Topic R3.1 · SL and HL

Proton transfer reactions: notes and practice questions

13 min read Jump to 20 practice questions

Summary

This topic covers Brønsted-Lowry acid-base theory and pH calculations.
A Brønsted-Lowry acid donates a proton; a base accepts one.
Conjugate acid-base pairs differ by a single proton.
Some species are amphiprotic.
pH measures $\text{[H}^+\text{]}$ : $\text{pH} = -\text{log}_{10}\text{[H}^+\text{]}$ .
The ion product constant of water is $K_\text{w} = \text{[H}^+\text{]} \text{[OH}^-\text{]}$ .
Strong acids and bases ionize completely; weak acids and bases partially ionize.
Acids react with metal oxides, hydroxides, hydrogencarbonates, and carbonates.
Sketch and interpret pH curves for strong acid-strong base titrations.

How it is examined

Heavily examined at both levels. pH calculations are 1 to 2 marks and the log direction is the common error. The strong versus weak distinction is a recurring 1-mark part: May 2025 HL Paper 2 TZ1 4(b)(ii) accepted "ethanoic acid weak / HCl strong" or "ethanoic acid partially ionizes / HCl fully ionizes", with "lower [H⁺]" also accepted. pH curve sketches are [3] and want the starting pH, the shape through the buffer region, and a vertical section at the correct equivalence volume and pH. Buffer questions come as a pair: describe how to make the most effective buffer [2], then explain what happens when acid or base is added [2], and the explanation marks need equations or named species, not "it neutralizes it".

Given in the booklet

The pH and pOH equations, Kw and its value at 298 K, pKa and pKb values for common weak acids and bases, and indicator pH ranges with their colours. The list of strong acids and bases is recall. Which indicator suits which titration is a deduction from the booklet's range table, not a memory item.

Key ideas

3.1.1 A Brønsted-Lowry acid is a proton donor and a Brønsted-Lowry base is a proton acceptor. Students deduce the Brønsted-Lowry acid and base in a reaction.
3.1.2 A pair of species differing by a single proton is called a conjugate acid-base pair. Students deduce the formula of the conjugate acid or base of any Brønsted-Lowry base or acid.
3.1.3 Some species can act as both Brønsted-Lowry acids and bases. Students interpret and formulate equations to show acid-base reactions of these species.
3.1.4 The pH scale can be used to describe the [H⁺] of a solution: `pH = −log₁₀[H⁺]`; `[H⁺] = 10^(−pH)`. Students perform calculations involving the logarithmic relationship between pH and [H⁺].

Not assessed

HL: the use of quadratic equations is not expected in calculations.
HL: salt hydrolysis examples should include NH₄⁺, RCOO⁻, CO₃²⁻ and HCO₃⁻. The acidity of hydrated transition element ions and Al³⁺(aq) is not required.

At HL

3.1.9 The pOH scale describes the [OH⁻] of a solution: `pOH = −log₁₀[OH⁻]`; `[OH⁻] = 10^(−pOH)`. Students interconvert [H⁺], [OH⁻], pH and pOH values.
3.1.10 The strengths of weak acids and bases are described by their Ka, Kb, pKa or pKb values. Students interpret the relative strengths of acids and bases from these data.
3.1.11 For a conjugate acid-base pair, the relationship `Ka × Kb = Kw` can be derived from the expressions for Ka and Kb. Students solve problems involving these values.
3.1.12 The pH of a salt solution depends on the relative strengths of the parent acid and base. Students construct equations for the hydrolysis of ions in a salt and predict the effect of each ion on the pH of the salt solution.

Guiding questions

What happens when protons are transferred?

Linking questions

Nature of science, Reactivity 3.4 Why has the definition of acid evolved over time?
Structure 2.1 What are the conjugate acids of the polyatomic anions listed in Structure 2.1?
Structure 3.1 What is the periodic trend in the acid-base properties of metal and non-metal oxides? Why does the release of oxides of nitrogen and sulfur into the atmosphere cause acid rain?
Reactivity 2.3 How would you expect the equilibrium constants of strong and weak acids to compare? Why does the extent of ionization of water increase as temperature increases? (HL) Why must buffer solutions be composed of weak acid or base conjugate systems? How does Le Châtelier's principle enable us to interpret the behaviour of indicators and buffer solutions?
Reactivity 1.1 Why does the acid strength of the hydrogen halides increase down group 17? Neutralization reactions are exothermic. How can this be explained in terms of bond enthalpies?
Tools 1, 2, 3 What is the shape of a sketch graph of pH against [H⁺]? When are digital sensors more suitable than analogue methods? (HL) When collecting data to generate a pH curve, when should smaller volumes of titrant be added between each measurement?
Structure 1.3, Reactivity 3.1 Why are log scales useful when discussing [H⁺] and ionization energies?

