IB Chemistry SL 3.1 Proton Transfer Reactions Questions

Use IB Chemistry SL questions to identify proton-transfer pairs, calculate pH and interpret neutralization or titration evidence.

Exam points

Identify Brønsted–Lowry proton donors and acceptors, conjugate acid–base pairs and amphiprotic species, then write balanced proton-transfer equations.
Calculate pH from [H₃O⁺] or [H⁺], reverse pH to ion concentration, and compare acidity on the logarithmic scale.
Apply K_w = [H₃O⁺][OH⁻] to classify acidic, neutral and basic solutions and calculate the missing ion concentration with correct units.
Explain strong versus weak acids and bases by degree of ionization, concentration and conjugate-base behaviour, without confusing strength with dilution.
Write neutralization equations, use stoichiometry for acid–base amounts, and interpret a titration curve or equivalence-point volume.

Covered syllabus points

3.1.1—Brønsted-Lowry theory 3.1.2—Conjugate acid-base pairs 3.1.3—Amphiprotic species 3.1.4—pH scale 3.1.5—Ion product of water (Kw)3.1.6—Strong vs. weak acids/bases 3.1.7—Neutralization reactions 3.1.8—pH curves

[Maximum number: 1]

Two hydrides of nitrogen are ammonia and hydrazine, $\mathrm{N}_{2} \mathrm{H}_{4}$ . One derivative of ammonia is methanamine whose molecular structure is shown below.

Hydrazine reacts with water in a similar way to ammonia. Deduce an equation for the reaction of hydrazine with water.

$\mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{N}_{2} \mathrm{H}_{5}{ }^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$ $\text { Accept } \mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \rightleftharpoons$ $\mathrm{N}_{2} \mathrm{H}_{6}{ }^{2+}(\mathrm{aq})+2 \mathrm{OH}^{-}(\mathrm{aq})$ .

Equilibrium sign must be present.

[Maximum number: 3]

Chlorine undergoes many reactions.

(a)

Chlorine gas reacts with water to produce hypochlorous acid and hydrochloric acid.

\mathrm{Cl}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HClO}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq})

[ 3 ]

(i)

Hypochlorous acid is considered a weak acid. Outline what is meant by the term weak acid.

[ 1 ]

(ii)

State the formula of the conjugate base of hypochlorous acid.

[ 1 ]

$\mathrm{ClO}^{-}$

(iii)

Calculate the concentration of $\mathrm{H}^{+}(\mathrm{aq})$ in a HClO(aq) solution with a pH=3.61.

[ 1 ]

« $\left[\mathrm{H}^{+}\right]=10^{-3.61}=$ » $2.5 \times 10^{-4}$ «mol dm ${ }^{-3}$ »

[Maximum number: 3]

Soluble acids and bases ionize in water.

(a)

Sodium hypochlorite ionizes in water.

\mathrm{OCl}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{OH}^{-}(\mathrm{aq})+\mathrm{HOCl}(\mathrm{aq})

[ 2 ]

(i)

Identify the amphiprotic species.

[ 1 ]

water/ $\mathrm{H}_{2} \mathrm{O}$

Marking guidance:

Accept "hydroxide ion/OH".

(ii)

Identify one conjugate acid-base pair in the reaction.

Table for Question (ii) — IB Chemistry SL

[ 1 ]

(b)

A solution containing 0.510 g of an unknown monoprotic acid, HA , was titrated with $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaOH}(\mathrm{aq}) .25 .0 \mathrm{~cm}^{3}$ was required to reach the equivalence point.

[ 1 ]

(i)

Calculate $\left[\mathrm{H}^{+}\right]$ in the NaOH solution.

[ 1 ]

$1.00\times10^{-14}=[\mathrm{H^+}]\times0.100$ $[\mathrm{H^+}]=1.00\times10^{-13}\ \mathrm{mol\ dm^{-3}}$

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Covered syllabus points

Question 1

Question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question 3

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)