Define hydrogenation .

Hydrogenation is the addition reaction of an alkene with hydrogen gas in the presence of a finely divided Pt or Ni catalyst to produce an alkane .

What conditions are required for the hydrogenation of an alkene to form an alkane?

Hydrogen gas, heat, and a finely divided Pt or Ni catalyst are required. The catalyst is finely divided to increase its surface area and raise the rate of reaction.

True or False? Hydrogenation of an alkene is an endothermic reaction.

False. Hydrogenation is an exothermic reaction. Cracking, by contrast, is endothermic.

In the manufacture of margarine, vegetable oil is partially .......... ed so that C=C double bonds are reduced, making the hydrocarbon chains straighter and raising the .......... .

In the manufacture of margarine, vegetable oil is partially hydrogenat ed so that C=C double bonds are reduced, making the hydrocarbon chains straighter and raising the melting point .

What type of molecules are produced when a large alkane is cracked?

Cracking produces a smaller alkane and an alkene . For example, decane (C 10 H 22 ) can be cracked to give octane (C 8 H 18 ) and ethene (C 2 H 4 ).

True or False? The cracking chamber contains oxygen to combust the hydrocarbon and release energy.

False. The chamber contains no oxygen . Oxygen would cause the hydrocarbon to combust to water and carbon dioxide rather than cracking into smaller molecules.

In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over an .......... catalyst to break them into smaller molecules.

In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over an aluminium oxide (Al 2 O 3 ) catalyst to break them into smaller molecules.

Cracking is the process in which large, less useful hydrocarbon molecules from crude oil are broken down into smaller, more useful alkane and alkene molecules using heat and an Al 2 O 3 catalyst.

Why is the Pt or Ni catalyst used in hydrogenation finely divided?

The catalyst is finely divided to increase its surface area , which increases the overall rate of the hydrogenation reaction.

What products are formed during the complete combustion of an alkane?

Complete combustion of an alkane produces carbon dioxide and water . All the carbon and hydrogen are fully oxidised. This requires a plentiful supply of oxygen.

True or False? Carbon monoxide is produced during the complete combustion of an alkane.

False. Carbon monoxide is produced during incomplete combustion, which occurs when the oxygen supply is limited. Complete combustion produces carbon dioxide.

Define incomplete combustion .

Incomplete combustion is the burning of a hydrocarbon in a limited supply of oxygen, where carbon is only partially oxidised to form carbon monoxide rather than carbon dioxide.

Carbon monoxide is toxic because it binds to .......... in blood, preventing it from carrying .......... around the body.

Carbon monoxide is toxic because it binds to haemoglobin in blood, preventing it from carrying oxygen around the body.

Why is carbon monoxide particularly dangerous as a toxic gas?

Carbon monoxide is odourless and therefore cannot be detected by smell. It binds to haemoglobin in the blood, preventing oxygen transport, and can cause dizziness, loss of consciousness and death.

True or False? Ultraviolet light is required to initiate free-radical substitution of alkanes.

True. Alkanes are very unreactive and UV light provides the energy needed to break the halogen bond (Cl–Cl or Br–Br) in the initiation step.

In free-radical substitution, a hydrogen atom on an alkane is substituted by a .......... The three steps of the mechanism are .......... , propagation and termination.

In free-radical substitution, a hydrogen atom on an alkane is substituted by a halogen . The three steps of the mechanism are initiation , propagation and termination.

Define termination step .

A termination step is the final stage of free-radical substitution in which two radicals collide and combine, ending the chain reaction.

What happens in the initiation step of free-radical substitution?

The halogen–halogen bond (e.g. Cl–Cl or Br–Br) is broken by UV energy to produce two halogen radicals . These radicals then drive the propagation steps of the chain reaction.

Define homolytic fission .

Homolytic fission is the breaking of a covalent bond in which each atom takes one electron from the shared pair, producing two radicals .

What is produced in the initiation step of free-radical substitution of chlorine with an alkane?