Practice questions

31 questions · 14 easy · 13 medium · 4 hard

Showing 20 of 20

Question 1

EasyPaper 2 · calculator1 mark

Which pair represents a Brønsted-Lowry conjugate acid-base pair?

A. $H_3PO_4 / PO_4^{3-}$

B. $H_2PO_4^- / HPO_4^{2-}$

C. $NH_4^+ / OH^-$

D. $SO_3 / H_2SO_4$

Question 2

MediumPaper 2 · calculator1 mark

Which of the following aqueous solutions, each with a concentration of $0.50 \text{ mol dm}^{-3}$ , has the highest electrical conductivity?

A. $CH_3COOH$

B. $NH_3$

C. $C_6H_{12}O_6$

D. $MgCl_2$

Question 3

HardPaper 1B · calculator16 marks

(a)

Nitrogen dioxide, a pollutant from car exhausts, reacts with water in the atmosphere to form nitric acid and nitrous acid. This is a disproportionation reaction.

$2NO_{2}(g) + H_{2}O(l) \rightleftharpoons HNO_{3}(aq) + HNO_{2}(aq)$

(a) Deduce the oxidation states of nitrogen in the reactant and products.

Reactant: $NO_{2}$

Products: $HNO_{3}$ , $HNO_{2}$

[2]

(b)

(b) Explain, with reference to the equilibrium, why more nitrogen dioxide gas dissolves when the reaction occurs in alkaline rainwater.

[1]

(c)(i)

(c) The solubility of nitrogen dioxide gas in water was measured by different scientific groups. A summary of their results is shown.

Source	Temperature / °C	Solubility of $NO_{2}$ gas in 0.100 dm³ of water
A	0	0.380 dm³
B	10	0.28 dm³
C	20	200 cm³
D	25	0.15 L
E	30	0.120 dm³

(i) Identify a problem in comparing the data from the different sources as presented in the table.

[1]

(c)(ii)

(ii) The units of solubility are converted to mol dm⁻³. Complete the table by calculating the value for source A. Assume the atmospheric pressure is 100 kPa and the density of the resulting solution is 1.00 g cm⁻³.

[2]

(c)(iii)

(iii) Suggest an explanation for the effect of temperature on the solubility of nitrogen dioxide gas.

[1]

(d)

(d) Suggest one reason why nitrogen dioxide is considered a major air pollutant.

[1]

(e)(i)

(e) Nitrous acid, $HNO_2$ , is a weak acid. The graph shows the percentage of nitrous acid and its conjugate base, the nitrite ion ( $NO_2^-$ ), present at different pH values.

(GRAPH IS A STANDARD SPECIATION PLOT FOR A WEAK ACID. X-AXIS: pH from 0 to 8. Y-AXIS: Percentage from 0 to 100. A curve for HA starts at 100% and goes down, a curve for A- starts at 0% and goes up. The two curves cross at pH = 3.3, where each is at 50%.)

(i) Deduce the pH range where nitrous acid, $HNO_{2}$ , is the dominant nitrogen-containing species in the solution.

[1]

(e)(ii)

(ii) Determine, with reference to the graph, the $pK_a$ of nitrous acid.

[2]

(f)(i)

(f) Nitrous acid can react with secondary amines to form N-nitrosamines, which are potent carcinogens. An example is the reaction with dimethylamine, $(CH_3)_2NH$ .

(i) Deduce a balanced chemical equation for the formation of N-nitrosodimethylamine, $(CH_3)_2NNO$ , from dimethylamine and nitrous acid.

[1]

(f)(ii)

(ii) The rate of N-nitrosamine formation is highly dependent on pH. The reaction rate is highest under mildly acidic conditions where there is a sufficient concentration of both the unprotonated amine and nitrous acid. State two conditions that could be maintained in an industrial process to minimize the formation of N-nitrosamines.

[2]

(g)

(g) To combat the effects of acid rain, powdered limestone ( $CaCO_3$ ) is sometimes added to lakes. Suggest two distinct reasons why this 'liming' process is effective at restoring the aquatic ecosystem.

[2]

Question 4

EasyPaper 2 · calculator9 marks

(a)

A student investigates the reactions of dilute hydrochloric acid with four different substances: solid sodium carbonate ( $Na_2CO_3$ ), solid copper(II) oxide ( $CuO$ ), magnesium ribbon ( $Mg$ ), and aqueous potassium hydroxide ( $KOH$ ).

