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Question 1
A radical is best identified by the presence of what feature?
A.
A positive charge
B.
A carbon-carbon double bond
C.
A lone pair on oxygen
D.
An unpaired electron
Question 2
The ethyl radical has the unpaired electron on the terminal carbon atom. What is the correct representation?
A.
B.
C.
D.
Question 3
Bromine molecules undergo homolytic fission in ultraviolet light. What equation represents this initiation step?
A.
B.
C.
D.
Question 4
Methane reacts with chlorine in ultraviolet light by radical substitution. What is the overall equation for monosubstitution?
A.
B.
C.
D.
Question 5
Radicals are important intermediates in some substitution reactions.
Define the term radical.
[1]
Represent the methyl radical, showing the position of the unpaired electron.
[1]
0
Question 6
A student draws the homolytic fission of . What arrow description is correct for the electron movement?
A.
Two double-barbed arrows, each starting at a chlorine atom and ending at the bond
B.
One single-barbed arrow starting at one chlorine atom and ending at the other chlorine atom
C.
One double-barbed arrow starting at the bond and ending at one chlorine atom
D.
Two single-barbed arrows, each starting at the bond and ending at a different chlorine atom
Question 7
The following steps may occur during bromination of ethane.
I.
II.
III.
What are the stages represented by I, II and III?
A.
I propagation; II initiation; III termination
B.
I initiation; II propagation; III termination
C.
I termination; II propagation; III initiation
D.
I initiation; II termination; III propagation
Question 8
Chlorofluorocarbons in the atmosphere can release chlorine radicals more readily than fluorine radicals. What is the best explanation?
A.
The bond is more polar than the bond, so it undergoes heterolytic fission more readily.
B.
The bond is weaker than the bond, so it requires less energy for homolytic fission.
C.
The chlorine atom has fewer electrons than fluorine, so it forms radicals more easily.
D.
The bond absorbs ultraviolet light and reforms the bond.
Question 9
Chlorine radicals in the stratosphere react with ozone, , more readily than with oxygen, . What does this suggest?
A.
The molecule is thermodynamically unstable with respect to chlorine radicals.
B.
The bonding in is more readily disrupted than the bonding in .
C.
The molecule cannot form radicals under any conditions.
D.
The molecule contains no covalent bonds.
Question 10
Alkanes are described as kinetically stable but thermodynamically unstable with respect to combustion. What statement gives the correct reason?
A.
Combustion is rapid at room temperature because alkanes contain radicals.
B.
Combustion is energetically unfavourable, but weak polar bonds give a low activation energy.
C.
Combustion is prevented because carbon-carbon bonds cannot be broken.
D.
Combustion is energetically favourable, but strong non-polar bonds give a high activation energy.
Question 11
What equation represents a termination step in the radical chlorination of ethane?
A.
B.
C.
D.
Question 12
Chlorine molecules can form chlorine radicals in the presence of ultraviolet light.
State the type of bond fission that occurs.
[1]
Write an equation for the initiation step.
[1]
Explain why single-barbed arrows are used in this step.
[1]
0
Question 13
Methane reacts with chlorine in the presence of UV light to form chloromethane.
Write the overall equation for the formation of chloromethane from methane.
[1]
Write the two propagation steps in this reaction.
[2]
0
Question 14
A bond can break by homolytic or heterolytic fission.
Compare the electron movement in homolytic and heterolytic fission.
[2]
State the type of arrow used to show the movement of one electron in radical mechanisms.
[1]
0
Question 15
A mass spectrometer experiment detected several short-lived molecular entities during irradiation of a chloromethane sample. The table shows their formulae and simplified electron information.
| Species | Simplified electron information |
|---|---|
| CH3Cl | 14 valence electrons; all paired |
| Cl | 7 valence electrons; one unpaired electron |
| HCl | 8 valence electrons; all paired |
| CH3 | 7 valence electrons; one unpaired electron |
State the feature that identifies a species as a radical.
[1]
Identify the two radicals in the table and represent each using a dot.
[1]
Explain why the species identified in part (b) are more reactive than chloromethane.
[1]
0
Question 16
The graph shows the rate of radical formation from gaseous chlorine as the intensity of ultraviolet radiation is increased at constant temperature.
Describe the relationship shown between ultraviolet radiation intensity and the rate of radical formation.
[1]
Write the equation for the initiation step that produces the radicals from chlorine.
