Construct

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