UV energy causes homolytic fission of the Cl–Cl bond, producing two chlorine radicals (Cl • ). Each chlorine atom takes one electron from the bond.

True or False? In the propagation step, the chlorine radical is consumed and not regenerated.

False. The chlorine radical is regenerated in the second propagation step when an alkyl radical reacts with Cl 2 , producing the halogenoalkane and a new Cl • radical.

In the termination step of free-radical substitution, two .......... collide and combine to form a single .......... molecule, ending the chain reaction.

In the termination step of free-radical substitution, two radicals collide and combine to form a single unreactive molecule, ending the chain reaction.

Why is free-radical substitution not a good method for preparing a specific halogenoalkane?

A mixture of substitution products is formed. If excess halogen is present, all hydrogen atoms can eventually be substituted, giving a range of products rather than one specific compound.

True or False? Half-headed (fish hook) arrows are used in free-radical mechanisms to show the movement of a single electron.

True. Half-headed arrows show the movement of one electron . Double-headed (curly) arrows show the movement of an electron pair and are used in ionic mechanisms.

In the chlorination of methane, complete substitution produces .......... . The intermediate after three substitutions is .......... (CHCl 3 ).

In the chlorination of methane, complete substitution produces CCl 4 . The intermediate after three substitutions is trichloromethane (CHCl 3 ).

Define propagation step .

A propagation step is the repeating stage of free-radical substitution in which a radical reacts with the alkane to form an alkyl radical and HX, then the alkyl radical reacts with X 2 to regenerate the halogen radical .

In the termination step, give one example of a product other than the expected halogenoalkane that can form.

Two alkyl radicals can combine to form a longer alkane . For example, two ethyl radicals ( • CH 2 CH 3 ) combine to give butane (CH 3 CH 2 CH 2 CH 3 ).

Crude oil is a mixture of hydrocarbons — including alkanes , cycloalkanes and arenes — extracted from the earth by drilling and refined by fractional distillation.

Why are heavier fractions from fractional distillation subjected to cracking?

The smaller hydrocarbon fractions such as gasoline are in high demand, but heavier fractions are in excess. Cracking breaks these down into smaller, more useful alkanes and alkenes with lower M r values.

In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over .......... catalyst. Oxygen is excluded to prevent .......... of the hydrocarbons.

In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over aluminium oxide (Al 2 O 3 ) catalyst. Oxygen is excluded to prevent combustion of the hydrocarbons.

True or False? Fractional distillation of crude oil separates hydrocarbons based on their molecular mass only.

False. Fractional distillation separates hydrocarbons based on their boiling points . Boiling point is related to molecular mass, but it is boiling point that determines the separation into fractions.

Why are the low-molecular-mass alkenes formed in cracking particularly valuable to the chemical industry?

Alkenes are more reactive than alkanes due to their C=C double bond. This makes them useful feedstock — for example, as monomers in polymerisation reactions and as starting compounds for many other syntheses.

True or False? Cracking of decane (C 10 H 22 ) can produce octane and ethene.

True. Decane (C 10 H 22 ) can be cracked to give octane (C 8 H 18 ) and ethene (C 2 H 4 ) — a smaller alkane and an alkene.

A feedstock is a raw material used as the starting compound for an industrial chemical process. Alkenes from cracking are valuable feedstocks for polymerisation and other synthesis reactions.

Crude oil contains .......... , cycloalkanes and .......... (compounds with a benzene ring).

Crude oil contains alkanes , cycloalkanes and arenes (compounds with a benzene ring).

What two types of organic product are always formed when a large alkane is cracked?

Cracking always produces a smaller alkane and an alkene . The alkane fraction makes a useful fuel and the alkene is reactive enough to be used as a chemical feedstock.

Give two reasons why alkanes are chemically unreactive.

Alkanes have strong C–C and C–H covalent bonds that require a lot of energy to break. Alkanes are nonpolar molecules, so they do not react with polar reagents, nucleophiles or electrophiles.

