IB Chemistry SL 3.1 Proton Transfer Reactions Questions

Practise IB Chemistry SL 3.1 with questions on Brønsted–Lowry acids and bases, pH, neutralization, indicators and titration curves.

Exam points

Identify Brønsted–Lowry acids, bases and conjugate pairs, then write amphiprotic and neutralization equations.
Calculate pH, [H⁺] and K_w, and interpret strong versus weak acids, bases and titration curves.
Sketch and read strong-acid/strong-base pH curves, locating intercept and equivalence point.

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

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Study 3.1 Proton transfer reactions conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[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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