Structure and Formulae of Alkenes, and Reactions of Alkenes [chemistry only]

SPEC 8462 • LESSON 7.04 • PAPER 2 • CHEMISTRY ONLY

Structure & Reactions of Alkenes

The C=C functional group, the first four alkenes, and three addition reactions: with hydrogen, water, and halogens. Spec 4.7.2.1 + 4.7.2.2. ⭐ Priority — chemistry-only content.

What This Lesson Covers

This is chemistry-only content — Combined Science students don't sit it. The trade-off: it's high-yield. Circle-the-C=C and bromine water explanation questions appear nearly every Paper 2.

CₙH₂ₙ

Alkene general formula

3

Addition reactions to know

~60%

Scored 1/1 on circle-C=C (Jun24 Q08.1)

Why This Matters

The Jun24 examiner report noted that "around 60% of students correctly identified the C=C bond" — meaning 40% lost a free mark on circle-the-functional-group, usually by including attached H atoms or only circling the double bond without the carbons. This lesson fixes the common errors precisely.

Key Definitions

Three terms anchor this whole topic. Every mark-scoring answer in 7.04 uses at least one of them.

Alkene

An unsaturated hydrocarbon with a C=C double bond. General formula CₙH₂ₙ — two fewer hydrogens than the corresponding alkane.

Functional Group

A specific arrangement of atoms in a molecule that gives it characteristic chemical properties. For alkenes, the functional group is the C=C double bond — this is what makes them more reactive than alkanes.

Addition Reaction

A reaction in which two or more atoms add across the C=C double bond, converting it to a single C–C bond. No atoms are lost — every atom in the reactants ends up in the single product.

The First Four Alkenes

For chemistry-only spec 4.7.2.1, you must know the names, formulae, and structures of the first four alkenes.

Ethene

C₂H₄

n = 2

Propene

C₃H₆

n = 3

Butene

C₄H₈

n = 4

Pentene

C₅H₁₀

n = 5

Displayed formulae · ethene (C₂H₄) and propene (C₃H₆)

Circle the C=C

One of the most reliable Paper 2 marks — and one of the most commonly missed. The Jun24 examiner report on Q08.1 was unusually specific about exactly what the circle must include.

"Around 60% of students correctly identified the C=C bond. For students who were not awarded the mark common mistakes were incorporating the 3 Hs and 1 O atom attached to the C=C group in their circle or also circling the C=O group. Others circled only the double bond and not the carbon atoms on either side." — AQA Examiner Report, P2 Jun 2024 Q08.1

The Rule · Three Things In, Nothing Else

Your circle must contain only three things: (1) the left carbon, (2) the right carbon, (3) the double bond between them — i.e. C=C. NO hydrogens. NO attached oxygens. NO other carbons in the chain.

Circle examples · propene with one correct and two incorrect circles

Context-specific trap

In Jun24 Q08.1, "Compound A" had both an alkene C=C AND an ester C=O. Some students circled the C=O instead of the C=C. If a question gives a molecule with multiple double bonds (alkene C=C, carbonyl C=O, ester, etc.), only the C=C between two carbons is the alkene functional group. C=O is the carbonyl group (a different functional group entirely).

Why Alkenes Are Reactive

Alkenes are more reactive than alkanes. The reason traces directly back to the C=C double bond.

Single vs Double Bond

An alkane has only single C–C and C–H bonds. To react, those strong single bonds must break — energetically expensive, so alkanes are unreactive. An alkene has a C=C double bond, which is a region of high electron density. The double bond can open up — letting two atoms add across without breaking the molecule apart.

Combustion · Two Modes

Spec verbatim:"Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons , but they tend to burn in air with smoky flames because of incomplete combustion."

So alkenes can undergo complete combustion (with plenty of O₂) → CO₂ + H₂O only. They just tend to do incomplete combustion in air → smoky flame, with some unburnt carbon (soot) and CO mixed in. Both forms are examinable.