True or False? Alkanes are polar molecules because carbon is more electronegative than hydrogen.

False. Although carbon is slightly more electronegative than hydrogen (difference of only 0.4 on the Pauling scale), alkanes are considered nonpolar molecules. There are no significant partial charges on the C–H or C–C bonds.

A nucleophile is a species that is attracted to electron-deficient regions. Alkanes do not react with nucleophiles because they have no electron-deficient areas.

Alkanes do not react with polar reagents because they are .......... molecules and have no .......... or electrophilic sites.

Alkanes do not react with polar reagents because they are nonpolar molecules and have no nucleophilic or electrophilic sites.

True or False? Alkanes can react with electrophiles because they have electron-rich areas in the C–H bonds.

False. Alkanes lack electron-rich areas to attract electrophiles . They are nonpolar, so neither electrophiles nor nucleophiles are drawn to them.

What are the only two types of reaction that alkanes typically undergo?

Alkanes undergo combustion (complete and incomplete) and free-radical substitution with halogens. Their strong, nonpolar bonds make them resistant to all other reaction types.

The difference in electronegativity between carbon and hydrogen is only .......... on the Pauling scale, which means C–H bonds are essentially .......... .

The difference in electronegativity between carbon and hydrogen is only 0.4 on the Pauling scale, which means C–H bonds are essentially nonpolar .

Define electrophile .

An electrophile is a species that is attracted to electron-rich regions. Alkanes do not react with electrophiles because they have no areas of high electron density.

Why do strong C–C and C–H bonds reduce the reactivity of alkanes?

A large amount of energy is required to break these strong bonds. Because the activation energy is high and there are no weak or polar bonds to facilitate attack, alkanes rarely react under mild conditions.

Define photochemical smog .

Photochemical smog is an atmospheric pollutant formed when volatile organic compounds (VOCs) react with oxides of nitrogen in sunlight to produce peroxyacetyl nitrate (PAN), which is harmful to the lungs, eyes and plant life.

Why is carbon monoxide particularly dangerous as an exhaust gas pollutant?

Carbon monoxide is colourless and odourless , so it cannot be detected without a sensor. It binds to haemoglobin with very high affinity (much higher than O 2 ), preventing oxygen transport, and can cause dizziness, loss of consciousness and death.

True or False? Nitrogen is reactive enough to combine with oxygen under normal atmospheric conditions.

False. Nitrogen is normally too unreactive to oxidise under atmospheric conditions. It only reacts with oxygen inside a car engine where the high temperatures and pressures are sufficient to drive the reaction.

Oxides of nitrogen from car exhaust dissolve in water and react with oxygen to form .......... , a cause of .......... rain.

Oxides of nitrogen from car exhaust dissolve in water and react with oxygen to form nitric acid , a cause of acid rain.

How does a catalytic converter reduce pollutants from car exhaust fumes?

Precious metals (e.g. platinum ) coated on a honeycomb structure provide a large surface area. CO is oxidised to CO 2 and NO/NO 2 is reduced to N 2 . Unburnt hydrocarbons (VOCs) are also oxidised to CO 2 and H 2 O.

True or False? Carbon dioxide is a toxic gas and a pollutant that causes direct harm to human health.

False. CO 2 is not toxic but it is still a pollutant that contributes to global warming and climate change. It is CO (carbon monoxide) that is directly toxic.

A LevelChemistryCambridge (CIE)Flashcards14. HydrocarbonsAlkanes

Alkanes (Cambridge (CIE) A Level Chemistry): Flashcards

Exam code: 9701

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Define hydrogenation.

