Learning loop0/4 stages · 0%
Stage 1
Comprehend
Build the core concepts, explanations and evidence.
1.1 Why Carbon is the Centre of Life
Think about this: there are over 118 elements on the periodic table. Yet the chemistry of living things — your body, food, fuel, medicines, plastics — is built almost entirely from one element: carbon. Carbon sits in Group IV of the periodic table. It has 4 valence electrons, meaning it needs 4 more to complete its outer shell. So it forms 4 covalent bonds. This simple fact unlocks millions of compounds.
Carbon's Four Superpowers
1.2 Key Vocabulary
1.3 Alkanes — General Formula CnH(2n+2)
Alkanes are saturated hydrocarbons — every C-C bond is single. First ten: Methane CH4 -162°C | Ethane C2H6 -89°C | Propane C3H8 -42°C | Butane C4H10 0°C | Pentane C5H12 36°C | Hexane C6H14 69°C | Heptane C7H16 98°C | Octane C8H18 126°C | Nonane C9H20 151°C | Decane C10H22 174°C. Memory: My Enormous Purple Butterfly Perches High Here On New Desks.
Physical Properties of Alkanes
Boiling point increases with chain length: More carbons = more electrons = stronger Van der Waals forces = more energy needed to separate = higher BP. Branching lowers BP: n-Butane (0°C) vs 2-Methylpropane (-12°C) — same formula C4H10 but branched is more compact, less surface area, weaker Van der Waals. Solubility: insoluble in water (non-polar cannot H-bond with water), soluble in organic solvents. Like dissolves like. Oil floats on water — never use water on oil fire. Density: less dense than water (0.6-0.8 g/cm3).
Reaction 1: Combustion
Complete combustion (excess O2): CH4 + 2O2 → CO2 + 2H2O | C3H8 + 5O2 → 3CO2 + 4H2O | 2C4H10 + 13O2 → 8CO2 + 10H2O. Balancing steps: (1) Balance C. (2) Balance H. (3) Count O on right divide by 2 for O2. (4) Multiply to clear fractions. (5) Verify. Incomplete combustion (limited O2): 2CH4 + 3O2 → 2CO + 4H2O. Signs: yellow flame, black soot, less heat.
Reaction 2: Free Radical Substitution — Full Mechanism
Alkanes react with halogens (Cl2, Br2) in UV light — NOT heat, NOT darkness. Overall: CH4 + Cl2 →(UV)→ CH3Cl + HCl. UV is REQUIRED — without it the reaction does NOT occur.
Stage 1 INITIATION: UV breaks Cl-Cl bond homolytically (equally). Each Cl gets one electron → Cl• radicals. Cl-Cl →(UV)→ Cl• + Cl•. Only needs to happen once to start chain.
Stage 2 PROPAGATION: Cl• + CH4 → HCl + CH3•. Then CH3• + Cl2 → CH3Cl + Cl•. The Cl• regenerated goes back to attack more CH4. One radical causes thousands of substitutions — chain reaction.
Stage 3 TERMINATION: Two radicals combine and chain ends. Cl• + Cl• → Cl2 | CH3• + Cl• → CH3Cl | CH3• + CH3• → C2H6. (Traces of ethane in products explains the last equation.)
💡 NOTE
CRITICAL TEST: Alkanes do NOT decolourise bromine water. Bromine water stays orange with alkanes — no UV light, no reaction. This is the key difference from alkenes and alkynes.
1.4 Alkenes — General Formula CnH(2n)
Alkenes are unsaturated hydrocarbons containing C=C double bond. The double bond = one sigma bond (strong, direct overlap) + one pi bond (weaker, sideways overlap, electron-rich, exposed above and below the bond). The pi bond is the reactive centre — why alkenes are far more reactive than alkanes. Think: sigma bond is the foundation buried underground; pi bond is the open window that reagents attack.
First six alkenes: Ethene C2H4: CH2=CH2 | Propene C3H6: CH3-CH=CH2 | But-1-ene C4H8: CH2=CH-CH2-CH3 | But-2-ene C4H8: CH3-CH=CH-CH3 | Pent-1-ene C5H10: CH2=CH-CH2-CH2-CH3 | Pent-2-ene C5H10: CH3-CH=CH-CH2-CH3. Naming: number chain so double bond gets lowest number.
Cis-Trans Isomerism: No rotation around C=C so groups lock in place. cis-But-2-ene: both CH3 on SAME side (BP 4°C). trans-But-2-ene: CH3 on OPPOSITE sides (BP 1°C). Different compounds — different boiling points.