(a) Write a balanced chemical equation, including state symbols, for the reaction of hydrochloric acid with solid sodium carbonate.

[2]

(b)

(b) Write a balanced chemical equation, including state symbols, for the reaction of hydrochloric acid with solid copper(II) oxide.

[2]

(c)

(c) Write a balanced chemical equation, including state symbols, for the reaction of hydrochloric acid with aqueous potassium hydroxide.

[2]

(d)(i)

(d) (i) The reaction with magnesium ribbon is the only one of the four that does not produce water. Identify the gaseous product of this reaction.

[1]

(d)(ii)

(d) (ii) Write the balanced chemical equation, including state symbols, for the reaction between magnesium and hydrochloric acid.

[1]

(e)

(e) State the type of reaction that occurs between hydrochloric acid and potassium hydroxide.

[1]

Question 5

MediumPaper 2 · calculator1 mark

Aqueous equimolar solutions of a strong base and a weak base can be distinguished using several properties. Which of the following methods would successfully distinguish between $0.10 \text{ mol dm}^{-3} \text{ KOH(aq)}$ and $0.10 \text{ mol dm}^{-3} \text{ NH}_3\text{(aq)}$ ?

I. Measuring their electrical conductivity.

II. Measuring their pH.

III. Measuring the volume of $0.10 \text{ mol dm}^{-3} \text{ HCl(aq)}$ needed for neutralization.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

Question 6

HardPaper 2 · calculator24 marks

(a)(i)

Antimony (Sb) and Bismuth (Bi) are elements in group 15 of the periodic table.

(a) Antimony has two stable isotopes. 57.21% of antimony atoms contain 70 neutrons and the remainder contain 72 neutrons.

(i) Deduce the nuclear symbol of the isotope of antimony containing 72 neutrons. Use section 6 of the data booklet.

[1]

(a)(ii)

(ii) Calculate, to two decimal places, the relative atomic mass of antimony.

[2]

(b)(i)

Bismuth(III) nitrate, $Bi(NO_3)_3$ , is a common salt of bismuth.

(b) (i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.

[2]

(b)(ii)

(ii) Distinguish between covalent and ionic bonding in terms of electron distribution.

[2]

(b)(iii)

(iii) State the enthalpy term that characterizes the strength of the bonding between the ions in an ionic solid.

[1]

(b)(iv)

(iv) Write an equation for the formation of aqueous bismuth(III) nitrate from solid bismuth(III) oxide and nitric acid.

[2]

(b)(v)

(v) Calculate the volume, in $cm^3$ , of $1.50 \text{ mol dm}^{-3}$ nitric acid required to react completely with $5.00 \text{ g}$ of solid bismuth(III) oxide.

[3]

(b)(vi)

(vi) Predict, with a reason, whether bismuth(III) oxide is expected to be primarily acidic, basic or amphoteric.

[1]

(b)(vii)

(vii) Discuss how the relative reactivity of zinc and bismuth could be established using the metals and aqueous solutions of their nitrates.

[2]

(b)(viii)

(viii) Discuss the products formed at the electrodes during the electrolysis of aqueous bismuth(III) nitrate. Use the standard electrode potential $E^\ominus(Bi^{3+}/Bi) = +0.32 \text{ V}$ and section 24 of the data booklet.

[2]

(c)(i)

Bismuth compounds are sometimes used in fireworks to produce special effects.

[1]

(c)(ii)

(ii) Calculate the wavelength, in nm, that corresponds to the first ionization energy of bismuth. Use sections 1, 2 and 8 of the data booklet.

[3]

(c)(iii)

(iii) Explain why the first ionization energy of bismuth is lower than that of polonium (Po), in terms of nuclear charge and electron shielding.

[2]

Question 7

EasyPaper 2 · calculator1 mark

Which two species act as Brønsted-Lowry acids in the following equilibrium?

$NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$

A. $NH_3$ and $H_2O$

B. $NH_4^+$ and $OH^-$

C. $H_2O$ and $NH_4^+$

D. $NH_3$ and $OH^-$

Question 8

MediumPaper 2 · calculator5 marks

(a)

Boron trifluoride, $BF_3$ , is a versatile industrial catalyst. It reacts with ammonia, $NH_3$ , to form a stable adduct as shown in the equation below.

$BF_3 + NH_3 \rightarrow F_3BNH_3$

(a) Define the term Lewis acid.

