Alkenes

1. Overview

Alkenes are a homologous series of unsaturated hydrocarbons that are significantly more reactive than alkanes due to the presence of a carbon-carbon double bond ($C=C$). They are essential raw materials in the chemical industry, used primarily to produce plastics (polymers) and alcohols.

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Key Definitions

• Alkene: A homologous series of unsaturated hydrocarbons containing at least one carbon-carbon double bond.
• Unsaturated: A molecule containing one or more double (or triple) covalent bonds between carbon atoms.
• Hydrocarbon: A compound consisting only of carbon and hydrogen atoms.
• Cracking: The thermal decomposition of long-chain alkanes into shorter, more useful alkanes and alkenes.
• Addition Reaction: A chemical reaction where two or more substances combine to form a single product.

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Core Content

Bonding and Structure

• Alkenes contain a double covalent bond between two carbon atoms ($C=C$).
• They follow the general formula: $C_nH_{2n}$.
• Because they contain a double bond, they are described as unsaturated (they do not have the maximum possible number of hydrogen atoms).

Manufacture of Alkenes: Cracking

Large alkane molecules obtained from crude oil are often in low demand. Cracking breaks these into smaller, high-demand molecules.

• Conditions: High temperature (approx. 600–700°C) and a catalyst (silica/silicon dioxide, $SiO_2$, or alumina/aluminum oxide, $Al_2O_3$).
• Products: A shorter-chain alkane + an alkene (or sometimes an alkene + hydrogen).

Example: Cracking of Decane

• Word Equation: Decane → Octane + Ethene
• Symbol Equation: $C_{10}H_{22}(l) \rightarrow C_8H_{18}(l) + C_2H_4(g)$

Reasons for Cracking

1. To match supply with demand: Crude oil provides more long-chain alkanes than the market needs, and not enough short-chain alkanes (like gasoline/petrol).
2. To produce alkenes: Alkenes are needed to make plastics (polyethene) and other chemicals.
3. To produce hydrogen: Hydrogen gas is used in the Haber process to make ammonia.

Test for Unsaturation

To distinguish between an alkane (saturated) and an alkene (unsaturated), use aqueous bromine (bromine water).

• Procedure: Add bromine water to the sample and shake.
• Result for Alkenes: The orange/brown bromine water turns colourless (decolourises). This happens because bromine adds across the C=C double bond (an addition reaction).
• Result for Alkanes: The bromine water remains orange/brown. Alkanes have no double bond to react with.

This test works because the C=C double bond is the reactive part of an alkene. When bromine adds across it, the Br atoms become bonded to the carbon atoms, and the coloured Br₂ molecules are used up. This is why the colour disappears.

Types of addition reactions alkenes undergo:

• + Bromine ($Br_2$) → dibromoalkane (the test above)
• + Hydrogen ($H_2$, with a nickel catalyst) → alkane (hydrogenation — this is how margarine is made from vegetable oils)
• + Steam ($H_2O$, with an acid catalyst) → alcohol (hydration — this is how ethanol is manufactured industrially)
• + Hydrogen halide (e.g. $HBr$) → halogenoalkane

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Extended Content (Extended Only)

Addition Reactions

In an addition reaction, the $C=C$ double bond "opens up" to form a single $C-C$ bond, allowing new atoms to bond to the carbon atoms. Only one product is formed.

(a) Reaction with Bromine ($Br_2$)

• Word Equation: Ethene + Bromine → 1,2-dibromoethane
• Symbol Equation: $C_2H_4(g) + Br_2(aq) \rightarrow C_2H_4Br_2(l)$
• Structural Formula of Product: H H | | H–C – C–H | | Br Br

(b) Reaction with Hydrogen (Hydrogenation)

• Conditions: Nickel catalyst, temperature of 150°C.
• Word Equation: Ethene + Hydrogen → Ethane
• Symbol Equation: $C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)$
• Structural Formula of Product: H H | | H–C – C–H | | H H

(c) Reaction with Steam (Hydration)