[1]
Explain, in terms of electron movement, why this step is described as homolytic fission.
[2]
0
Question 17
Propane reacts with chlorine in ultraviolet light. The reaction is not perfectly selective. What product mixture is expected after monosubstitution?
A.
and
B.
and
C.
and
D.
and
Question 18
The propagation steps in bromination of ethane are shown.
What overall equation is obtained by adding these propagation steps?
A.
B.
C.
D.
Question 19
Ethane undergoes radical substitution with bromine under suitable conditions.
State one suitable condition for this reaction.
[1]
Write one termination step that forms bromoethane.
[1]
Explain why the reaction is described as a chain reaction.
[2]
0
Question 20
Alkanes are described as kinetically stable but thermodynamically unstable with respect to combustion.
Explain why alkanes are kinetically stable at room temperature.
[2]
State why they are thermodynamically unstable with respect to combustion.
[1]
0
Question 21
Some chlorofluorocarbons release chlorine radicals in the stratosphere. The table gives mean bond enthalpies for two carbon-halogen bonds.
| Bond | Mean bond enthalpy / kJ mol^-1 |
|---|---|
| C–Cl | 338 |
| C–F | 485 |
Use the bond enthalpy data to explain why chlorine radicals are released more readily than fluorine radicals.
[2]
Write a general equation for homolytic fission of a carbon-chlorine bond in an organic molecule represented by .
[1]
State the role of UV radiation in this process.
[1]
0
Question 22
The radical substitution of butane with bromine forms several products. One propagation pathway produces -bromobutane.
Write the propagation step in which a bromine radical forms the secondary butyl radical from butane.
[1]
Write the propagation step in which this secondary butyl radical forms -bromobutane.
[1]
Explain why -bromobutane can also form.
[2]
0
Question 23
The reverse of homolytic fission can occur during a radical chain reaction.
Write an equation for the reverse of homolytic fission of bromine.
[1]
Identify the stage of a radical substitution mechanism represented by this equation.
[1]
Explain why this stage slows the chain reaction.
[1]
0
Question 24
Methane and chlorine were mixed and exposed to ultraviolet light. The flowchart summarizes the radical chain mechanism, with some species omitted.
Identify the stage of the mechanism in which ultraviolet light is used to produce the first radicals.
[1]
Complete the two propagation equations for this reaction.
[2]
Suggest one termination equation that could form a product with two carbon atoms.
[1]
Explain why propagation allows a small number of radicals to produce many product molecules.
[1]
0
Question 25
Propane was reacted with chlorine under ultraviolet light. The graph shows the percentage composition of organic products as reaction time increases.
Describe how the proportion of polychlorinated products changes during the reaction.
[1]
State why radical substitution of propane can form more than one monochlorinated product.
[1]
Suggest why increasing reaction time leads to polychlorinated products.
[2]
0
Question 26
Propane reacts with chlorine under UV light. A mixture of organic products is obtained.
Suggest two monochlorinated structural isomers that can form.
[2]
Explain why products containing more than one chlorine atom may also be present.
[2]
0
Question 27
A student suggests using fluorine instead of chlorine for the radical substitution of hexane to prepare a single fluorohexane product.
Evaluate this suggestion.
[3]
State why iodine is not normally used for radical substitution of alkanes.
[1]
0
Question 28
Chlorine radicals released in the stratosphere can react with ozone, , but normally do not break down oxygen, , in the same way.
State what this suggests about the relative strength of bonding in and .
[1]
Explain why a chlorine radical is highly reactive.
[1]
Chlorine radicals can be regenerated after reacting with ozone. Explain why regeneration makes radicals effective in chain processes.
[2]
0
Question 29
The table compares selected bond enthalpies in a chlorofluorocarbon and the energy available from ultraviolet radiation in the stratosphere.
| Bond or radiation | Energy / kJ mol^-1 |
|---|---|
| C–F bond in CCl3F | 485 |
| C–Cl bond in CCl3F | 340 |
| UV photon in the stratosphere | 400 |
Identify the carbon-halogen bond that is more likely to undergo homolytic fission in the stratosphere.
[1]
Write an equation for homolytic fission of one bond in .
[1]
Explain why chlorine radicals are released more readily than fluorine radicals from the molecule.
