Not all chemical reactions are the same. In this chapter we classify reactions by what is transferred between species — protons (acid-base) or electrons (redox). Both classification systems are essential for CAPS Grade 11 and 12 chemistry.
10.1 Brønsted-Lowry Acids and Bases
Definition
Acids (Brønsted-Lowry)
A Brønsted-Lowry acid is a substance that gives away protons (H⁺), and is therefore called a proton donor.
Definition
Bases (Brønsted-Lowry)
A Brønsted-Lowry base is a substance that takes up protons (H⁺), and is therefore called a proton acceptor.
Definition
Amphoteric
An amphoteric substance is one that can react as either an acid or base.
Definition
Amphiprotic
An amphiprotic substance is one that can act as either a proton donor (acid) or a proton acceptor (base) in a chemical reaction. Water is the most common amphiprotic substance: it donates a proton to form OH⁻, or accepts a proton to form H₃O⁺. Other examples: HCO₃⁻, HSO₄⁻, HS⁻.
In a Brønsted-Lowry acid-base reaction, a proton (H⁺) transfers from the acid to the base. This creates a conjugate base (the acid after losing H⁺) and a conjugate acid (the base after gaining H⁺). These pairs differ by only one proton.
Identifying conjugate acid-base pairs
- Find the species that donates H⁺ → this is the acid.
- Find the species that receives H⁺ → this is the base.
- The acid after donating H⁺ becomes its conjugate base.
- The base after accepting H⁺ becomes its conjugate acid.
- Conjugate pairs differ by exactly ONE proton (H⁺).
Worked Example
For the reaction HCl + H₂O ⇌ H₃O⁺ + Cl⁻, identify the acid, base, conjugate acid, and conjugate base, and state the two conjugate acid-base pairs.
Given
- HCl + H₂O ⇌ H₃O⁺ + Cl⁻
Find
Acid, base, conjugate acid, conjugate base, conjugate pairs
Solution
- 1HCl donates H⁺ → HCl is the acid
- 2H₂O accepts H⁺ → H₂O is the base
- 3H₃O⁺ can donate H⁺ back → H₃O⁺ is the conjugate acid of H₂O
- 4Cl⁻ can accept H⁺ back → Cl⁻ is the conjugate base of HCl
Exam Tip
Water is the most common amphiprotic substance. With an acid it acts as a base (H₂O + H⁺ → H₃O⁺). With a base it acts as an acid (H₂O → OH⁻ + H⁺). HCO₃⁻ and HSO₄⁻ behave the same way.
Practice Question
In the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the Brønsted-Lowry acid and base and state both conjugate acid-base pairs.
(5 marks)
10.2 Oxidation States and Redox
In redox reactions, electrons are transferred between species. An oxidation state (oxidation number) is a bookkeeping tool that tracks how electrons are distributed. If an atom's oxidation state increases, it has been oxidised (lost electrons). If it decreases, it has been reduced (gained electrons).
Rules for assigning oxidation states
- Elements in their standard state have an oxidation state of 0 (e.g. Na, Cl₂, O₂).
- Monoatomic ions have an oxidation state equal to their charge (Na⁺ = +1, Cl⁻ = −1).
- Oxygen is usually −2 (except in peroxides where it is −1).
- Hydrogen is usually +1 (except in metal hydrides where it is −1).
- The sum of oxidation states in a neutral compound is 0.
- The sum of oxidation states in a polyatomic ion equals the ion charge.
Oxidising Agent vs Reducing Agent
| Property | Oxidising Agent | Reducing Agent |
|---|---|---|
| What it does | Oxidises another species | Reduces another species |
| Electrons | Gains electrons (is reduced) | Loses electrons (is oxidised) |
| Oxidation state | Decreases (becomes less positive) | Increases (becomes more positive) |
| Example | O₂, Cl₂, MnO₄⁻ | Metals, H₂, C |
Exam Tip
Mnemonic: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). The oxidising agent causes oxidation and is itself reduced. The reducing agent causes reduction and is itself oxidised.
Worked Example
In the reaction CuO + H₂ → Cu + H₂O, identify what is oxidised, what is reduced, and name the oxidising and reducing agents.
Given
- CuO + H₂ → Cu + H₂O
Find
Oxidised species, reduced species, oxidising agent, reducing agent
Solution
- 1In CuO: Cu = +2, O = −2. In Cu: Cu = 0. Cu²⁺ → Cu: decrease in oxidation state → Cu is REDUCED.
- 2In H₂: H = 0. In H₂O: H = +1. H → H⁺: increase in oxidation state → H is OXIDISED.
- 3CuO is the oxidising agent (causes H₂ to be oxidised; Cu in CuO is reduced).
- 4H₂ is the reducing agent (causes Cu to be reduced; H in H₂ is oxidised).
Practice Question
In the reaction 2Na + Cl₂ → 2NaCl, state the oxidation state change for Na and Cl, and identify the oxidising and reducing agents.
(5 marks)
Real World
Redox reactions power batteries. In a lithium-ion battery, lithium is oxidised (reducing agent) at the anode, releasing electrons that drive current through the circuit.
10.3 Amphiprotic Substances in Detail
An amphiprotic substance contains both a transferable proton (can act as acid) AND a lone pair to accept a proton (can act as base). Water is the classic example. Hydrogen carbonate (HCO₃⁻) is an important exam favourite because it appears in blood buffering and in the carbon cycle.
Common amphiprotic substances
- H₂O — Acts as acid with NH₃; acts as base with HCl
- HCO₃⁻ — Acts as acid: HCO₃⁻ → CO₃²⁻ + H⁺; acts as base: HCO₃⁻ + H⁺ → H₂CO₃
- HSO₄⁻ — Acts as acid: HSO₄⁻ → SO₄²⁻ + H⁺; acts as base: HSO₄⁻ + H⁺ → H₂SO₄
- H₂PO₄⁻ — can both donate and accept a proton
Worked Example
Write two equations to show that HCO₃⁻ is amphiprotic: one where it acts as an acid and one where it acts as a base.
Given
- HCO₃⁻ can donate or accept H⁺
Find
Two equations
Solution
- 1As an acid (proton donor): HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O
- 2As a base (proton acceptor): HCO₃⁻ + HCl → H₂CO₃ + Cl⁻
Watch Out
Amphiprotic and amphoteric are often confused. Amphiprotic specifically refers to proton transfer (Brønsted-Lowry). Amphoteric is broader and includes Lewis acid-base behaviour. For Grade 11, treat them as equivalent.
Practice Question
Write equations to show that water acts as (a) a Brønsted-Lowry acid with ammonia (NH₃) and (b) a Brønsted-Lowry base with hydrogen chloride (HCl).
(4 marks)