This set covers the five skills in organic reactions: combustion, addition, alcohol and acid changes, ester formation and polymers. The questions run from straightforward to more demanding, with no marks shown. All data are invented for practice, and the set is original.
Write each answer in full before opening the working. Use the equation balance reasoning trainer to check atom counts and the mole and equation-ratio tutor for amounts.
Relative atomic masses used: H 1, C 12, O 16, Br 80.
The questions
1. Balance the complete combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O
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Carbon gives 3CO₂, hydrogen gives 4H₂O, oxygen on the right is 6 + 4 = 10, so 5O₂.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Check: 3 C, 8 H, 10 O on each side.
2. A gas from a burning fuel turns limewater milky, and a cold tube held above the flame becomes misty with droplets. What can you conclude about the fuel?
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Carbon dioxide and water formed, so the fuel contained carbon and hydrogen. The oxygen came from the air. The evidence does not tell you whether oxygen was also in the fuel.
3. A gas flame burns yellow and leaves a black deposit. Suggest why, and write a balanced equation for methane forming carbon and water.
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The oxygen supply is limited, so combustion is incomplete and solid carbon (soot) forms.
CH₄ + O₂ → C + 2H₂O
Check: 1 C, 4 H, 2 O on each side.
4. Write the equation for ethene reacting with hydrogen, and name the product.
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C₂H₄ + H₂ → C₂H₆. Check: 2 C and 6 H on each side. The product is ethane.
5. Two colourless liquids are hexane, C₆H₁₄, and hexene, C₆H₁₂. Describe how orange-brown bromine water distinguishes them.
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Shake each with bromine water. The hexene turns it colourless, because bromine adds across the double bond. The hexane leaves it orange-brown, because it is saturated and has no double bond.
6. Propene reacts with bromine. Give the equation and the name of the product.
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C₃H₆ + Br₂ → C₃H₆Br₂, which is CH₃-CHBr-CH₂Br, 1,2-dibromopropane.
Check: 3 C, 6 H, 2 Br on each side.
7. 5.6 g of ethene reacts completely with hydrogen. Calculate the mass of ethane formed.
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Mr of ethene C₂H₄ = 28, so 5.6 ÷ 28 = 0.20 mol. The ratio is 1 : 1, so 0.20 mol of ethane forms. Mr of C₂H₆ = 30.
Mass = 0.20 × 30 = 6.0 g.
Check: 0.20 mol of H₂ is 0.40 g, and 5.6 + 0.40 = 6.0 g.
8. Glucose ferments to ethanol and carbon dioxide. Write the balanced equation and calculate the mass of ethanol from 36 g of glucose, assuming complete conversion.
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C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Check: 6 C, 12 H, 6 O on each side.
Mr of glucose = 180, so 36 ÷ 180 = 0.20 mol. The ratio glucose to ethanol is 1 : 2, so 0.40 mol of ethanol. Mr of ethanol = 46.
Mass = 0.40 × 46 = 18.4 g.
Check: carbon dioxide is 0.40 × 44 = 17.6 g, and 18.4 + 17.6 = 36.0 g.
9. Propan-1-ol is oxidised to a carboxylic acid. Name the acid and write the equation using oxygen.
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The acid is propanoic acid.
C₃H₇OH + O₂ → C₂H₅COOH + H₂O
Check: 3 C, 8 H, 3 O on each side.
10. Methanol reacts with ethanoic acid in the presence of an acid catalyst. Name the ester, give its formula and calculate its relative formula mass.
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The alcohol gives methyl and the acid gives ethanoate: methyl ethanoate.
CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O. Check: 3 C, 8 H, 3 O on each side.
The formula is C₃H₆O₂, so Mr = 3(12) + 6(1) + 2(16) = 74.
11. Draw the repeat unit of poly(chloroethene), and calculate how many repeat units are in a chain of relative molecular mass 25 000. Use Cl = 35.5.
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The monomer is CH₂=CHCl. The repeat unit is [-CH₂-CHCl-]n.
Mr of one unit, C₂H₃Cl, is 24 + 3 + 35.5 = 62.5. The number of units is 25 000 ÷ 62.5 = 400.
Check: 400 × 62.5 = 25 000.
If you got these wrong
| What went wrong | Questions | Where to revisit |
|---|---|---|
| Unbalanced equation or wrong oxygen count | 1, 3, 8 | Explain combustion using products and evidence |
| Conclusion not matched to the evidence | 2, 5 | Combustion and evidence |
| Wrong atom count in an addition product | 4, 6, 7 | Represent addition to an unsaturated molecule |
| Confusing fermentation, oxidation and combustion | 8, 9 | Alcohol and acid transformations |
| Ester name or formula wrong, water forgotten | 10 | Interpret an ester-forming relationship |
| Double bond kept in the repeat unit, or Mr slips | 11 | Recognise a polymer repeat unit |
Record each miss in the mistake log and retest queue and retest that pattern a week later with a different molecule. For a wider check, return to the module overview.
If one error type keeps coming back, our teachers can look at your written working in online one-to-one Chemistry tuition.