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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

1.12 Acids and Bases (A-level only)

1.12.1 Bronsted-Lowry Acid-Base Equilibria 1.12.2 Definition and Determination of pH 1.12.3 The Ionic Product of Water, Kw 1.12.4 Weak Acids and Bases Ka for Weak Acids 1.12.5 pH Curves, Titrations and Indicators 1.12.6 Buffer Action

Brønsted-Lowry Acid-Base Equilibria

Specification Reference Physical Chemistry, Acids and bases 3.1.12.1

Quick Notes

  • Brønsted-Lowry acids are proton (H+) donors
  • Brønsted-Lowry bases are proton (H+) acceptors
  • Acid-base equilibrium reactions involve conjugate acid-base pairs.
  • Strong acids fully dissociate, while weak acids partially dissociate in solution.

Full Notes

Acid-Base theory has been covered in more detail here.
This page is just what you need to know for AQA A-level Chemistry :)

Definition of Brønsted-Lowry Acids and Bases

There are several different ways to describe acids and bases in chemistry. At this level, we use the Brønsted-Lowry theory.

Brønsted-Lowry acid: A substance that donates a proton (H+).

Brønsted-Lowry base: A substance that accepts a proton (H+).

Example In the reaction between HCl(aq) and NaOH(aq)

AQA A-Level Chemistry equation HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) used to illustrate an acid donating H+ and a base accepting H+.

HCl is the acid (proton donor) and NaOH is the base (proton acceptor).

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Remember a H+ ion is a proton - meaning both terms can be used when talking about acids and bases.


H+ and H3O+ ions

In aqueous solution (dissolved in water), acids actually give a H+ ion to a water molecule, forming H3O+ (called a hydroxonium ion).

AQA A-Level Chemistry note that H+(aq) is equivalent to H3O+(aq), the hydronium ion, in aqueous solution.

The H3O+ ion then reacts with hydroxide ions (from the NaOH) to form water.

AQA A-Level Chemistry reaction H3O+(aq) + OH−(aq) → 2H2O(l) showing hydronium reacting with hydroxide to form water.

When combined together, the overall reaction cancels out the water molecule that transferred the H+ ion from the HCl to OH−.

AQA A-Level Chemistry reminder that writing H+(aq) is shorthand for the hydronium ion H3O+(aq).

When you see H+(aq) written in equations, it represents a H3O+ ion, however as the H2O is just ‘carrying’ the H+ ion, we don’t usually include it.

Acid-base equilibrium reactions involve conjugate acid-base pairs.

In a reaction, when an acid dissociates and releases a H+ ion, it forms a negatively charged ion. If this negative ion was to accept a H+, it would re-form the original acid and therefore would act as a base. Equally, if a base accepts a H+ it could then later react and release the H+ again, acting as an acid!

We call these conjugate pairs.

AQA A-Level Chemistry diagram showing HA(aq) ⇌ H+(aq) + A−(aq), labelling HA as weak acid and A− as conjugate base (conjugate pair).

An acid dissociates, forming its conjugate base. In reverse, a base can accept a proton (H+ ion) and form its conjugate acid.

Summary