Worked Example · Complete Combustion of Ethene

Apply the C → H → O method (Lesson 7.01):

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Check: C: 2 = 2 ✓ | H: 4 = 4 ✓ | O: 6 = 4 + 2 = 6 ✓. P2 Jun 2024 Q08.4 tested complete combustion of an alkene-containing compound (3-mark, spec ref 4.7.2.1) — so balanced complete-combustion equations are within scope.

EXAMINER TIP

"Smoky flame" can be used to distinguish alkenes from alkanes — but the AQA-preferred test is still bromine water (more reliable, observable colour change). If asked to distinguish, default to bromine water unless the question forces otherwise.

Addition Reactions · Overview

The C=C double bond opens up, and two atoms (or two groups) add across the bond — one to each carbon. The product has only single C–C bonds. The double bond is gone.

Addition · C=C opens, X-X adds across, leaving C–C with X on each side

Addition · Hydrogen (H₂)

Alkene + hydrogen → alkane. The reaction needs a nickel catalyst at warm temperatures. Industrially, this "hydrogenation" turns vegetable oils (containing C=C) into solid fats (saturated) — used in margarine production.

alkene + H₂ → alkane

Worked Example · Ethene

C₂H₄ + H₂ → C₂H₆

(ethene + hydrogen → ethane)

Conditions to Memorise

Catalyst: nickel (Ni). The H₂ molecule splits and adds across the C=C, converting it to C–C. Atom count check: C₂H₄ + H₂ has C: 2, H: 6 → ethane (C₂H₆) has C: 2, H: 6 ✓.

Real-World Application

Margarine manufacture: liquid vegetable oils (with C=C bonds) are hydrogenated → solid fats with single C–C bonds. The product has higher melting point (solid at room temp) and longer shelf life (single bonds are less prone to oxidation).

Drawing Displayed Formulae of Products

The spec explicitly requires students to "draw fully displayed structural formulae of the first four members of the alkenes and the products of their addition reactions with hydrogen, water, chlorine, bromine and iodine." Here's the method.

Worked example · drawing the displayed formula of ethane from ethene + H₂

The 3-Step Method

Step 1. Draw the alkene displayed formula with C=C. Step 2. Replace C=C with a single C–C bond. Step 3. Add one new atom (or group) to each carbon — H+H for hydrogenation, Br+Br for bromination, H+OH for hydration. Apply the same logic for propene + H₂ → propane, butene + Br₂ → 1,2-dibromobutane, etc.

Addition · Halogens (Cl₂, Br₂, I₂)

Alkene + halogen → dihaloalkane. No catalyst needed — alkenes react readily with halogens at room temperature. The spec mandates students can write addition equations with chlorine, bromine, and iodine (4.7.2.2).

alkene + X₂ → dihaloalkane

Three Worked Examples · Ethene + each halogen

C₂H₄ + Cl₂ → C₂H₄Cl₂

(ethene + chlorine → 1,2-dichloroethane)

C₂H₄ + Br₂ → C₂H₄Br₂

(ethene + bromine → 1,2-dibromoethane — this is the bromine water test)

C₂H₄ + I₂ → C₂H₄I₂

(ethene + iodine → 1,2-diiodoethane)

What's Happening (all three halogens)

The C=C double bond opens up. One halogen atom adds to each carbon — giving X–CH₂–CH₂–X. The pattern is identical for all three halogens: same reaction type, just different X. Atom check (Br₂): C: 2=2 ✓; H: 4=4 ✓; Br: 2=2 ✓.

Why Bromine Specifically?

All three halogens react the same way, but only bromine is used as the standard test — because Br₂(aq) has a distinctive orange/brown colour that changes to colourless on reaction. Cl₂(aq) is pale green-yellow (less easy to read); I₂(aq) is brown (also reads but slower reaction). Bromine water hits the sweet spot for an A-Level lab observation.

