R3.3 Hydrogen sharing reactions
Practice exam-style IB Chemistry questions for Hydrogen sharing reactions, aligned with the syllabus and grouped by topic.
Question 1
A radical is best described as a molecular entity that contains what feature?
A.
An ionic bond between two atoms
B.
At least one unpaired electron
C.
A full positive charge on carbon
D.
A lone pair of electrons on an electronegative atom
Question 2
What is the correct representation of a methyl radical?
A.
CH4•
B.
CH3+
C.
CH3−
D.
•CH3
Question 3
What type of bond breaking produces two radicals from a covalent molecule?
A.
Homolytic fission
B.
Hydrolysis
C.
Heterolytic fission
D.
Condensation
Question 4
Which condition is commonly used for the initiation step in the chlorination of methane?
A.
Ultraviolet light
B.
Aqueous sodium hydroxide
C.
Nickel catalyst at high pressure
D.
Acidified potassium dichromate(VI)
Question 5
Ethane reacts with bromine under ultraviolet light. What is the organic product after one substitution?
A.
Ethanoic acid
B.
Ethanol
C.
Bromoethane
D.
Ethene
Question 6
Define the term radical.
[1]
Give one example of a radical.
[1]
Question 7
Bromine molecules can form bromine radicals.
Write the equation for this process.
[1]
State one condition that causes this process.
[1]
Question 8
Ethane undergoes monosubstitution with chlorine.
Write the overall equation.
[1]
State the type of organic reaction.
[1]
Question 9
Which equation represents a propagation step in the chlorination of methane?
A.
Cl2 → 2Cl•
B.
Cl• + CH4 → HCl + •CH3
C.
•CH3 + Cl• → CH3Cl
D.
CH4 + Cl2 → CH3Cl + HCl
Question 10
Which equation represents a termination step in methane chlorination?
A.
•CH3 + Cl2 → CH3Cl + Cl•
B.
Cl2 → 2Cl•
C.
Cl• + CH4 → HCl + •CH3
D.
•CH3 + •CH3 → C2H6
Question 11
Why do alkanes require radical conditions to react with chlorine at an appreciable rate?
A.
They contain weak C=C bonds that need UV light to open.
B.
They are ionic compounds with strong electrostatic attractions.
C.
They contain polar O–H bonds that repel chlorine.
D.
They contain strong, essentially non-polar C–C and C–H bonds.
Question 12
What product forms when two methyl radicals combine in a termination step?
A.
Ethene
B.
Chloromethane
C.
Methane
D.
Ethane
Question 13
Which statement best compares fluorine and iodine in radical substitution of alkanes?
A.
Fluorine and iodine react only by nucleophilic substitution.
B.
Fluorine and iodine are both ideal for controlled monosubstitution.
C.
Fluorine is not reactive enough and iodine is too reactive.
D.
Fluorine is too reactive and iodine is not reactive enough for controlled substitution.
Question 14
The diagram shows a chlorine molecule before bond fission.
Describe how fish-hook arrows should be drawn for homolytic fission of Cl2.
[1]
Question 15
Methane reacts with chlorine in a radical chain reaction.
Write the two propagation equations that form chloromethane.
[1]
State why these steps allow the chain reaction to continue.
[1]
Question 16
In the chlorination of methane, termination steps remove radicals.
State three possible termination equations. [3]
Question 17
Explain why alkanes are relatively unreactive under ordinary laboratory conditions. [2]
Question 18
Radicals can be neutral or charged.
Identify the feature that makes O2•− a radical.
[1]
State why the term radical does not mean the species must be neutral.
[1]
Question 19
A sample of methane and chlorine was exposed to different irradiation conditions. The amount of chloromethane formed after the same time was measured.
| Irradiation condition | Chloromethane formed / mmol |
|---|---|
| Dark | 0.1 |
| Visible light | 0.2 |
| Low UV intensity | 2.4 |
| High UV intensity | 5.3 |
Identify the condition giving the greatest formation of chloromethane.
[1]
Describe the trend shown by the data.
[1]
Explain why ultraviolet light increases the amount of chloromethane formed.
[1]
Question 20
The rate of chlorine radical formation was studied at different ultraviolet intensities.
State the relationship between UV intensity and the initial rate of Cl• formation.
[1]
Write the equation for formation of chlorine radicals.
