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
Use IB Chemistry HL questions to identify proton-transfer pairs, calculate pH and interpret neutralization or titration evidence.
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.
Chloric(I) acid is a weak acid, but hydrochloric acid is a strong acid. Outline how this is indicated in the equation above.
chloric(I) acid (shown as) a molecule/molecular, but hydrochloric acid (shown as being) split into ions / chloric(I) acid is partially dissociated and hydrochloric acid is fully dissociated;
State a balanced equation for the reaction of chloric(I) acid with water.
HOCl(aq)⇌H+(aq)+ClO−(aq)/HOCl(aq)+H2O(l)⇌H3O+(aq)+ClO−(aq);
Partial neutralization of chloric(I) acid creates a buffer solution. Given that the pKa of chloric(I) acid is 7.53 , determine the pH of a solution that has [HOCl]=0.100 moldm−3 and [ClO−]=0.0500 moldm−3.
Ka=10−7.53=2.95×10−8moldm−3;
Ka=[HOCl][H+][ClO−]=(0.1)[H+](0.05)≈2[H+]=2.95×10−8moldm−3;
[H+]=2×2.95×10−8=5.9×10−8moldm−3;
pH=−log(5.9×10−8)=7.23;
Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.
HIn⇌H++In−;
(weak acid in which the) acid/HIn and conjugate base/In- have different colours / OWTTE;
excess alkali shifts the equilibrium to the RHS/towards the conjugate base;
Nitrous acid, HNO2, is a weak acid which can be used to make acidic buffers.
A 1.00 moldm−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:
conjugate acid H3O+«(aq)» AND conjugate base H2O « (l)»
conjugate acid HNO2 «(aq)» AND conjugate base NO2−«(aq)»
This 1.00 moldm−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 of nitrous acid is 3.25.
Concentration of conjugate base:
Ka<10−3.25>=5.62×10−4
« [H+]=10−3.00 » =0.001 «mol dm −3 »
« [A−]=(5.62×10−4×1.00)/0.001»=0.562 «mol dm−3 »
Alternative solution:
pH=pKa+log10[ salt ]/[ acid ] « log10[ salt ]/[ acid ] 》 =−0.25
OR
«[salt]/[acid]» = 0.562、
《 [A−]=0.562/1.00»=0.562 «mol dm−3 » ↓
Award[3]for correct final answer.
Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula PH3.
Ammonia acts as a weak Brønsted-Lowry base when dissolved in water.
Outline what is meant by the terms "weak" and "Brønsted-Lowry base".
Weak:
Brønsted-Lowry base:
Weak: only partially dissociated/ionized «in dilute aqueous solution»
Brønsted-Lowry base: an acceptor of protons/ H^+/hydrogen ions
Marking guidance:
Accept reaction with water is reversible/an equilibrium.
Accept "water is partially dissociated «by the weak base»".
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:
The ion H2PO2− is amphiprotic. Outline what is meant by amphiprotic, giving the formulas of both species it is converted to when it behaves in this manner.
can act as both a «Brønsted-Lowry» acid and a «Brønsted-Lowry» base OR can accept and/or donate a hydrogen ion/proton/ H+HPO22− AND H3PO2
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.
State a balanced equation for the reaction of chloric(I) acid with water.
HOCl(aq)⇌H+(aq)+ClO−(aq)/HOCl(aq)+H2O(l)⇌H3O+(aq)+ClO−(aq);
Nitrous acid, HNO2, is a weak acid which can be used to make acidic buffers.
A 1.00 moldm−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:
conjugate acid H3O+«(aq)» AND conjugate base H2O « (l)»
conjugate acid HNO2 «(aq)» AND conjugate base NO2−«(aq)»
Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula PH3.
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:
The ion H2PO2− is amphiprotic. Outline what is meant by amphiprotic, giving the formulas of both species it is converted to when it behaves in this manner.
can act as both a «Brønsted-Lowry» acid and a «Brønsted-Lowry» base OR can accept and/or donate a hydrogen ion/proton/ H+HPO22− AND H3PO2
A group of students investigated the rate of the reaction between aqueous sodium thiosulfate and hydrochloric acid according to the equation below.
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.
Initially they measured out 10.0 cm3 of 0.500moldm−3 hydrochloric acid and then added 40.0 cm3 of 0.0200 moldmm−3 aqueous sodium thiosulfate. The mark on the paper was obscured 47 seconds after the solutions were mixed.
The teacher asked the students to devise another technique to measure the rate of this reaction.
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.
[H+]=0.5×5010=0.1(moldm−3);pH(=−log[H+]=−log(0.10))=1;
Deduce the percentage of hydrochloric acid that would have to be used up for the pH to change by one unit.
90%;
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, 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−
Half-equation 2: O2(aq)+4e−+2H2O(l)→4OH−(aq)
State a chemical equation for the partial dissociation of water into ions, including state symbols.
H2O(l)⇌H+(aq)+OH−(aq)/2H2O(l)⇌H3O+(aq)+OH−(aq);
The dissociation of water into ions is reversible. State the expression for the ionic product constant of water.