• Conditions: Phosphoric acid ($H_3PO_4$) catalyst, 300°C, 60 atm pressure.
• Word Equation: Ethene + Steam → Ethanol
• Symbol Equation: $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$
• Structural Formula of Product: H H | | H–C – C–O–H | | H H

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Key Equations

Process	Equation	Key Symbols
General Formula	$C_nH_{2n}$	$n$ = number of carbon atoms
Cracking	$C_{15}H_{32}(l) \rightarrow C_{13}H_{28}(l) + C_2H_4(g)$	Alkane → Alkane + Alkene
Bromination	$C_2H_4(g) + Br_2(aq) \rightarrow C_2H_4Br_2(l)$	1,2-dibromoethane (colorless)
Hydrogenation	$C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)$	Nickel catalyst required
Hydration	$C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$	$H_3PO_4$ catalyst required

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Common Mistakes to Avoid

• ❌ Wrong: Describing the bromine water test result as "clear."
• ✅ Right: Use the word "colorless." (A solution can be clear but still have a color, like tea).
• ❌ Wrong: Forgetting to show the double bond when drawing an alkene.
• ✅ Right: Always ensure the $C=C$ bond is visible in alkene structural formulas.
• ❌ Wrong: Keeping the double bond in the product of an addition reaction.
• ✅ Right: The $C=C$ bond always becomes a $C-C$ single bond after an addition reaction.

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Exam Tips

• Command Words: When asked to "describe" the test for an alkene, you must give the reagent (bromine water) AND the observation (orange to colorless).
• Unknowns: In questions involving an "unknown hydrocarbon," look for the mention of bromine water. If it decolorizes, the unknown is an alkene.
• Real-world Contexts: Cracking is frequently tested in the context of the oil industry and the economic need to produce petrol for cars and plastics for packaging.
• Displaying Structures: When drawing the product of hydration (ethanol), ensure the bond goes from the Carbon to the Oxygen (C—O—H), not the Hydrogen (C—H—O).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [6 marks]

Question:

But-1-ene is an alkene with the formula $C_4H_8$. It undergoes addition reactions.

(a) State what is meant by the term unsaturated hydrocarbon. [2]

(b) Draw the displayed formula of the product formed when but-1-ene reacts with bromine. [2]

(c) State the observation you would make when but-1-ene reacts with bromine water. [2]

Worked Solution:

(a)

1. Definition of hydrocarbon: A hydrocarbon is a compound containing carbon and hydrogen only.
2. Definition of unsaturated: Unsaturated means containing a carbon-carbon double or triple bond.

How to earn full marks:

• Must mention both carbon and hydrogen as the only elements present.
• Must mention either a double or triple carbon-carbon bond.

(b)

1. Draw the displayed formula:

How to earn full marks:

• Correct displayed formula showing all atoms and bonds.
• Correct addition of bromine atoms to the first and second carbon atoms.

(c)

1. Description of the observation: The bromine water changes from brown/orange to colourless.

How to earn full marks:

• Must state the initial and final colours of the bromine water clearly.
• Accept "decolourised" instead of "colourless".

Common Pitfall: Many students forget to include both the initial and final colours when describing the reaction of bromine water with an alkene. Also, make sure you can draw displayed formulas correctly, showing all bonds.

Exam-Style Question 2 — Short Answer [5 marks]

Question:

Ethene ($C_2H_4$) can be converted into ethanol ($C_2H_5OH$) in the presence of a catalyst.

(a) State the type of reaction that converts ethene to ethanol. [1]

(b) State two conditions required for this reaction to occur. [2]

(c) Write the balanced chemical equation for the complete combustion of ethanol. [2]

Worked Solution:

(a)

1. Type of reaction: Addition reaction / hydration.

How to earn full marks:

• Accept either "addition" or "hydration".

(b)

1. State the first condition: High temperature / between 250°C and 350°C.
2. State the second condition: High pressure / between 60 atm and 70 atm / presence of a catalyst.

How to earn full marks:

• Any two correct conditions.
• Must specify a value if stating a temperature or pressure.
• Accept phosphoric acid as a catalyst.

