CourseCourseConceptConcept

IB Chemistry HL 3.1 Proton Transfer Reactions Questions

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

Syllabus
First assessment 2025
Course
Chemistry HL
Level
HL

Exam points

  • Identify Brønsted–Lowry proton donors and acceptors, conjugate acid–base pairs and amphiprotic species, then write balanced proton-transfer equations.
  • Calculate pH and pOH from [H₃O⁺], [OH⁻], K_w or equilibrium data, and convert between logarithmic measures with stated temperature conditions.
  • Use Kₐ, Kᵦ, pKₐ, pKᵦ and KₐKᵦ = K_w to compare acid/base strength, conjugate pairs and weak-acid or weak-base equilibria.
  • Predict salt hydrolysis and the pH of a salt solution from parent acid/base strengths, including hydrolysis-equilibrium calculations.
  • Analyse strong- and weak-acid/base titration curves, locate equivalence or half-equivalence regions, and select an indicator whose transition range matches the endpoint.
  • Write buffer reactions and determine buffer composition or pH after neutralization, using conjugate-pair ratios and the Henderson–Hasselbalch relationship.
  • Explain how dilution changes buffer capacity while leaving the conjugate-pair ratio, and interpret the effect of added strong acid or base.

Question 1

[Maximum number: 9]

Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

Question (a)

(a)

In aqueous chlorine the equilibrium below produces chloric(I) acid (hypochlorous acid), HOCl , the active bleach.

Cl2(aq)+H2O(l)⇌HOCl(aq)+H+(aq)+Cl−(aq)\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})
[ 9 ]

Question (i)

(i)

Chloric(I) acid is a weak acid, but hydrochloric acid is a strong acid. Outline how this is indicated in the equation above.

[ 1 ]

Question (ii)

(ii)

State a balanced equation for the reaction of chloric(I) acid with water.

[ 1 ]

Question (iii)

(iii)

Partial neutralization of chloric(I) acid creates a buffer solution. Given that the pKa\mathrm{p} K_{\mathrm{a}} of chloric(I) acid is 7.53 , determine the pH of a solution that has [HOCl]=0.100 moldm−3[\mathrm{HOCl}]=0.100 \mathrm{~mol} \mathrm{dm}^{-3} and [ClO−]=0.0500 moldm−3\left[\mathrm{ClO}^{-}\right]=0.0500 \mathrm{~mol} \mathrm{dm}{ }^{-3}.

[ 4 ]

Question (iv)

(iv)

Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.

[ 3 ]

Question 2

[Maximum number: 5]

Nitrous acid, HNO2\mathrm{HNO}_{2}, is a weak acid which can be used to make acidic buffers.

Question (a)

(a)

A 1.00 moldm−31.00 \mathrm{~mol} \mathrm{dm}^{-3} solution of nitrous acid was prepared. This solution contains two conjugate acid-base pairs.

[ 5 ]

Question (i)

(i)

State the formulas of the conjugate acid and conjugate base in each pair.

Conjugate acid:
Conjugate base:
Conjugate acid:
Conjugate base:

[ 2 ]

Question (ii)

(ii)

This 1.00 moldm−31.00 \mathrm{~mol} \mathrm{dm}^{-3} solution of nitrous acid was used to prepare a buffer with pH 3.00 .

Calculate the concentration of the conjugate base of nitrous acid required to make this buffer. The pKa\mathrm{pK}_{\mathrm{a}} of nitrous acid is 3.25.

Concentration of conjugate base:

[ 3 ]

Question 3

[Maximum number: 4]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula PH3\mathrm{PH}_{3}.

Question (a)

(a)

Ammonia acts as a weak Brønsted-Lowry base when dissolved in water.

NH3(aq)+H2O(l)⇌NH4+(aq)+OH−(aq)\mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})

Outline what is meant by the terms "weak" and "Brønsted-Lowry base".

Weak:

Brønsted-Lowry base:

[ 2 ]

Question (b)

(b)

Phosphine is usually prepared by heating white phosphorus, one of the allotropes of phosphorus, with concentrated aqueous sodium hydroxide. The equation for the reaction is:

P4( s)+3OH−(aq)+3H2O(l)→PH3( g)+3H2PO2−(aq)\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})
[ 2 ]

Question (i)

(i)

The ion H2PO2−\mathrm{H}_{2} \mathrm{PO}_{2}^{-} is amphiprotic. Outline what is meant by amphiprotic, giving the formulas of both species it is converted to when it behaves in this manner.

[ 2 ]

Question 4

[Maximum number: 1]

Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

In aqueous chlorine the equilibrium below produces chloric(I) acid (hypochlorous acid), HOCl , the active bleach.