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Define hydrogenation.
Hydrogenation is the addition reaction of an alkene with hydrogen gas in the presence of a finely divided Pt or Ni catalyst to produce an alkane.
What conditions are required for the hydrogenation of an alkene to form an alkane?
Hydrogen gas, heat, and a finely divided Pt or Ni catalyst are required. The catalyst is finely divided to increase its surface area and raise the rate of reaction.
True or False?
Hydrogenation of an alkene is an endothermic reaction.
False. Hydrogenation is an exothermic reaction. Cracking, by contrast, is endothermic.
In the manufacture of margarine, vegetable oil is partially ..........ed so that C=C double bonds are reduced, making the hydrocarbon chains straighter and raising the ...........
In the manufacture of margarine, vegetable oil is partially hydrogenated so that C=C double bonds are reduced, making the hydrocarbon chains straighter and raising the melting point.
What type of molecules are produced when a large alkane is cracked?
Cracking produces a smaller alkane and an alkene. For example, decane (C₁₀H₂₂) can be cracked to give octane (C₈H₁₈) and ethene (C₂H₄).
True or False?
The cracking chamber contains oxygen to combust the hydrocarbon and release energy.
False. The chamber contains no oxygen. Oxygen would cause the hydrocarbon to combust to water and carbon dioxide rather than cracking into smaller molecules.
In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over an .......... catalyst to break them into smaller molecules.
In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over an aluminium oxide (Al₂O₃) catalyst to break them into smaller molecules.
Define cracking.
Cracking is the process in which large, less useful hydrocarbon molecules from crude oil are broken down into smaller, more useful alkane and alkene molecules using heat and an Al₂O₃ catalyst.
Why is the Pt or Ni catalyst used in hydrogenation finely divided?
The catalyst is finely divided to increase its surface area, which increases the overall rate of the hydrogenation reaction.
What products are formed during the complete combustion of an alkane?
Complete combustion of an alkane produces carbon dioxide and water. All the carbon and hydrogen are fully oxidised. This requires a plentiful supply of oxygen.
True or False?
Carbon monoxide is produced during the complete combustion of an alkane.
False. Carbon monoxide is produced during incomplete combustion, which occurs when the oxygen supply is limited. Complete combustion produces carbon dioxide.
Define incomplete combustion.
Incomplete combustion is the burning of a hydrocarbon in a limited supply of oxygen, where carbon is only partially oxidised to form carbon monoxide rather than carbon dioxide.
Carbon monoxide is toxic because it binds to ..........in blood, preventing it from carrying .......... around the body.
Carbon monoxide is toxic because it binds to haemoglobin in blood, preventing it from carrying oxygen around the body.
Why is carbon monoxide particularly dangerous as a toxic gas?
Carbon monoxide is odourless and therefore cannot be detected by smell. It binds to haemoglobin in the blood, preventing oxygen transport, and can cause dizziness, loss of consciousness and death.
True or False?
Ultraviolet light is required to initiate free-radical substitution of alkanes.
True. Alkanes are very unreactive and UV light provides the energy needed to break the halogen bond (Cl–Cl or Br–Br) in the initiation step.
In free-radical substitution, a hydrogen atom on an alkane is substituted by a .......... The three steps of the mechanism are .........., propagation and termination.
In free-radical substitution, a hydrogen atom on an alkane is substituted by a halogen. The three steps of the mechanism are initiation, propagation and termination.
Define termination step.
A termination step is the final stage of free-radical substitution in which two radicals collide and combine, ending the chain reaction.
What happens in the initiation step of free-radical substitution?
The halogen–halogen bond (e.g. Cl–Cl or Br–Br) is broken by UV energy to produce two halogen radicals. These radicals then drive the propagation steps of the chain reaction.
Define homolytic fission.
Homolytic fission is the breaking of a covalent bond in which each atom takes one electron from the shared pair, producing two radicals.
What is produced in the initiation step of free-radical substitution of chlorine with an alkane?