(c)

1. Write the balanced equation: $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)$

How to earn full marks:

• Correct formulae for all reactants and products.
• Correct balancing of the equation.
• State symbols are not required.

Common Pitfall: Students often forget the specific conditions (temperature, pressure, catalyst) needed for the hydration of ethene. Also, double-check that your combustion equations are balanced correctly.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Decane ($C_{10}H_{22}$) is a long-chain alkane. It can be cracked to produce smaller, more useful hydrocarbons.

(a) Define the term cracking. [2]

(b) State two reasons why cracking is important in the oil industry. [2]

(c) When decane is cracked, one possible product is propene ($C_3H_6$).

(i) Draw the structural formula of propene. [1]

(ii) Describe a test to distinguish between propene and propane, including the expected observations. [3]

Worked Solution:

(a)

1. Definition of cracking: Cracking is the breaking down of long-chain hydrocarbons into shorter-chain hydrocarbons.
2. Using heat and catalyst: Using heat and/or a catalyst.

How to earn full marks:

• Must mention breaking down of long-chain hydrocarbons.
• Must mention formation of shorter-chain hydrocarbons.
• Must mention the use of heat and/or a catalyst.

(b)

1. Reason 1: To produce smaller, more useful hydrocarbons.
2. Reason 2: To produce alkenes (for polymerisation).

How to earn full marks:

• Any two correct reasons.
• Accept answers relating to matching supply with demand.

(c)(i)

1. Structural formula: $CH_3CH=CH_2$

How to earn full marks:

• Correct structural formula showing the double bond.

(c)(ii)

1. Add bromine water: Add bromine water to both propene and propane.
2. Observation with propene: The bromine water will decolourise with propene.
3. Observation with propane: The bromine water will remain brown/orange with propane.

How to earn full marks:

• Must state that bromine water is added.
• Must state the observation for propene (decolourisation).
• Must state the observation for propane (no change).

Common Pitfall: Make sure you know the definition of cracking, including the conditions required. When describing the bromine water test, be sure to state what you would observe with BOTH the alkene and the alkane.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Ethene can be polymerised to form poly(ethene).

(a) Define the term polymerisation. [2]

(b) State two conditions required for the polymerisation of ethene. [2]

(c) Describe the process of addition polymerisation of ethene to form poly(ethene). Include the following in your answer: * the structure of the monomer * the repeating unit of the polymer * the type of bond formed between the monomers [5]

Worked Solution:

(a)

1. Definition of polymerisation: Polymerisation is the joining of many small molecules (monomers).
2. To form a large molecule: To form a large molecule (polymer).

How to earn full marks:

• Must mention joining of small molecules.
• Must mention formation of a large molecule.

(b)

1. Condition 1: High temperature (150-300°C).
2. Condition 2: High pressure (1000-2000 atm) / presence of a catalyst.

How to earn full marks:

• Any two correct conditions.
• Must specify a value if stating a temperature or pressure.

(c)

1. Monomer structure: The monomer is ethene, $CH_2=CH_2$
2. Repeating unit:
   📊The repeating unit of poly(ethene) with the double bond broken and a bond extending from each carbon atom. The unit should be enclosed in brackets with an 'n' subscript outside the brackets. The structure should show two carbon atoms each bonded to two hydrogen atoms, with single bonds connecting the carbon atoms in the chain.
3. Description of the process: Many ethene molecules join together.
4. Type of bond: Covalent bonds form between the carbon atoms.
5. Breaking of the double bond: The double bond in ethene breaks.

How to earn full marks:

• Correct monomer structure or formula.
• Correct repeating unit, including brackets and 'n'.
• Mention that many ethene molecules join together.
• Mention that covalent bonds are formed.
• Mention the breaking of the double bond.

Common Pitfall: Be careful when drawing the repeating unit of the polymer. Remember to break the double bond in the monomer and show the bonds extending from the carbon atoms to indicate the continuation of the chain. Also, remember to include the 'n' outside the brackets.