[2]
0
Question 30
Chlorine radicals react with ozone in the stratosphere. The table compares two possible reactions of under the same conditions.
| Reaction at 220 K | Product(s) | k / cm³ molecule⁻¹ s⁻¹ |
|---|---|---|
| Cl· + O3 | ClO· + O2 | 1.4 × 10^-11 |
| Cl· + O2 | No detectable reaction | <1.0 × 10^-24 |
Identify which allotrope reacts more readily with chlorine radicals under these conditions.
[1]
State what the data suggest about the relative strengths of bonding in and .
[1]
Explain why chlorine radicals can contribute to ozone depletion even when present in low concentration.
[2]
0
Question 31
A mixture of chlorine and ethane was irradiated for a short time and then kept in the dark. The graph shows the relative concentration of chlorine radicals during and after irradiation.
Describe the change in chlorine radical concentration after the ultraviolet light is switched off.
[1]
Explain why the concentration of radicals decreases in the dark.
[2]
Write one termination equation involving an ethyl radical that could occur in this mixture.
[1]
0
Question 32
The diagram shows four proposed arrow-pushing representations for breaking a bond in a chlorofluorocarbon under ultraviolet light.
State the feature of the arrow-pushing diagram that correctly represents homolytic fission of the bond.
[1]
Explain why a fish-hook arrow is used rather than a double-barbed curly arrow.
[1]
State the two products expected from homolytic fission of at a bond.
[1]
Suggest one error that would make an arrow-pushing diagram for homolytic fission incorrect.
[1]
0
Question 33
Radicals are involved in many atmospheric and organic reactions. Four species are shown: , , and .
Radicals can be atoms, molecules or ions.
I.
State the feature common to all radicals.
[1]
II.
Identify one radical cation and one radical anion from the species shown.
[2]
Explain why radicals are usually highly reactive and short-lived intermediates in reaction mechanisms.
[3]
0
Question 34
The initiation step in the chlorination of an alkane involves the homolytic fission of a chlorine molecule.
The equation for the initiation step is represented as .
I.
Define homolytic fission.
[1]
II.
Explain the use of single-barbed arrows in a mechanism for this step.
[2]
Explain why ultraviolet light or heat is needed for this initiation step.
[3]
0
Question 35
A student compares heterolytic fission and homolytic fission using chlorine-containing molecules.
Bond fission can produce different types of reactive species.
I.
Write an equation for the homolytic fission of bromine.
[1]
II.
State the type of arrow used to show the movement of electrons in homolytic fission and the number of electrons represented by each arrow.
[2]
Compare and contrast homolytic and heterolytic fission.
[3]
0
Question 36
Methane and deuterated methane were separately chlorinated under identical ultraviolet conditions. The graph shows the initial rate of first monochlorination for each substrate.
Estimate the ratio of the initial rate for methane to that for deuterated methane from the graph.
[1]
Identify the propagation step most directly affected by replacing with .
[1]
Suggest why the rate is lower for deuterated methane.
[2]
Evaluate whether the data support a radical substitution mechanism.
[1]
0
Question 37
Butane was separately chlorinated and brominated under radical conditions. The table shows the percentage of substitution at primary and secondary carbon atoms in each reaction.
| Reaction | Primary substitution / % | Secondary substitution / % |
|---|---|---|
| Chlorination | 30 | 70 |
| Bromination | 3 | 97 |
State the number of primary and secondary hydrogen atoms in butane.
[1]
Using the table, determine which halogenation is more selective for secondary substitution.
[1]
Calculate the relative reactivity of one secondary bond compared with one primary bond in the bromination experiment.
[2]
Suggest why both reactions still form mixtures of products.
[1]
0
Question 38
In a photochemical chlorination experiment, the number of photons absorbed and the amount of chloromethane formed were measured at different irradiation times.
| Irradiation time / min | Photons absorbed / photons | CH3Cl formed / mol |
|---|---|---|
| 5 | 2.5 × 10^11 | 5.0 × 10^-9 |
| 10 | 5.0 × 10^11 | 1.0 × 10^-8 |
| 15 | 7.5 × 10^11 | 1.5 × 10^-8 |
Use the table to calculate the approximate number of chloromethane molecules formed per photon absorbed for the middle irradiation time.
[2]
Explain why the value calculated in part (a) is greater than one.
[2]
State the type of step that ultimately stops an individual radical chain.