Addition · Water (Steam)

Alkene + water (steam) → alcohol. This is industrial hydration — the main route to ethanol used in solvents and fuels.

alkene + H₂O → alcohol

Worked Example · Ethene + Steam

C₂H₄ + H₂O → C₂H₅OH

(ethene + steam → ethanol)

Conditions to Memorise

High temperature · high pressure · catalyst (usually phosphoric acid — accepted by mark schemes; "catalyst" alone is also acceptable). The H–OH adds across the C=C: H to one carbon, OH (the alcohol's hydroxyl group) to the other.

Looking Ahead · Lesson 7.05

Ethanol production has two industrial routes: (1) hydration of ethene (this lesson — fast, pure, uses non-renewable feedstock); (2) fermentation of sugars (slow, renewable, but produces dilute ethanol). Compare and contrast in 7.05.

Bromine Water · Mechanism

The bromine water test combines an observation (colour change) with a mechanism (addition reaction). At chemistry-only level, you should be able to explain both.

ORANGE→COLOURLESS

SubstanceWhat you seeWhy

Alkene (C=C present)Bromine water turns from orange/brown to colourlessBr₂ adds across the C=C double bond → dibromoalkane (no Br₂ left in solution)

Alkane (no C=C)Bromine water stays orange/brownNo reactive site for Br₂ to add across — no addition occurs

PEE Pattern for "Why does X decolourise but Y doesn't?"

(P) X has a C=C double bond; Y does not. (E) Bromine adds across the C=C in an addition reaction (CH₂=CH₂ + Br₂ → CH₂BrCH₂Br) — the bromine is consumed, so the solution turns colourless. (E) Y has only single bonds, so bromine cannot add — the orange colour remains.

Examiner Alert · Bromine Water

The Jun24 Q08.2 examiner report shows precisely how marks are split — and how to lose them.

"Those students who were awarded marking point 1 used a range of the colours allowed in the mark scheme to describe the starting colour of bromine water. The most common colours were orange or brown. A small number of students described the bromine water as turning clear rather than colourless. For students who were awarded one mark there seemed to be an even split between those awarded marking point 1 and those awarded marking point 2. Just over half scored both." — AQA Examiner Report, P2 Jun 2024 Q08.2

Mark Scheme · Verbatim

MP1 (start colour): orange (allow yellow / brown). MP2 (end colour): turns colourless (allow "is decolourised"). "Ignore clear."

WrongRightWhy it matters

"Bromine water goes clear ""Bromine water turns colourless ""Clear" means transparent, not colour. MS literally "ignores clear".

"Goes red" / "purple""Orange" / "yellow" / "brown"Wrong starting colour fails MP1.

"Disappears""Decolourised" / "colourless"The bromine doesn't disappear — its colour is consumed.

State only one colourState both — start AND end2 marks = 2 colour states.

Practice · Functional Group

(a)

(b)

3 MARKS

Inspired by AQA Q08.1 (Jun24 / Jun23) circle-the-functional-group pattern. Tests both identification and the trap-recognition skill.

(a)

Functional group: the C=C double bond (the alkene functional group).

(b)

Student B would score the mark.

Why

The functional group is only the C=C unit (the two carbons + the bond between them). Student A's circle is too big — it includes attached H atoms that are not part of the functional group. The Jun24 examiner report flagged this exact error: ~40% of students lost the mark by including attached atoms.

⚠

Trap reminder: "C=C" means the bond AND both carbons. NOT the bond alone (that's only half the functional group). NOT the carbons + attached Hs (that's too much). Just the 2 Cs + the double bond between them.

Practice · Addition Reaction

(a)

(b)

(c)

3 MARKS

Inspired by AQA addition-reaction pattern (spec 4.7.2.2).

(a)

Type: addition (reaction).

(b)

Product: propane (C₃H₈). Atom check: C₃H₆ + H₂ → C₃H₈. C: 3 ✓ | H: 6+2 = 8 ✓.

(c)

The C=C double bond opens up. One H atom adds to each of the two carbons. The result is a single C–C bond. The product is a saturated alkane.

✓

Mark-scoring vocabulary: "addition reaction"; "C=C opens"; "H adds across the double bond"; "single bond / saturated".