Cl2(aq)+H2O(l)⇌HOCl(aq)+H+(aq)+Cl−(aq)\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})

State a balanced equation for the reaction of chloric(I) acid with water.

Question 5

[Maximum number: 2]

Nitrous acid, HNO2\mathrm{HNO}_{2}, is a weak acid which can be used to make acidic buffers.

A 1.00 moldm−31.00 \mathrm{~mol} \mathrm{dm}^{-3} solution of nitrous acid was prepared. This solution contains two conjugate acid-base pairs.

State the formulas of the conjugate acid and conjugate base in each pair.

Conjugate acid:
Conjugate base:
Conjugate acid:
Conjugate base:

Question 6

[Maximum number: 2]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula PH3\mathrm{PH}_{3}.

Phosphine is usually prepared by heating white phosphorus, one of the allotropes of phosphorus, with concentrated aqueous sodium hydroxide. The equation for the reaction is:

P4( s)+3OH−(aq)+3H2O(l)→PH3( g)+3H2PO2−(aq)\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

The ion H2PO2−\mathrm{H}_{2} \mathrm{PO}_{2}^{-} is amphiprotic. Outline what is meant by amphiprotic, giving the formulas of both species it is converted to when it behaves in this manner.

Question 7

[Maximum number: 3]

A group of students investigated the rate of the reaction between aqueous sodium thiosulfate and hydrochloric acid according to the equation below.

Na2 S2O3(aq)+2HCl(aq)→2NaCl(aq)+SO2( g)+S( s)+H2O(1)\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{NaCl}(\mathrm{aq})+\mathrm{SO}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(1)

The two reagents were rapidly mixed together in a beaker and placed over a mark on a piece of paper. The time taken for the precipitate of sulfur to obscure the mark when viewed through the reaction mixture was recorded.

Figure for Question 7 — IB Chemistry HL
Figure for Question 7 — IB Chemistry HL

Initially they measured out 10.0 cm310.0 \mathrm{~cm}^{3} of 0.500moldm−30.500 \mathrm{moldm}^{-3} hydrochloric acid and then added 40.0 cm340.0 \mathrm{~cm}^{3} of 0.0200 moldmm−30.0200 \mathrm{~mol} \mathrm{dm} \mathrm{m}^{-3} aqueous sodium thiosulfate. The mark on the paper was obscured 47 seconds after the solutions were mixed.

Question (a)

(a)

The teacher asked the students to devise another technique to measure the rate of this reaction.

[ 3 ]

Question (i)

(i)

One group suggested recording how long it takes for the pH of the solution to change by one unit. Calculate the initial pH of the original reaction mixture.

[ 2 ]

Question (ii)

(ii)

Deduce the percentage of hydrochloric acid that would have to be used up for the pH to change by one unit.

[ 1 ]

Question 8

[Maximum number: 6]

Iron rusts in the presence of oxygen and water. Rusting is a redox process involving several steps that produces hydrated iron(III) oxide, Fe2O3∙nH2O\mathrm{Fe}_{2} \mathrm{O}_{3} \bullet \mathrm{nH}_{2} \mathrm{O}, as the final product. The half-equations involved for the first step of rusting are given below.

Half-equation 1: Fe(s)→Fe2+(aq)+2e−\quad \mathrm{Fe}(\mathrm{s}) \rightarrow \mathrm{Fe}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}

Half-equation 2: O2(aq)+4e−+2H2O(l)→4OH−(aq)\quad \mathrm{O}_{2}(\mathrm{aq})+4 \mathrm{e}^{-}+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 4 \mathrm{OH}^{-}(\mathrm{aq})

Question (a)

(a)

State a chemical equation for the partial dissociation of water into ions, including state symbols.

[ 1 ]

Question (b)

(b)

The dissociation of water into ions is reversible. State the expression for the ionic product constant of water.

[ 1 ]

Question (c)

(c)

The ionic product constant of water was measured at three different temperatures.

Table for Question (c) — IB Chemistry HL

Deduce whether the ionization of water is exothermic or endothermic, giving your reason.

[ 2 ]

Question (d)

(d)

Use the data in part (iii) to determine the pH of water at 373 K , correct to two decimal places.

[ 2 ]

Question 9

[Maximum number: 1]

When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92.6 kJ\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92.6 \mathrm{~kJ}

Define the term weak base according to the Brønsted-Lowry theory.

Question 10

[Maximum number: 1]

Methanoic acid is a monoprotic weak acid.

The concentration of methanoic acid was found by titration with a 0.200 moldm−30.200 \mathrm{~mol} \mathrm{dm}^{-3} standard solution of sodium hydroxide, NaOH(aq), using an indicator to determine the end point.