[1]

(b)

(b) Explain, with reference to the electronic structures of the reactants, why this reaction is classified as a Lewis acid-base reaction. Identify the Lewis acid and the Lewis base.

[3]

(c)

[1]

Question 9

HardPaper 2 · calculator6 marks

(a)

Caffeine is a stimulant found in tea leaves. A student carried out an experiment to extract caffeine from tea. The procedure involved adding solid sodium carbonate ( $Na_2CO_3$ ) to boiling water with tea leaves, followed by a liquid-liquid extraction using dichloromethane ( $CH_2Cl_2$ ) as the organic solvent. The crude caffeine obtained after evaporating the dichloromethane was then purified by recrystallization.

(a) State one key property of a solvent required for a liquid-liquid extraction and suggest why dichloromethane is a suitable choice for extracting caffeine from the aqueous solution.

[2]

(b)

(b) The aqueous solution from the tea leaves is alkaline due to the presence of sodium carbonate. Suggest how the addition of sodium carbonate improves the separation of caffeine from other acidic compounds, such as tannins, also present in tea.

[2]

(c)

(c) During the final recrystallization step, the student cooled the hot solution of crude caffeine rapidly in an ice bath. Outline how this would affect the purity of the caffeine obtained.

[1]

(d)

(d) Dichloromethane is a chlorinated hydrocarbon. State one reason, other than its toxicity, why the use of such solvents is being phased out.

[1]

Question 10

EasyPaper 1A · calculator1 mark

Which two species can act as both Brønsted–Lowry acids and bases?

A. $H_2O$ and $H_3O^+$

B. $HCO_3^-$ and $CO_3^{2-}$

C. $H_2O$ and $HCO_3^-$

D. $H_3O^+$ and $CO_3^{2-}$

Question 11

MediumPaper 1A · calculator1 mark

Solutions of nitric acid ( $HNO_3$ ) and hydrocyanic acid ( $HCN$ ) are prepared at the same concentration. Why is the pH of the nitric acid solution significantly lower than the pH of the hydrocyanic acid solution?

A. Nitric acid has a greater molar mass than hydrocyanic acid.

B. Nitric acid ionizes to a much greater extent than hydrocyanic acid.

C. The nitrate ion ( $NO_3^{-}$ ) is a stronger conjugate base than the cyanide ion ( $CN^{-}$ ).

D. Nitric acid contains oxygen atoms, while hydrocyanic acid does not.

Question 12

HardPaper 2 · calculator16 marks

(a)(i)

Strontium is an alkaline earth metal in group 2 of the periodic table, used to produce the red colour in fireworks.

(a) In a sample of strontium, 10.0% of the atoms have a mass number of 86 and the remainder have a mass number of 88.

(i) Deduce the nuclear symbol of the isotope of strontium with mass number 86. Use section 6 of the data booklet.

[1]

(a)(ii)

(ii) Calculate, to two decimal places, the relative atomic mass of this sample of strontium.

[2]

(a)(iii)

(iii) Explain why the first ionization energy of strontium is greater than that of rubidium.

[2]

(b)(i)

(b) Strontium nitrate has the formula $Sr(NO_3)_2$ .

(i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.

[2]

(b)(ii)

(ii) Contrast covalent and ionic bonds in terms of how the valence electrons are involved.

[1]

(b)(iii)

(iii) Write a balanced chemical equation for the formation of strontium nitrate solution from the reaction of solid strontium hydroxide with nitric acid.

[2]

(b)(iv)

(iv) Calculate the volume, in $cm^3$ , of $1.50 \text{ mol dm}^{-3}$ nitric acid required to react completely with $5.00 \text{ g}$ of solid strontium hydroxide.

[3]

(b)(v)

(v) Predict, with a reason, whether strontium oxide is acidic, basic or amphoteric.

[1]

(b)(vi)

(vi) Describe an experiment to establish the relative reactivity of strontium and zinc using the metals and aqueous solutions of their nitrates.

[2]

Question 13

EasyPaper 1A · calculator1 mark

According to the Brønsted–Lowry theory, what is the species that remains after an acid has donated a proton?

A. Its conjugate base

B. Its conjugate acid

C. A hydroxide ion

D. An oxonium ion

Question 14

MediumPaper 1A · calculator1 mark

In which of the following reactions does water act as a Brønsted–Lowry acid?