Practice · Bromine Water Explanation

(a)

(b)

3 MARKS

Inspired by AQA Q08.2 (Jun24) bromine water observation pattern.

(a)

Liquid X is an alkene (or contains a C=C double bond). Liquid Y is an alkane (or has no C=C).

(b)

X has a C=C double bond. Bromine adds across the C=C in an addition reaction (CH₂=CHR + Br₂ → CH₂BrCHBrR). The bromine is consumed, so the orange colour disappears and the solution becomes colourless.

⚠

Mark-scoring rules: always state both colours ("orange to colourless"). Never use "clear" alone — MS says "ignore clear". Always link the colour change to addition across the C=C.

Mark-Scheme Language

The exact phrasing AQA accepts for chemistry-only 7.04 questions vs the loose wording that loses marks.

ConceptMark-scoring phraseDo not accept

Alkene functional group" C=C double bond " (circle only the 2 Cs + bond)Circling C=C with attached H or O atoms

Addition reaction"atoms add across the C=C double bond""react with"; "join with"

Hydrogenation"alkene + H₂ → alkane (with nickel catalyst)"Forgetting the catalyst

Hydration of ethene"alkene + steam → alcohol (catalyst, high T & P)""ethene + water → ethanol" without conditions

Bromine start colour" orange " / "yellow" / " brown ""red"; "purple"

Bromine end colour" colourless " (or "decolourised")"clear"; "disappears"

Why decolourises"Br₂ adds across the C=C double bond""Bromine reacts with the alkene" (vague)

Combustion of alkenes"smoky flame" (incomplete combustion)"black smoke" alone

Common Errors

The four mistakes that account for most lost marks across recent chemistry-only Paper 2 alkene questions. Pattern-match these in your own answers.

✗ Common error

Circle C=C plus attached H atoms

✓ Mark-scheme

Circle only the 2 carbons + double bond — nothing else

✗ Common error

"Bromine water goes clear"

✓ Mark-scheme

"Turns from orange to colourless" — MS "ignores clear"

✗ Common error

"Bromine reacts with the alkene"

✓ Mark-scheme

"Bromine adds across the C=C double bond"

✗ Common error

Confusing ethane (C₂H₆) with ethene (C₂H₄)

✓ Mark-scheme

"-ane" = saturated alkane; "-ene" = unsaturated alkene

Exam Strategy

Three habits that lock in the chemistry-only 7.04 marks.

Strategy 1 · Circle C=C, Nothing Else

For "circle the functional group" questions: include exactly 2 carbon atoms + the double bond between them. No hydrogens. No oxygens. No carbon-chain atoms. The Jun24 examiner report flagged this as the #1 mistake.

Strategy 2 · Use the Phrase "Adds Across"

For any addition reaction explanation: "X adds across the C=C double bond". This phrase scores the mechanism mark. "Reacts with" or "joins with" is too vague.

Strategy 3 · Always Both Colours

For bromine water: state both orange (or brown) → colourless. Never just one colour. Never "clear", "disappears", or "decolourised" alone. Two marks = two colour states linked by the colour change.

Lesson Summary

Key Definitions

Alkene — unsaturated hydrocarbon with C=C double bond; CₙH₂ₙ.

Functional group — atoms giving characteristic properties; for alkenes, the C=C.

Addition reaction — atoms add across the C=C, converting it to a single C–C bond. No atoms lost.

First 4 Alkenes · CₙH₂ₙ

C₂H₄ ethene

C₃H₆ propene

C₄H₈ butene

C₅H₁₀ pentene

3 Addition Reactions

+ H₂ (Ni cat) → alkane

+ Br₂ → dibromoalkane

+ H₂O (steam, cat) → alcohol

Top Exam Tip

For circle-the-C=C: only the 2 carbons + double bond, nothing else. For bromine water: orange → colourless (never "clear"). For any addition explanation: use "adds across the C=C" — this is the mark-scoring phrase.