[1]
0
Question 39
A radical chlorination of methane produced minor termination products. The table shows selected peaks from gas chromatography-mass spectrometry of the organic products.
| Peak | Retention time / min | Selected m/z (rel. int. / %) |
|---|---|---|
| A | 1.10 | 30 (100), 29 (14), 15 (78) |
| B | 1.95 | 50 (100), 52 (33), 35 (38), 37 (12), 15 (16) |
Deduce the termination product formed by combination of two methyl radicals.
[1]
Write the equation for this termination step.
[1]
Deduce the radical pair that combines to form chloromethane in a termination step.
[1]
Explain why termination products are usually minor products in a successful chain reaction.
[1]
0
Question 40
Ethane reacts with chlorine in the presence of ultraviolet light to form chloroethane and other products.
Consider the propagation steps in the formation of chloroethane.
I.
Write the equation for the propagation step in which a chlorine radical reacts with ethane.
[2]
II.
Write the equation for the propagation step in which the ethyl radical forms chloroethane.
[2]
Explain why these two steps allow the reaction to continue as a chain reaction.
[2]
0
Question 41
Pentane undergoes radical bromination under suitable conditions. A mixture of monobrominated and more highly brominated products is obtained.
Monobromination of pentane can occur at different positions in the carbon chain.
I.
State the conditions required to initiate radical bromination.
[1]
II.
Explain why pentane is relatively unreactive under ordinary conditions.
[2]
Discuss why radical bromination of pentane gives a mixture rather than a single pure product.
[4]
0
Question 42
A chemist proposes preparing chloromethane from methane and chlorine using ultraviolet light. The desired overall reaction is .
The reaction proceeds by a radical chain mechanism.
I.
Write the initiation step.
[1]
II.
Write one termination step that forms a product other than chloromethane.
[2]
Evaluate the suitability of this method for preparing a pure sample of chloromethane.
[4]
0
Question 43
Chlorofluorocarbons, CFCs, can release chlorine radicals in the stratosphere. A simplified representation is under ultraviolet radiation.
The release of involves homolytic fission.
I.
State what is meant by a chlorine radical.
[1]
II.
Explain why the dot must be shown in the formula of a radical intermediate.
[2]
Discuss why CFCs release chlorine radicals more readily than fluorine radicals under atmospheric ultraviolet radiation.
[4]
0
Question 44
Chlorine radicals in the stratosphere can react with ozone, , but they typically do not break down oxygen, , in the same way.
Consider the bonding in and in relation to radical attack.
I.
Suggest what the different behaviour of and indicates about their relative resistance to radical attack.
[1]
II.
Explain this difference in terms of bond strength.
[2]
Evaluate the significance of chlorine radicals being regenerated during radical reactions in the atmosphere.
[3]
0
Question 45
Fluorine, chlorine, bromine and iodine show different suitability for radical substitution reactions with alkanes.
Chlorine and bromine are commonly used in controlled radical substitution of alkanes.
I.
State the initiation equation for bromination.
[1]
II.
State the type of reaction by which an alkane forms a bromoalkane.
[1]
Compare and contrast the suitability of fluorine, chlorine, bromine and iodine for radical substitution with alkanes.
[4]
0
Question 46
The radical chlorination of propane gives a mixture of organic products. Two different propyl radicals can be formed during propagation.
Formation of different radicals leads to different substitution products.
I.
Write the formula of the radical formed when a terminal hydrogen atom is removed from propane.
[1]
II.
Write the formula of the radical formed when a hydrogen atom is removed from the middle carbon of propane.
[1]
III.
State the names of the two monochlorinated products formed from these radicals.
[1]
Discuss why the product mixture from propane chlorination is difficult to control.
[4]
0
Question 47
A reaction vessel contains methane and chlorine. The reaction is exposed to ultraviolet light for a short time and then kept in the dark. The following reactions may occur:
The two equations shown are propagation steps.
I.
Explain why each equation is classified as a propagation step.
[2]
II.
Write two possible termination equations for this system.
[2]
Evaluate the effect of turning off the ultraviolet light after the reaction has started.
[4]
0
Question 48
A student describes alkanes as “unreactive compounds”. Another student states that alkanes are thermodynamically unstable because they burn readily once ignited.
The reactivity of alkanes depends on the reaction conditions.
I.
Explain why alkanes are described as kinetically stable under ordinary conditions.
[2]
II.
Explain why radical substitution provides a route to more reactive organic products.
[2]
Discuss the statement that alkanes are kinetically stable but thermodynamically unstable, using combustion and radical substitution as examples.
[4]
0
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