A. $H_2O(l) + CH_3COOH(aq) \rightleftharpoons H_3O^+(aq) + CH_3COO^-(aq)$

B. $2H_2O(l) + 2F_2(g) \rightarrow 4HF(aq) + O_2(g)$

C. $H_2O(l) + CN^-(aq) \rightleftharpoons OH^-(aq) + HCN(aq)$

D. $[CoCl_4]^{2-}(aq) + 6H_2O(l) \rightarrow [Co(H_2O)_6]^{2+}(aq) + 4Cl^-(aq)$

Question 15

EasyPaper 1A · calculator1 mark

What is the pH of $0.0050$ $mol$ $dm^{-3}$ $HNO_3(aq)$ ?

A. $2.0$

B. $2.3$

C. $2.7$

D. $3.0$

Question 16

MediumPaper 1A · calculator1 mark

A sample of rainwater collected in an industrial area was found to have a pH of 5.5. What is the hydrogen ion concentration in this rainwater sample in $\text{mol dm}^{-3}$ ?

A. $5.5 \times 10^{-6}$

B. $1.0 \times 10^{-5}$

C. $3.2 \times 10^{-6}$

D. $1.0 \times 10^{-7}$

Question 17

EasyPaper 1A · calculator1 mark

Which solution is basic at $25$ °C?

A. $[H_3O^+] = 1.0 \times 10^{-2}$ mol dm $^{-3}$

B. $[OH^-] = 1.0 \times 10^{-7}$ mol dm $^{-3}$

C. $[H^+] = 1.0 \times 10^{-10}$ mol dm $^{-3}$

D. $[OH^-] = 1.0 \times 10^{-9}$ mol dm $^{-3}$

Question 18

MediumPaper 1A · calculator1 mark

A chemist is preparing solutions for a reaction that requires a specific alkaline environment. Identify which of the following solutions would have a $pH$ of $12.0$ .

A. $1.0 \times 10^{-2}$ $mol$ $dm^{-3}$ $NaOH(aq)$

B. $1.0 \times 10^{-2}$ $mol$ $dm^{-3}$ $NH_3(aq)$ ( $K_b = 1.8 \times 10^{-5}$ )

C. $1.0 \times 10^{-12}$ $mol$ $dm^{-3}$ $NaOH(aq)$

D. $1.0 \times 10^{-1}$ $mol$ $dm^{-3}$ $NH_3(aq)$ ( $K_b = 1.8 \times 10^{-5}$ )

Question 19

EasyPaper 1A · calculator1 mark

Which two species can act as both Brønsted-Lowry acids and bases?

A. $H_2SO_4$ and $SO_4^{2-}$

B. $HSO_4^-$ and $HCO_3^-$

C. $HSO_4^-$ and $CO_3^{2-}$

D. $HCO_3^-$ and $H_2SO_4$

Question 20

MediumPaper 1A · calculator1 mark

Which reactions involve the transfer of a proton?

I. $2HNO_3(aq) + Zn(s) \rightarrow Zn(NO_3)_2(aq) + H_2(g)$

II. $2HNO_3(aq) + Zn(OH)_2(s) \rightarrow Zn(NO_3)_2(aq) + 2H_2O(l)$

III. $HNO_3(aq) + NH_3(aq) \rightarrow NH_4NO_3(aq)$

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

11 more Proton transfer reactions questions in the app

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
Joining the dots instead of drawing a curve.
Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.

FAQ

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What does Proton transfer reactions cover in IB Chemistry?

This topic covers Brønsted-Lowry acid-base theory and pH calculations. A Brønsted-Lowry acid donates a proton; a base accepts one. Conjugate acid-base pairs differ by a single proton.

Is Proton transfer reactions SL or HL?

Both. SL and HL students study Proton transfer reactions, and HL goes further: 3.1.9 The pOH scale describes the [OH⁻] of a solution: `pOH = −log₁₀[OH⁻]`; `[OH⁻] = 10^(−pOH)`. Students interconvert [H⁺], [OH⁻], pH and pOH values.

How do I revise Proton transfer reactions for IB Chemistry?

Start from the core idea: this topic covers Brønsted-Lowry acid-base theory and pH calculations. In the exam: heavily examined at both levels. pH calculations are 1 to 2 marks and the log direction is the common error. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

How does FourtyFive help me practise Proton transfer reactions?

FourtyFive has 31 Proton transfer reactions questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Proton transfer reactions practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Proton transfer reactions answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

Proton transfer reactions: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

11 more Proton transfer reactions questions in the app

Where marks are lost

Practise this topic as a session

Questions,
answered.

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bank built for you.

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

11 more Proton transfer reactions questions in the app

Where marks are lost

Questions,answered.

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Questions,
answered.

Start with the IB question
bank built for you.