UV energy causes homolytic fission of the Cl–Cl bond, producing two chlorine radicals (Cl^•). Each chlorine atom takes one electron from the bond.
True or False?
In the propagation step, the chlorine radical is consumed and not regenerated.
False. The chlorine radical is regenerated in the second propagation step when an alkyl radical reacts with Cl₂, producing the halogenoalkane and a new Cl^• radical.
In the termination step of free-radical substitution, two .......... collide and combine to form a single .......... molecule, ending the chain reaction.
In the termination step of free-radical substitution, two radicals collide and combine to form a single unreactive molecule, ending the chain reaction.
Why is free-radical substitution not a good method for preparing a specific halogenoalkane?
A mixture of substitution products is formed. If excess halogen is present, all hydrogen atoms can eventually be substituted, giving a range of products rather than one specific compound.
True or False?
Half-headed (fish hook) arrows are used in free-radical mechanisms to show the movement of a single electron.
True. Half-headed arrows show the movement of one electron. Double-headed (curly) arrows show the movement of an electron pair and are used in ionic mechanisms.
In the chlorination of methane, complete substitution produces ........... The intermediate after three substitutions is .......... (CHCl₃).
In the chlorination of methane, complete substitution produces CCl₄. The intermediate after three substitutions is trichloromethane (CHCl₃).
Define propagation step.
A propagation step is the repeating stage of free-radical substitution in which a radical reacts with the alkane to form an alkyl radical and HX, then the alkyl radical reacts with X₂ to regenerate the halogen radical.
In the termination step, give one example of a product other than the expected halogenoalkane that can form.
Two alkyl radicals can combine to form a longer alkane. For example, two ethyl radicals (^•CH₂CH₃) combine to give butane (CH₃CH₂CH₂CH₃).
Define crude oil.
Crude oil is a mixture of hydrocarbons — including alkanes, cycloalkanes and arenes — extracted from the earth by drilling and refined by fractional distillation.
Why are heavier fractions from fractional distillation subjected to cracking?
The smaller hydrocarbon fractions such as gasoline are in high demand, but heavier fractions are in excess. Cracking breaks these down into smaller, more useful alkanes and alkenes with lower M_r values.
In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over .......... catalyst. Oxygen is excluded to prevent .......... of the hydrocarbons.
In cracking, large hydrocarbon molecules are heated in a steel chamber and passed over aluminium oxide (Al₂O₃) catalyst. Oxygen is excluded to prevent combustion of the hydrocarbons.
True or False?
Fractional distillation of crude oil separates hydrocarbons based on their molecular mass only.
False. Fractional distillation separates hydrocarbons based on their boiling points. Boiling point is related to molecular mass, but it is boiling point that determines the separation into fractions.
Why are the low-molecular-mass alkenes formed in cracking particularly valuable to the chemical industry?
Alkenes are more reactive than alkanes due to their C=C double bond. This makes them useful feedstock — for example, as monomers in polymerisation reactions and as starting compounds for many other syntheses.
True or False?
Cracking of decane (C₁₀H₂₂) can produce octane and ethene.
True. Decane (C₁₀H₂₂) can be cracked to give octane (C₈H₁₈) and ethene (C₂H₄) — a smaller alkane and an alkene.
Define feedstock.
A feedstock is a raw material used as the starting compound for an industrial chemical process. Alkenes from cracking are valuable feedstocks for polymerisation and other synthesis reactions.
Crude oil contains .........., cycloalkanes and .......... (compounds with a benzene ring).
Crude oil contains alkanes, cycloalkanes and arenes (compounds with a benzene ring).
What two types of organic product are always formed when a large alkane is cracked?
Cracking always produces a smaller alkane and an alkene. The alkane fraction makes a useful fuel and the alkene is reactive enough to be used as a chemical feedstock.
Give two reasons why alkanes are chemically unreactive.
1. Alkanes have strong C–C and C–H covalent bonds that require a lot of energy to break.
2. Alkanes are nonpolar molecules, so they do not react with polar reagents, nucleophiles or electrophiles.
True or False?
