An alkene is an unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C). The general formula of the alkenes is C n H 2n .

How is bromine water used to distinguish an alkene from an alkane?

A few drops of bromine water (orange) are added to the sample and shaken. With an alkene , the bromine water is decolourised (turns colourless) as bromine reacts with the C=C double bond. With an alkane , there is no colour change and the solution stays orange.

Alkenes are produced by .......... , which is the thermal decomposition of long-chain alkanes using a high temperature and a catalyst. (Extended Tier Only)

Alkenes are produced by cracking , which is the thermal decomposition of long-chain alkanes using a high temperature and a catalyst.

True or False? When bromine water decolourises with an alkene, the result is described as the solution turning clear.

False. The correct term is colourless , not clear. A solution can be clear (transparent) but still coloured.

Why are alkenes more reactive than alkanes?

Alkenes are more reactive because they contain a carbon-carbon double bond (C=C) that can open up and form new bonds. Alkanes only contain strong single bonds, which are harder to break.

Why is cracking important in the petroleum industry? (Extended Tier Only)

Cracking converts excess long-chain hydrocarbons into more useful short-chain molecules such as petrol (alkanes) and alkenes . Alkenes are used to make polymers and hydrogen is used in ammonia manufacture.

The cracking of decane can be represented as: C 10 H 22 → .......... + C 2 H 4 , showing that an alkane and an .......... are formed. (Extended Tier Only)

The cracking of decane can be represented as: C 10 H 22 → C 8 H 18 + C 2 H 4 , showing that an alkane and an alkene are formed.

True or False? Cracking requires a high temperature and an aluminium oxide or silicon dioxide catalyst. (Extended Tier Only)

True. Cracking requires a temperature of around 600-700 °C and a catalyst such as aluminium oxide (alumina) or silicon dioxide (silica) to break the carbon-carbon bonds in long-chain alkanes.

Give the names and molecular formulae of the first three alkenes.

Ethene : C 2 H 4 Propene : C 3 H 6 But-1-ene : C 4 H 8

Define addition reaction . (Extended Tier Only)

An addition reaction is a reaction in which atoms from a small molecule add across the C=C double bond of an alkene, forming only one product .

What type of compound is formed when bromine reacts with an alkene in an addition reaction? (Extended Tier Only)

A saturated compound is formed. The bromine atoms add across the C=C double bond, breaking the double bond and converting the alkene into a compound with only single carbon-carbon bonds.

When ethene reacts with hydrogen in an addition reaction, an .......... catalyst is needed and the product formed is .......... . (Extended Tier Only)

When ethene reacts with hydrogen in an addition reaction, a nickel catalyst is needed and the product formed is ethane .

True or False? In an addition reaction, two products are always formed. (Extended Tier Only)

False. In an addition reaction, only one product is formed. Atoms add across the C=C double bond and the double bond breaks to form a single product.

What product is formed when an alkene reacts with steam in an addition reaction, and what catalyst is required? (Extended Tier Only)

When an alkene reacts with steam, an alcohol is formed. This is also called a hydration reaction . An acid catalyst is required. For example, ethene + steam → ethanol.

Why do alkenes undergo addition reactions, whereas alkanes do not? (Extended Tier Only)

Alkenes contain a C=C double bond that can break open to form new bonds with other molecules. Alkanes contain only strong single covalent bonds, which do not open up to add other atoms in this way.

The addition of bromine to ethene decolourises bromine water from .......... to .......... , because bromine reacts with the C=C bond and is no longer .......... . (Extended Tier Only)

The addition of bromine to ethene decolourises bromine water from orange to colourless , because bromine reacts with the C=C bond and is no longer in solution .

True or False? The reaction of propene with hydrogen produces propane. (Extended Tier Only)

True. When propene (C 3 H 6 ) reacts with hydrogen (H 2 ) in the presence of a nickel catalyst, propane (C 3 H 8 ) is formed. Hydrogen adds across the C=C double bond.