[1]
Explain the type of bond fission occurring.
[1]
Question 21
A mixture of hexane and bromine water was kept under different conditions and its colour was recorded.
| Condition | Initial colour | Colour after 5 min | Colour after 15 min |
|---|---|---|---|
| Dark | Orange | Orange | Orange |
| Visible light | Orange | Pale orange | Very pale orange |
| Ultraviolet light | Orange | Colourless | Colourless |
State the observation that indicates reaction with bromine.
[1]
Identify the condition under which the reaction is expected to occur most rapidly.
[1]
Explain the role of this condition.
[1]
Question 22
In a fish-hook mechanism for Cl2 → 2Cl•, where should the two single-barbed arrows start?
A.
From the Cl–Cl bond, one to each chlorine atom
B.
From one chlorine atom to the other chlorine atom
C.
From the lone pairs on chlorine to the Cl–Cl bond
D.
From each chlorine atom towards the Cl–Cl bond
Question 23
CFC molecules release chlorine radicals more readily than fluorine radicals in the atmosphere. What is the best explanation?
A.
C–Cl bonds are weaker than C–F bonds.
B.
Fluorine atoms do not contain electrons.
C.
Chlorine radicals are formed only by heterolytic fission.
D.
C–Cl bonds are more ionic than C–F bonds.
Question 24
Chlorine radicals react with ozone, O3, in the stratosphere but typically not with oxygen, O2, in the same way. What does this suggest?
A.
O2 is more readily disrupted than O3.
B.
O3 is more vulnerable to radical attack than O2.
C.
O2 contains no covalent bonds.
D.
O3 cannot undergo any radical reactions.
Question 25
Why can monochlorination of propane give more than one organic product?
A.
Propane contains a C=C bond.
B.
Propane contains more than one type of C–H bond.
C.
All termination steps give the same molecule.
D.
Chlorine cannot form radicals.
Question 26
What is the reverse process of homolytic fission?
A.
A molecule absorbs water and splits into ions.
B.
One ion transfers both electrons to another ion.
C.
A double bond opens to form a polymer chain.
D.
Two radicals combine to form a covalent bond.
Question 27
In the reaction R• + Br2 → RBr + Br•, what stage of a radical chain reaction is represented?
A.
Initiation
B.
Termination
C.
Hydration
D.
Propagation
Question 28
Butane reacts with chlorine under UV light and undergoes one substitution.
State the number of different monochlorinated structural isomers formed.
[1]
Give the names of these isomers.
[1]
Question 29
CFC molecules in the atmosphere can form chlorine radicals but do not usually form fluorine radicals as readily.
Explain this difference. [3]
Question 30
Chlorine radicals react with ozone, O3, in the stratosphere but typically do not break down oxygen, O2, in the same way.
What does this suggest about the relative susceptibility of O3 and O2 to radical attack? [2]
Question 31
The chlorination of methane often gives a mixture of organic products rather than only chloromethane.
Explain why. [3]
Question 32
Alkanes are described as kinetically stable but thermodynamically unstable with respect to combustion.
Distinguish between these two descriptions. [3]
Question 33
Compare homolytic fission and heterolytic fission of a covalent bond. [3]
Question 34
A methane–chlorine mixture was irradiated for increasing times. The organic products were analysed.
Identify the product that appears first in the largest amount.
[1]
Describe how the proportion of more highly chlorinated products changes with irradiation time.
[1]
Suggest why the product mixture changes with time.
[1]
State one termination product that is not a chloromethane.
[1]
Question 35
The relative rates of reaction of methane with different halogens under radical conditions were compared.
Identify the halogen with the fastest reaction.
[1]
Identify the halogen for which radical substitution is least effective.
[1]
Explain why chlorine and bromine are usually preferred for controlled alkane halogenation.
[1]
Question 36
A simplified simulation shows the concentrations of Cl•, •CH3, CH3Cl and C2H6 during chlorination of methane.
Identify the species whose concentration remains very low because it is an intermediate.
[1]
Explain why CH3Cl concentration increases during the reaction.
[1]
Explain the formation of C2H6 in the simulation.
[1]
Question 37
Propane reacts with bromine under UV light.
Write a propagation equation leading to the propan-2-yl radical.
[1]
Write the propagation equation forming 2-bromopropane from this radical.
