Proton transfer reactions (acids and bases)

Chemistry· Reactivity 3.1· 12 min read

1. Bronsted-Lowry Acid and Base Definitions★★☆☆☆⏱ 10 min

The Bronsted-Lowry model of acid-base chemistry centers entirely on the transfer of a single proton (H⁺ ion) between two species, rather than the older Arrhenius definition that only applies to aqueous solutions. This model works for all solvent systems, including non-aqueous reactions.

📘 Definition

Bronsted-Lowry Acid

HA

Proton donor, releases one H⁺ ion during reaction

Example:

Hydrochloric acid HCl, ethanoic acid CH₃COOH

📘 Definition

Bronsted-Lowry Base

B

Proton acceptor, uses a lone electron pair to bind the incoming H⁺

Example:

Ammonia NH₃, hydroxide ion OH⁻

📐 Worked Example

Classify the following species as Bronsted-Lowry acid, base, or both: H₂O, HNO₃, NH₂⁻

1
Step 1: Check if the species can donate an H⁺
2
$HNO_3 \rightarrow H^+ + NO_3^-$
3
HNO₃ can only donate a proton, so it is a Bronsted-Lowry acid.
4
Step 2: Check if the species can accept an H⁺
5
$NH_2^- + H^+ \rightarrow NH_3$
6
NH₂⁻ can only accept a proton, so it is a Bronsted-Lowry base.
7
Step 3: H₂O can both donate and accept protons, so it is amphiprotic, acting as acid or base depending on the reaction partner.

2. Conjugate Acid-Base Pairs★★★☆☆⏱ 12 min

When an acid donates a proton, the remaining species is its conjugate base, which can accept a proton to reform the original acid. When a base accepts a proton, the resulting species is its conjugate acid, which can donate a proton to reform the original base. The two species in a pair differ by exactly one H⁺ unit.

HA + B \rightleftharpoons A^- + HB^+

📐 Worked Example

Identify all conjugate acid-base pairs in the reaction between ethanoic acid and water: CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

1
Step 1: Find the species that loses an H⁺ on the left side
2
CH₃COOH loses an H⁺ to become CH₃COO⁻, so this is the acid → conjugate base pair.
3
Step 2: Find the species that gains an H⁺ on the left side
4
H₂O gains an H⁺ to become H₃O⁺, so this is the base → conjugate acid pair.
5
Step 3: Confirm each pair differs by exactly one H⁺, no other atom or charge changes.

✓ Quick check

Test your understanding:

Which of the following is a valid conjugate pair?
- H₂O and OH⁻
- HCl and H₂O
- NH₃ and O²⁻
- CH₃COOH and H₂SO₄
Reveal answer
H₂O and OH⁻ —
H₂O donates a proton to form OH⁻, so they differ by exactly one H⁺.

3. Writing Balanced Proton Transfer Equations★★★☆☆⏱ 10 min

All proton transfer reactions must balance for atoms and total charge on both sides of the equation. The total charge on the reactant side must equal the total charge on the product side, as no electrons are lost or gained in the proton transfer process.

🔬 Derivation

Goal:

Derive the full balanced equation for the reaction between hydrochloric acid and ammonia

Starting from:

Half equation 1: HCl donates a proton

1
$HCl \rightarrow H^+ + Cl^-$
2
Half equation 2: NH₃ accepts the proton
3
$NH_3 + H^+ \rightarrow NH_4^+$
4
Add the two half equations together, cancel the free H⁺ term on both sides

Result:

Full balanced equation: $H C l + N H_{3} \to N H_{4}^{+} + C l^{-}$

4. Common Proton Transfer Reaction Examples★★☆☆☆⏱ 8 min

IB SL Chemistry exam questions almost exclusively use a small set of common proton transfer reactions, including neutralisation between strong acids and strong bases, reaction of weak acids with carbonates, and aqueous dissociation of weak bases.

Reaction Type	Example Equation	Acid	Base
Strong acid + strong base	$H C l + N a O H \to N a C l + H_{2} O$	HCl	OH⁻
Weak acid + carbonate	$2 C H_{3} C O O H + N a_{2} C O_{3} \to 2 C H_{3} C O O N a + H_{2} O + C O_{2}$	CH₃COOH	CO₃²⁻
Ammonia in water	$N H_{3} + H_{2} O ⇌ N H_{4}^{+} + O H^{-}$	H₂O	NH₃

5. Common Pitfalls

Wrong move:

Identifying H₂O as always an acid or always a base

Why:

Water is amphiprotic, so its role depends entirely on the other reaction partner

Correct move:

Check if water loses an H⁺ (acts as acid) or gains an H⁺ (acts as base) in the specific reaction given

Wrong move:

Counting conjugate pairs that differ by more than one H⁺

Why:

Conjugate pairs must differ by exactly one proton, no more no less

Correct move:

Count the number of H atoms and the charge difference between the two species to confirm they only differ by one H⁺

Wrong move:

Forgetting to balance total charge across the proton transfer equation

Why:

Unbalanced charge is an automatic mark deduction in IB P2 questions

Correct move:

Sum all charges on reactant side and product side before finalising your equation to confirm they are equal

Wrong move:

Using the Arrhenius definition to classify bases in non-aqueous reactions

Why:

Arrhenius bases only produce OH⁻ in water, which does not apply to reactions in other solvents

Correct move:

Always use the Bronsted-Lowry proton donor/acceptor definition for all IB SL acid-base classification questions

Wrong move:

Classifying the conjugate base of a strong acid as a strong base

Why:

Conjugate bases of strong acids are extremely weak, almost neutral species

Correct move:

Remember: strong acid → very weak conjugate base, weak acid → stronger conjugate base

6. Quick Reference Cheatsheet

Term	Definition	Key Rule
Bronsted-Lowry Acid	Proton donor	Loses 1 H⁺
Bronsted-Lowry Base	Proton acceptor	Gains 1 H⁺
Conjugate Pair	Two species differing by 1 H⁺	Charge differs by +1 between base and its conjugate acid
Amphiprotic	Can donate or accept H⁺	Most common example = H₂O

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · P2
Identify conjugate pairs in neutralisation
2024 · P1
Classify Bronsted-Lowry acid and base
2023 · P2
Write proton transfer half equations

What's Next

Now that you have mastered the core proton transfer model of Bronsted-Lowry acids and bases, you are ready to explore the properties of strong and weak acids, including pH calculations, dissociation constants, and neutralisation reaction stoichiometry. These concepts are heavily weighted in Paper 1 multiple choice and Paper 2 structured response questions, and build directly on the conjugate pair identification skills you practiced in this guide. Mastering proton transfer first will make all subsequent acid-base topics far easier to learn, and will help you avoid common mistakes that cost marks on exam day.