Kw=[H+][OH−]/Kw=[H3O+][OH−];
The ionic product constant of water was measured at three different temperatures.
Deduce whether the ionization of water is exothermic or endothermic, giving your reason.
at higher temperatures ionization increases / at higher temperatures equilibrium shifts to right;
ionization is endothermic;
Use the data in part (iii) to determine the pH of water at 373 K , correct to two decimal places.
[H3O+]=[H+]=5.13×10−13=7.16×10−7moldm−3;
pH=6.14 / 6.15;
When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.
Define the term weak base according to the Brønsted-Lowry theory.
proton acceptor and partially/slightly ionized;
Accept "proton acceptor and partially/slightly dissociated".
Methanoic acid is a monoprotic weak acid.
The concentration of methanoic acid was found by titration with a 0.200 moldm−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.
HCOOH(aq)+NaOH(aq)→HCOONa(aq)+H2O(l)
Which chart shows the neutralization curve of a strong base with a strong acid?
C
Methanoic acid is a monoprotic weak acid.
The concentration of methanoic acid was found by titration with a 0.200 moldm−3 standard solution of sodium hydroxide, NaOH(aq), using an indicator to determine the end point.
Calculate the pH of the sodium hydroxide solution.
[OH−]=0.200 mol dm−3
Alternative 1:
pOH=−log(0.200)=0.699pH=14.000−0.699=13.301
Alternative 2:
[H+]=0.2001.00×10−14=5.00×10−14 mol dm−3pH=−log(5.00×10−14)=13.301
Two hydrides of nitrogen are ammonia and hydrazine, N2H4. One derivative of ammonia is methanamine whose molecular structure is shown below.
Hydrazine reacts with water in a similar way to ammonia. (The association of a molecule of hydrazine with a second H+ is so small it can be neglected.)
Calculate the pH of a 0.0100 moldm−3 solution of hydrazine.
Kb=10−5.77=1.698×10−6
or:
Kb=[N2H4][N2H5+][OH−][OH−]2=1.698×10−6×0.0100=1.698×10−8[OH−]=1.698×10−8=1.303×10−4 mol dm−3pOH=3.89,pH=10.11
Accept answers in the range 10.1-10.2.
When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.
Determine the pH of a 0.100 moldm−3 solution of ammonia, NH3(aq), using tables 2 and 15 of the data booklet.
Kb=[NH3][NH4+][OH−]=1.8×10−5/10−4.75;
[ NH4+] =[OH−]and [NH3]≈1.00×10−1( moldm−3);
[OH−]=(1.8×10−6=)1.3×10−3( moldm−3)/pOH=2.89;
pH=(14.0-2.89=) 11.1;
Award [4] for correct final answer.
Phenylethene (styrene) is produced from ethylbenzene in a gas-phase equilibrium.
Benzoic acid is a weak acid. The graph shows how the pH changes during the titration of a 10.0 cm3 aqueous solution of benzoic acid with aqueous sodium hydroxide.
Explain, with reference to acid-base equilibria, why the sodium benzoate solution formed has a pH>7.
«benzoate ion» hydrolysis/reaction with water AND forms OH− OR
C6H5COO−(aq)+H2O(l)⇌C6H5COOH(aq)+OH−(aq)
When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.
Sketch the pH titration curve obtained when 50.0 cm3 of 0.100moldm−3NH3(aq) is added to 25.0 cm3 of 0.100 moldm−3HCl (aq).
For volume =0: pH=1;
vertical jump should be positioned in volume range 24 cm3 to 26 cm3 and include pH range between 3 to 6;
For volume =50 : pH between 8 to 11;
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.
Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.
HIn⇌H++In−;
(weak acid in which the) acid/HIn and conjugate base/In- have different colours / OWTTE;
excess alkali shifts the equilibrium to the RHS/towards the conjugate base;
When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.
Identify an indicator from table 16 of the data booklet that could be used for this titration.
methyl orange / bromophenol blue / bromocresol green / methyl red;
Phenylethene (styrene) is produced from ethylbenzene in a gas-phase equilibrium.
Describe how a buffer solution could be made from aqueous benzoic acid and aqueous sodium hydroxide.
partially neutralize benzoic acid with NaOH
OR
amount/moles of NaOH added is smaller «than amount of benzoic acid»
Nitrous acid, HNO2, is a weak acid which can be used to make acidic buffers.
A 1.00 moldm−3 solution of nitrous acid was prepared. This solution contains two conjugate acid-base pairs.
This 1.00 moldm−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 of nitrous acid is 3.25.
Concentration of conjugate base:
Ka<10−3.25>=5.62×10−4
« [H+]=10−3.00 » =0.001 «mol dm −3 »
« [A−]=(5.62×10−4×1.00)/0.001»=0.562 «mol dm−3 »
Alternative solution:
pH=pKa+log10[ salt ]/[ acid ] « log10[ salt ]/[ acid ] 》 =−0.25
OR
«[salt]/[acid]» = 0.562、
《 [A−]=0.562/1.00»=0.562 «mol dm−3 » ↓
Award[3]for correct final answer.