R3.4 Electron-pair sharing reactions questions for IB Chemistry

136 questions on Electron-pair sharing reactions, written by Augment in the style of IB exam papers. Six of them are below in full, to answer on paper.

IB Chemistry
Part of R3 What are the mechanisms of chemical change?
SL and HL

Questions by paper and level

SL and HL questions are for students at either level. HL only questions cover what only HL students study.

Paper	SL and HL	HL only	All
Paper 1A	26	35	61
Paper 1B	4	12	16
Paper 2	21	38	59
Every paper	51	85	136

Counted on 10 November 2026

Questions to try

Written by Augment in the style of IB exams, not past papers. Answer them on paper, then check each one against its markscheme with a free account.

Question 1 · CHEM-00100Paper 1A · 1 mark

Ammonia reacts with boron trifluoride to form the addition compound $H_{3} N B F_{3}$ .

Which describes the bond formed between nitrogen and boron?

A
An ionic bond formed by transfer of an electron from N to B
B
A covalent bond in which N and B each provide one electron
C
A coordination bond in which both electrons come from N
D
A coordination bond in which both electrons come from B

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Question 2 · CHEM-01059Paper 1B · HL only · 10 marks

The copper content of brass was determined by colorimetry. 0.500 g of brass was dissolved in nitric acid, excess aqueous ammonia was added to form the deep blue complex $[C u (N H_{3})_{4} (H_{2} O)_{2}]^{2 +}$ , and the solution was made up to 250.0 cm³. Standard copper(II) solutions were treated with ammonia in the same way. The colorimeter was set to zero using a blank.

[Cu²⁺] / mol dm⁻³	0.0050	0.0100	0.0150	0.0200	0.0250
Absorbance ± 0.01	0.12	0.25	0.37	0.50	0.62

The brass solution had an absorbance of 0.52.

(a)
Using section 15 of the data booklet, suggest the colour of light the colorimeter should use.
1 mark
(b)
Suggest why ammonia is added rather than measuring the absorbance of aqueous copper(II) ions directly.
1 mark
(c)
Determine the gradient of the calibration line, including its units.
2 marks
(d)
Determine the percentage by mass of copper in the brass.
3 marks
(e)
State what the blank should contain.
1 mark
(f)
Brass also contains zinc. Explain why zinc ions do not affect the absorbance.
1 mark
(g)
Calculate the percentage uncertainty in the absorbance of the brass solution.
1 mark

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Question 3 · CHEM-00299Paper 2 · HL only · 10 marks

In 1865 Kekulé proposed that benzene, C₆H₆, has a ring of six carbon atoms joined by alternating single and double bonds (cyclohexa-1,3,5-triene). Some data are given.

C_{6} H_{10} (l) + H_{2} (g) \to C_{6} H_{12} (l) Δ H = - 120 {kJ mol}^{- 1}

C_{6} H_{6} (l) + 3 H_{2} (g) \to C_{6} H_{12} (l) Δ H = - 208 {kJ mol}^{- 1}

In the first equation C₆H₁₀ is cyclohexene, which has one C=C bond. X-ray diffraction shows that all the carbon–carbon bonds in benzene are 140 pm long.

(a)
Predict the enthalpy change, in kJ mol⁻¹, for the complete hydrogenation of the Kekulé structure.
1 mark
(b)
Calculate the difference between your answer to (a) and the experimental value, and state what it shows about benzene.
2 marks
(c)
Using section 11 of the data booklet, explain how the bond length data support a delocalized structure.
2 marks
(d)
Cyclohexene rapidly decolourizes bromine water but benzene does not. Explain this difference in reactivity.
2 marks
(e)
State the hybridization of the carbon atoms in benzene and describe how the delocalized π system is formed.
2 marks
(f)
The Kekulé structure predicts two different isomers of 1,2-dichlorobenzene. Only one is known. Outline what this shows.
1 mark

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Question 4 · CHEM-00196Paper 1A · 1 mark

Ethanol labelled with the isotope ¹⁸O, $C H_{3} C H_{2}^{18} O H$ , was used to investigate the reaction between ethanol and ethanoic acid. The ¹⁸O was found in the ester product and not in the water.

Why is ¹⁸O suitable for this investigation?

A
It reacts in the same way as ¹⁶O but its greater mass allows it to be detected in the products.
B
It reacts faster than ¹⁶O, so it ends up in the main product.
C
It has two more protons than ¹⁶O, so it can be identified.
D
It has a different electron configuration from ¹⁶O, so it forms different bonds.

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Question 5 · CHEM-01060Paper 1B · HL only · 10 marks

The iron content of an iron supplement tablet was determined by colorimetry. Iron(III) ions react with excess thiocyanate ions to form a red complex, $[F e (S C N) (H_{2} O)_{5}]^{2 +}$ .

The absorption spectrum of the complex was recorded first.

Wavelength / nm	400	425	450	475	500	525	550	575	600	650
Absorbance	0.21	0.38	0.55	0.63	0.58	0.41	0.22	0.10	0.05	0.02

Method:

One tablet was dissolved in dilute sulfuric acid and hydrogen peroxide was added to convert all Fe²⁺ to Fe³⁺.
The solution was made up to 500.0 cm³.
10.00 cm³ of this solution and 10.00 cm³ of potassium thiocyanate solution (excess) were diluted to 100.0 cm³.
The absorbance was measured at the wavelength of maximum absorbance: 0.58 ± 0.01.

A calibration graph gave the relationship: absorbance = 2.50 × 10³ dm³ mol⁻¹ × [Fe³⁺].

(a)
State the wavelength at which the measurements should be made.
1 mark
(b)
Outline why measurements are made at this wavelength.
1 mark
(c)
Using section 15 of the data booklet, deduce the colour of light absorbed most strongly and hence the colour expected for the complex.
1 mark
(d)
Deduce the change in oxidation state of iron when hydrogen peroxide is added.
1 mark
(e)
State the condensed electron configuration of Fe³⁺.
1 mark
(f)
Determine the mass of iron, in mg, in one tablet.
3 marks
(g)
The label states that each tablet contains 65 mg of iron. Comment on the result, with reference to the uncertainty in the absorbance.
2 marks

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Question 6 · CHEM-00552Paper 2 · 8 marks

There are four structural isomers with the molecular formula $C_{4} H_{9} B r$ .

(a)
Draw the structural formula of each isomer and state its IUPAC name.
4 marks
(b)
Classify each isomer as primary, secondary or tertiary.
2 marks
(c)
2-bromo-2-methylpropane reacts with aqueous sodium hydroxide to form an alcohol. State the IUPAC name of this alcohol and classify it as primary, secondary or tertiary.
2 marks

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