Write an equation for the reaction of methanoic acid with sodium hydroxide.

Question 11

[Maximum number: 1]

Which chart shows the neutralization curve of a strong base with a strong acid?

A
Figure for Question 11 — IB Chemistry HL — Option A
B
Figure for Question 11 — IB Chemistry HL — Option B
C
Figure for Question 11 — IB Chemistry HL — Option C
D
Figure for Question 11 — IB Chemistry HL — Option D

Question 12

[Maximum number: 2]

Methanoic acid is a monoprotic weak acid.

The concentration of methanoic acid was found by titration with a 0.200 moldm−30.200 \mathrm{~mol} \mathrm{dm}^{-3} standard solution of sodium hydroxide, NaOH(aq), using an indicator to determine the end point.

Calculate the pH of the sodium hydroxide solution.

Question 13

[Maximum number: 3]

Two hydrides of nitrogen are ammonia and hydrazine, N2H4\mathrm{N}_{2} \mathrm{H}_{4}. One derivative of ammonia is methanamine whose molecular structure is shown below.

Figure for Question 13 — IB Chemistry HL

Hydrazine reacts with water in a similar way to ammonia. (The association of a molecule of hydrazine with a second H+\mathrm{H}^{+} is so small it can be neglected.)

N2H4(aq)+H2O(l)⇌N2H5+(aq)+OH−(aq)pKb( hydrazine )=5.77\begin{gathered} \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}) \\ \mathrm{pK}_{\mathrm{b}}(\text { hydrazine })=5.77 \end{gathered}

Calculate the pH of a 0.0100 moldm−30.0100 \mathrm{~mol} \mathrm{dm}^{-3} solution of hydrazine.

Question 14

[Maximum number: 4]

When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92.6 kJ\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92.6 \mathrm{~kJ}

Determine the pH of a 0.100 moldm−30.100 \mathrm{~mol} \mathrm{dm}^{-3} solution of ammonia, NH3(aq)\mathrm{NH}_{3}(\mathrm{aq}), using tables 2 and 15 of the data booklet.

Question 15

[Maximum number: 1]

Phenylethene (styrene) is produced from ethylbenzene in a gas-phase equilibrium.

Figure for Question 15 — IB Chemistry HL

Benzoic acid is a weak acid. The graph shows how the pH changes during the titration of a 10.0 cm310.0 \mathrm{~cm}^{3} aqueous solution of benzoic acid with aqueous sodium hydroxide.

Figure for Question 15 — IB Chemistry HL

Explain, with reference to acid-base equilibria, why the sodium benzoate solution formed has a pH>7.

Question 16

[Maximum number: 3]

When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92.6 kJ\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92.6 \mathrm{~kJ}

Sketch the pH titration curve obtained when 50.0 cm350.0 \mathrm{~cm}^{3} of 0.100moldm−3NH3(aq)0.100 \mathrm{moldm}^{-3} \mathrm{NH}_{3}(\mathrm{aq}) is added to 25.0 cm325.0 \mathrm{~cm}^{3} of 0.100 moldm−3HCl0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HCl} (aq).

Figure for Question 16 — IB Chemistry HL

Question 17

[Maximum number: 3]

Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

In aqueous chlorine the equilibrium below produces chloric(I) acid (hypochlorous acid), HOCl , the active bleach.

Cl2(aq)+H2O(l)⇌HOCl(aq)+H+(aq)+Cl−(aq)\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})

Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.

Question 18

[Maximum number: 1]

When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92.6 kJ\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92.6 \mathrm{~kJ}

Identify an indicator from table 16 of the data booklet that could be used for this titration.

Question 19

[Maximum number: 1]

Phenylethene (styrene) is produced from ethylbenzene in a gas-phase equilibrium.

Figure for Question 19 — IB Chemistry HL

Describe how a buffer solution could be made from aqueous benzoic acid and aqueous sodium hydroxide.

Question 20

[Maximum number: 3]

Nitrous acid, HNO2\mathrm{HNO}_{2}, is a weak acid which can be used to make acidic buffers.

A 1.00 moldm−31.00 \mathrm{~mol} \mathrm{dm}^{-3} solution of nitrous acid was prepared. This solution contains two conjugate acid-base pairs.

This 1.00 moldm−31.00 \mathrm{~mol} \mathrm{dm}^{-3} solution of nitrous acid was used to prepare a buffer with pH 3.00 .

Calculate the concentration of the conjugate base of nitrous acid required to make this buffer. The pKa\mathrm{pK}_{\mathrm{a}} of nitrous acid is 3.25.

Concentration of conjugate base:

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