Name the three addition reactions that alkenes can undergo and the product of each. (Extended Tier Only)

Alkene + bromine → saturated dibromoalkane Alkene + hydrogen (nickel catalyst) → alkane Alkene + steam (acid catalyst) → alcohol

IGCSEScienceCambridge (CIE)Combined ScienceChemistryFlashcards11. Organic ChemistryAlkenes

Alkenes (Cambridge (CIE) IGCSE Combined Science: Chemistry): Flashcards

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Cards in this collection (18)

Define alkene.
An alkene is an unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C). The general formula of the alkenes is C_nH_2n.
How is bromine water used to distinguish an alkene from an alkane?
A few drops of bromine water (orange) are added to the sample and shaken. With an alkene, the bromine water is decolourised (turns colourless) as bromine reacts with the C=C double bond. With an alkane, there is no colour change and the solution stays orange.
Alkenes are produced by .........., which is the thermal decomposition of long-chain alkanes using a high temperature and a catalyst. (Extended Tier Only)
Alkenes are produced by cracking, which is the thermal decomposition of long-chain alkanes using a high temperature and a catalyst.
True or False?
When bromine water decolourises with an alkene, the result is described as the solution turning clear.
False.
The correct term is colourless, not clear. A solution can be clear (transparent) but still coloured.
Why are alkenes more reactive than alkanes?
Alkenes are more reactive because they contain a carbon-carbon double bond (C=C) that can open up and form new bonds. Alkanes only contain strong single bonds, which are harder to break.
Why is cracking important in the petroleum industry? (Extended Tier Only)
Cracking converts excess long-chain hydrocarbons into more useful short-chain molecules such as petrol (alkanes) and alkenes. Alkenes are used to make polymers and hydrogen is used in ammonia manufacture.
The cracking of decane can be represented as: C₁₀H₂₂ → .......... + C₂H₄, showing that an alkane and an .......... are formed. (Extended Tier Only)
The cracking of decane can be represented as: C₁₀H₂₂ → C₈H₁₈ + C₂H₄, showing that an alkane and an alkene are formed.
True or False?
Cracking requires a high temperature and an aluminium oxide or silicon dioxide catalyst. (Extended Tier Only)
True.
Cracking requires a temperature of around 600-700 °C and a catalyst such as aluminium oxide (alumina) or silicon dioxide (silica) to break the carbon-carbon bonds in long-chain alkanes.
Give the names and molecular formulae of the first three alkenes.
1. Ethene: C₂H₄
2. Propene: C₃H₆
3. But-1-ene: C₄H₈
Define addition reaction. (Extended Tier Only)
An addition reaction is a reaction in which atoms from a small molecule add across the C=C double bond of an alkene, forming only one product.
What type of compound is formed when bromine reacts with an alkene in an addition reaction? (Extended Tier Only)
A saturated compound is formed. The bromine atoms add across the C=C double bond, breaking the double bond and converting the alkene into a compound with only single carbon-carbon bonds.
When ethene reacts with hydrogen in an addition reaction, an .......... catalyst is needed and the product formed is ........... (Extended Tier Only)
When ethene reacts with hydrogen in an addition reaction, a nickel catalyst is needed and the product formed is ethane.
True or False?
In an addition reaction, two products are always formed. (Extended Tier Only)
False.
In an addition reaction, only one product is formed. Atoms add across the C=C double bond and the double bond breaks to form a single product.
What product is formed when an alkene reacts with steam in an addition reaction, and what catalyst is required? (Extended Tier Only)
When an alkene reacts with steam, an alcohol is formed. This is also called a hydration reaction. An acid catalyst is required. For example, ethene + steam → ethanol.
Why do alkenes undergo addition reactions, whereas alkanes do not? (Extended Tier Only)
Alkenes contain a C=C double bond that can break open to form new bonds with other molecules. Alkanes contain only strong single covalent bonds, which do not open up to add other atoms in this way.
The addition of bromine to ethene decolourises bromine water from .......... to .........., because bromine reacts with the C=C bond and is no longer ........... (Extended Tier Only)
The addition of bromine to ethene decolourises bromine water from orange to colourless, because bromine reacts with the C=C bond and is no longer in solution.
True or False?
The reaction of propene with hydrogen produces propane. (Extended Tier Only)
True.
When propene (C₃H₆) reacts with hydrogen (H₂) in the presence of a nickel catalyst, propane (C₃H₈) is formed. Hydrogen adds across the C=C double bond.
Name the three addition reactions that alkenes can undergo and the product of each. (Extended Tier Only)
1. Alkene + bromine → saturated dibromoalkane
2. Alkene + hydrogen (nickel catalyst) → alkane
3. Alkene + steam (acid catalyst) → alcohol

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