The Bromine Water Test — Most Tested Concept
Alkane + bromine water → NO CHANGE (stays orange). No double bond, no reaction. | Alkene + bromine water → DECOLOURISES (orange to colourless). Br2 adds across C=C → colourless 1,2-dibromoalkane. | Alkyne + bromine water → DECOLOURISES. Reacts twice across C≡C. This single test distinguishes saturated from unsaturated compounds.
Addition Reactions of Alkenes
Hydrogenation (+ H2): CH2=CH2 + H2 →(Ni, 150°C)→ CH3-CH3. Industrial use: converts vegetable oil to margarine (Bidco Cowboy, Kimbo, Blue Band all made this way).
Halogenation (+ Br2): CH2=CH2 + Br2 → CH2Br-CH2Br (1,2-dibromoethane). Room temperature, no UV. This is the bromine water test reaction.
Hydration (+ H2O → alcohol): CH2=CH2 + H2O →(H3PO4, 300°C, 60 atm)→ CH3CH2OH (ethanol). Industrial method for making ethanol.
Addition of HBr — Markovnikov's Rule: Propene + HBr → CH3-CHBr-CH3 (major product: 2-bromopropane). Rule: H adds to carbon with MORE hydrogen. The rich get richer. Reason: more stable secondary carbocation intermediate forms rather than less stable primary carbocation.
Addition Polymerisation
1.5 Alkynes — General Formula CnH(2n-2)
Alkynes contain C≡C triple bond = 1 sigma + 2 pi bonds. Most reactive of the three series. Shape: LINEAR (180°). First alkynes: Ethyne C2H2: H-C≡C-H (-84°C) | Propyne C3H4: CH3-C≡C-H (-23°C) | But-1-yne C4H6 (8°C) | But-2-yne C4H6 (27°C).
1.6 Comparison — The Three Series
Alkanes CnH(2n+2): C-C single bond | saturated | bromine water NO CHANGE | substitution (UV light) | low reactivity. Alkenes CnH(2n): C=C double bond | unsaturated | DECOLOURISES bromine water | addition | high reactivity. Alkynes CnH(2n-2): C≡C triple bond | unsaturated | DECOLOURISES bromine water reacts twice | addition | highest reactivity.
1.7 Calculations
- Alkane decolourises bromine water — WRONG. Stays orange.
- UV light just speeds up halogenation — WRONG. UV is REQUIRED. Without it no reaction.
- Unbalanced combustion equations — always verify C, H, O after balancing.
- Repeating unit without brackets and n — always show -[ ]n- with bonds outside.
- Substitution for alkenes or addition for alkanes — swap them over.
1.9 Practice Questions
Stage 2
Apply & check
Test understanding and surface misconceptions early.
❓ CHECK YOUR UNDERSTANDING
KCSE 3-mark question: Explain why pentane has a higher boiling point than butane.
❓ CHECK YOUR UNDERSTANDING
KCSE 6-mark mechanism question: Stage 1 Initiation — UV causes homolytic fission of Cl2 → 2Cl• (1 mark). Stage 2 Propagation — Cl• + CH4 → HCl + CH3• (1 mark); CH3• + Cl2 → CH3Cl + Cl• (1 mark); steps repeat in chain reaction (1 mark). Stage 3 Termination — radicals combine e.g. Cl• + Cl• → Cl2 ending the chain (1 mark). Reaction is called free radical substitution (1 mark).
This question is for reflection. No automatic marking is configured.
❓ CHECK YOUR UNDERSTANDING
KCSE 3-mark question: Describe a test to distinguish hexane from hex-1-ene.
❓ CHECK YOUR UNDERSTANDING
Worked Example — Empirical and Molecular Formula: Hydrocarbon contains 85.7% C and 14.3% H. Molar mass = 56 g/mol. Step 1 — Assume 100g: C=85.7g, H=14.3g. Step 2 — Moles: C=85.7÷12=7.14; H=14.3÷1=14.3. Step 3 — Ratio divide by 7.14: C=1, H=2. Empirical formula = CH2. Step 4 — Molecular formula: empirical mass=14; n=56÷14=4; molecular formula=C4H8. Step 5 — C4H8 fits CnH2n → Alkene → But-1-ene or But-2-ene.
This question is for reflection. No automatic marking is configured.
❓ CHECK YOUR UNDERSTANDING
Section A (1 mark each): 1. General formula of alkenes? 2. Name C7H16. 3. Structural formula of but-2-ene? 4. Observation when ethene bubbled into bromine water? 5. Reaction type when propene + H2? 6. Catalyst for hydrogenation of alkenes? 7. Molecular formula of alkyne with 5 carbons? 8. Conditions for methane + chlorine reaction? 9. Name polymer from chloroethene? 10. Bond type that breaks in initiation of free radical substitution? Answers: 1.CnH2n | 2.Heptane | 3.CH3-CH=CH-CH3 | 4.Decolourises/turns colourless | 5.Addition | 6.Nickel Ni | 7.C5H8 | 8.UV light | 9.PVC poly(chloroethene) | 10.Cl-Cl bond homolytic fission
This question is for reflection. No automatic marking is configured.