Alkanes are polar molecules because carbon is more electronegative than hydrogen.
False. Although carbon is slightly more electronegative than hydrogen (difference of only 0.4 on the Pauling scale), alkanes are considered nonpolar molecules. There are no significant partial charges on the C–H or C–C bonds.
Define nucleophile.
A nucleophile is a species that is attracted to electron-deficient regions. Alkanes do not react with nucleophiles because they have no electron-deficient areas.
Alkanes do not react with polar reagents because they are .......... molecules and have no .......... or electrophilic sites.
Alkanes do not react with polar reagents because they are nonpolar molecules and have no nucleophilic or electrophilic sites.
True or False?
Alkanes can react with electrophiles because they have electron-rich areas in the C–H bonds.
False. Alkanes lack electron-rich areas to attract electrophiles. They are nonpolar, so neither electrophiles nor nucleophiles are drawn to them.
What are the only two types of reaction that alkanes typically undergo?
Alkanes undergo combustion (complete and incomplete) and free-radical substitution with halogens. Their strong, nonpolar bonds make them resistant to all other reaction types.
The difference in electronegativity between carbon and hydrogen is only .......... on the Pauling scale, which means C–H bonds are essentially .......... .
The difference in electronegativity between carbon and hydrogen is only 0.4 on the Pauling scale, which means C–H bonds are essentially nonpolar.
Define electrophile.
An electrophile is a species that is attracted to electron-rich regions. Alkanes do not react with electrophiles because they have no areas of high electron density.
Why do strong C–C and C–H bonds reduce the reactivity of alkanes?
A large amount of energy is required to break these strong bonds. Because the activation energy is high and there are no weak or polar bonds to facilitate attack, alkanes rarely react under mild conditions.
Define photochemical smog.
Photochemical smog is an atmospheric pollutant formed when volatile organic compounds (VOCs) react with oxides of nitrogen in sunlight to produce peroxyacetyl nitrate (PAN), which is harmful to the lungs, eyes and plant life.
Why is carbon monoxide particularly dangerous as an exhaust gas pollutant?
Carbon monoxide is colourless and odourless, so it cannot be detected without a sensor. It binds to haemoglobin with very high affinity (much higher than O₂), preventing oxygen transport, and can cause dizziness, loss of consciousness and death.
True or False?
Nitrogen is reactive enough to combine with oxygen under normal atmospheric conditions.
False. Nitrogen is normally too unreactive to oxidise under atmospheric conditions. It only reacts with oxygen inside a car engine where the high temperatures and pressures are sufficient to drive the reaction.
Oxides of nitrogen from car exhaust dissolve in water and react with oxygen to form .........., a cause of .......... rain.
Oxides of nitrogen from car exhaust dissolve in water and react with oxygen to form nitric acid, a cause of acid rain.
How does a catalytic converter reduce pollutants from car exhaust fumes?
Precious metals (e.g. platinum) coated on a honeycomb structure provide a large surface area. CO is oxidised to CO₂ and NO/NO₂ is reduced to N₂. Unburnt hydrocarbons (VOCs) are also oxidised to CO₂ and H₂O.
True or False?
Carbon dioxide is a toxic gas and a pollutant that causes direct harm to human health.
False. CO₂ is not toxic but it is still a pollutant that contributes to global warming and climate change. It is CO (carbon monoxide) that is directly toxic.
In a catalytic converter, the reaction 2CO + 2NO → .......... + N₂ simultaneously removes .......... and nitrogen monoxide.
In a catalytic converter, the reaction 2CO + 2NO → 2CO₂ + N₂ simultaneously removes carbon monoxide and nitrogen monoxide.
Define volatile organic compounds (VOCs).
Volatile organic compounds (VOCs) are unburnt hydrocarbons and their oxides released in car exhaust fumes. They react with oxides of nitrogen in sunlight to produce PAN, contributing to photochemical smog.
What are the three main types of pollutant found in car exhaust fumes and what is one environmental consequence of each?
1. Carbon monoxide (CO) — toxic; prevents oxygen transport in blood.
2. Oxides of nitrogen (NO/NO₂) — form nitric acid; cause acid rain.
3. VOCs — react with NOx to form PAN; contribute to photochemical smog.

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