[1]
Explain why 1-bromopropane can also form.
[1]
Question 38
2-Methylpropane reacts with chlorine under UV light.
State why more than one monosubstituted product can form.
[1]
Draw or name the two possible monochloro products.
[1]
Explain why the reaction may not stop at monosubstitution.
[1]
Question 39
Selected bond enthalpies for bonds in halogenated methanes are shown.
| Bond | Average bond enthalpy / kJ mol⁻¹ |
|---|---|
| C–F | 485 |
| C–Cl | 330 |
| C–Br | 285 |
| C–H | 415 |
Identify the bond most likely to undergo homolytic fission under lower-energy UV radiation.
[1]
Explain your answer using the data.
[1]
Evaluate whether the data support the statement that CFCs release chlorine radicals more readily than fluorine radicals.
[1]
Question 40
The concentration of ozone was monitored in a chamber containing trace chlorine radicals.
Describe the change in ozone concentration after chlorine radicals are introduced.
[1]
State what happens to the chlorine radical concentration if chlorine radicals are regenerated during the process.
[1]
Suggest why a small concentration of chlorine radicals can have a large effect on ozone.
[1]
Suggest why O2 is less affected under the same conditions.
[1]
Question 41
Propane was chlorinated under UV light at two temperatures. The percentages of monochlorinated products were measured.
| Temp / K | Product A / % | Product B / % |
|---|---|---|
| 298 | 45 | 55 |
| 673 | 60 | 40 |
Identify the two monochlorinated products.
[1]
Describe one difference in product distribution between the two temperatures.
[1]
Explain why both products form.
[1]
Question 42
A model predicts the relative frequency of radical collisions during methane chlorination.
| Collision partners | Product formula(s) | Relative frequency / per 10^6 |
|---|---|---|
| Cl• + CH4 | HCl + CH3• | 620000 |
| CH3• + Cl2 | CH3Cl + Cl• | 350000 |
| Cl• + Cl• | Cl2 | 18000 |
| CH3• + Cl• | CH3Cl | 11000 |
| CH3• + CH3• | C2H6 | 1000 |
Identify the collision that forms the desired product chloromethane by termination.
[1]
Identify the collision that forms an organic side product.
[1]
Evaluate why termination alone cannot account for the main formation of chloromethane in a chain reaction.
[1]
Question 43
Methane reacts with chlorine in the presence of ultraviolet light.
State the overall equation and the role of ultraviolet light.
[1]
Explain the radical chain mechanism, including equations for initiation, propagation and termination.
[1]
Question 44
Alkanes can undergo radical substitution with halogens.
Explain why alkanes are relatively unreactive under ordinary conditions.
[1]
Discuss how radical substitution changes an alkane into a more useful organic compound and why mixtures of products may form.
[1]
Question 45
Butane reacts with bromine under ultraviolet light.
Deduce the two possible monosubstituted organic products.
[1]
Explain, with equations, how one of these products is formed by a radical chain mechanism and why a mixture may result.
[1]
Question 46
Halogens differ in their suitability for radical substitution of alkanes.
State the general propagation equations for reaction of an alkane, R–H, with a halogen, X2.
[1]
Compare fluorine, chlorine, bromine and iodine for use in controlled radical substitution of alkanes.
[1]
Question 47
Chlorofluorocarbons can release radicals in the stratosphere.
Explain, using bond fission, how a chlorine radical can be produced from a CFC.
[1]
Evaluate why chlorine radicals from CFCs are more significant than fluorine radicals in stratospheric ozone depletion.
[1]
Question 48
Fish-hook arrows are used in radical mechanisms.
Describe how fish-hook arrows show the homolytic fission of Br2.
[1]
Discuss how correct arrow use helps distinguish initiation, propagation and termination in the bromination of ethane.
[1]
Question 49
A flask of hexane is stable at room temperature but burns readily when ignited and reacts with chlorine under UV light.
Distinguish between kinetic stability and thermodynamic stability for hexane.
[1]
Discuss how radical substitution overcomes kinetic stability without implying that the alkane is thermodynamically stable.
[1]
Question 50
Radicals and ions can both be reactive intermediates.
Define radical and give two different types of radical species.
[1]
Compare and contrast radical formation by homolytic fission with ion formation by heterolytic fission, including expected products for X–Y bond cleavage.
[1]
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