❓ CHECK YOUR UNDERSTANDING
Section B: 11. Why is hexane insoluble in water but soluble in cyclohexane? 12. Why do branched alkanes have lower boiling points than straight-chain isomers? 13. Describe bromine water test — observation and conclusion for positive and negative. 14. Equation for but-1-ene + HBr — name major product and explain using Markovnikov's rule. 15. Why is incomplete combustion dangerous? Include chemistry.
This question is for reflection. No automatic marking is configured.
❓ CHECK YOUR UNDERSTANDING
Section C KCSE Style: Q1. Hydrocarbon X has molecular formula C4H8. (a) Homologous series — name and reason. (2 marks) (b) Draw and name TWO structural isomers. (2 marks) (c) Equation for one isomer + bromine. Name product. (2 marks). Q2. (a) Equation for complete combustion of octane C8H18. (2 marks) (b) TWO products of incomplete combustion. Danger of ONE to human health. (3 marks). Q3. Full mechanism of ethane + bromine in UV light with equations for each stage. (6 marks).
This question is for reflection. No automatic marking is configured.
Stage 3
Connect
Relate the learning to Kenya, Africa and connected ideas where relevant.
Tetravalency: Carbon forms exactly 4 bonds. This gives maximum flexibility to connect with other atoms. Catenation: Carbon bonds strongly with itself — C-C bond energy is 348 kJ/mol. Think of carbon atoms as LEGO bricks connecting endlessly in chains, branches, and rings. Multiple Bonding: Carbon forms C-C (single), C=C (double), and C≡C (triple) bonds — each with different reactivity. Bond with Almost Anything: Carbon bonds readily with H, O, N, S, Cl, Br creating millions of different compounds.
💡 NOTE
Why CO kills: Carbon monoxide is odourless, colourless, tasteless — you cannot detect it. CO binds to haemoglobin 200 times more strongly than oxygen. Haemoglobin is blocked → cells starved of oxygen → unconsciousness → death. Never use charcoal jikos in closed rooms. Many Kenyans die from CO poisoning every year — this is preventable chemistry.
💡 NOTE
Environmental impact: Addition polymers are non-biodegradable — persist hundreds of years. Kenya banned plastic bags under 30 microns in 2017. Poly(ethene) blocks Nairobi drainage and harms wildlife.
Reactions: Alkynes decolourise bromine water — react TWICE. First: H-C≡C-H + Br2 → CHBr=CHBr (still has double bond). Second (excess Br2): CHBr=CHBr + Br2 → CHBr2-CHBr2 (colourless). Combustion: 2C2H2 + 5O2 → 4CO2 + 2H2O. Oxy-acetylene flame (ethyne + pure O2) reaches 3500°C — cuts and welds steel. Every welding workshop in Kenya uses this.
Stage 4
Extend
Deepen learning through mastery practice, reflection and teacher-ready application.
💡 NOTE
By the end of this chapter you will be able to: name and write formulae of alkanes, alkenes and alkynes; describe their physical and chemical properties; explain free radical substitution and addition reactions; distinguish saturated from unsaturated compounds; and answer any KCSE question on hydrocarbons with confidence.
💡 NOTE
KCSE EXAMINER NOTE: Why does carbon form so many compounds? Answer must mention: tetravalency, catenation, and ability to form multiple bonds. Three points = three marks.
Hydrocarbon: compound containing C and H ONLY. Ethanol is NOT a hydrocarbon — it contains oxygen. KCSE trap — many students get this wrong. Homologous Series: family sharing same general formula, similar chemical properties, gradual change in physical properties, members differ by CH2. Saturated: only C-C single bonds — cannot add more hydrogen. Unsaturated: contains C=C or C≡C — can add more atoms. Memory: saturated sponge cannot absorb more water. Isomers: same molecular formula but different structural formulae — different compounds with different properties.
Many alkene monomers join under high pressure and catalyst to form one giant polymer. n(CH2=CH2) → -[CH2-CH2]n- (poly(ethene)/polythene). Polymers: Ethene → Poly(ethene) — plastic bags | Propene → Poly(propene) — crates, ropes | Chloroethene → PVC — pipes, cables. Repeating unit MUST show: square brackets [ ], letter n outside, bonds extending outside both sides. Missing these loses marks.
Preparation of Ethyne (KCSE must know): CaC2 + 2H2O → Ca(OH)2 + C2H2 gas. Calcium carbide (grey solid) reacts with water — carbide ion [C≡C]2- takes protons → ethyne released.
1.8 KCSE Mistakes — Avoid These