Organic Chemistry Exam Practice | AQA GCSE Chemistry Topic 7

Petrol contains a hydrocarbon with the formula C₉H₂₀.

(a) Complete and balance the equation for the complete combustion of C₉H₂₀. [2]

C₉H₂₀ + _____ → _____ + _____

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C₉H₂₀ + 14O₂ → 9CO₂ + 10H₂O [2]
Allow 1 mark for correct products (CO₂ and H₂O) but incorrect balancing

Examiner tip: Take care writing numbers clearly. A quickly written "14" can look like "19". Count oxygen atoms on both sides carefully: right side has (9×2) + 10 = 28 oxygens, so 28÷2 = 14 O₂.

Compound A has an alkene functional group.

(a) Describe what will be seen when compound A is shaken with bromine water. [2]

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Orange bromine water (allow yellow/brown) [1]
Turns colourless (allow "is decolourised") [1]

Examiner tip: You must state both the starting colour AND the end result. Do NOT write "clear" instead of "colourless". A solution can be clear but still coloured. The word colourless is required.

Heavy fuel oil consists of molecules with 20-40 carbon atoms, while kerosene consists of molecules with 11-15 carbon atoms. A student predicted that heavy fuel oil is more viscous than kerosene. The prediction was correct.

(a) Justify the student's prediction. [2]

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As molecular size increases, viscosity increases [1]
Heavy fuel oil has larger molecules than kerosene [1]

Examiner tip: For 2 marks, establish the general rule first (bigger molecules = more viscous), then link it to the specific data given. Just saying "heavy fuel oil has more carbons" without stating the rule won't get full marks.

Central heating boilers can burn kerosene. Kerosene is produced from crude oil in a fractionating column. In the first step, crude oil is heated and hydrocarbon vapours are formed.

(a) Explain how kerosene is produced from these hydrocarbon vapours. [3]

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There is a temperature gradient in the column (it gets cooler going up) [1]
The kerosene vapour condenses [1]
At the level in the column that matches its boiling point range [1]

Examiner tip: You must say vapours condense. Not just "separate". Don't say the crude oil is heated inside the column. It's heated before entering. State that different fractions condense at different heights because the column has a temperature gradient.

Ethene is an alkene produced from large hydrocarbon molecules.

(a) Name the process used to produce ethene and describe the conditions used. [3]

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Cracking (accept steam cracking or catalytic cracking) [1]
High temperature (allow a value in the range 300-900 °C) [1]
Steam or a catalyst (e.g. Aluminium oxide / zeolite) [1]

Examiner tip: Simply writing "heat" or "hot" is insufficient. You must say "high temperature". You also need either steam or a catalyst as the second condition to get full marks.

Methylated spirit contains ethanol.

(a) Describe how ethanol is produced from a sugar solution. Give the name of this process. [3]

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The process is fermentation [1]
Add yeast (as the catalyst/biological agent) [1]
Anaerobic conditions (absence of oxygen) and warm temperatures (allow 5-45 °C) [1]

Examiner tip: Do NOT say "high temperature". High temperatures would denature the enzymes in yeast. Use "warm" or give a specific temperature in the 25-40 °C range. You also need to mention the absence of oxygen (anaerobic) for full marks.

Crude oil is a complex finite resource made mostly of hydrocarbons.

(a) Describe and explain how crude oil is separated into its different fractions using fractional distillation. [6]

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Level 3 (5-6 marks): Clear, logically sequenced explanation of how fractions are separated, correctly using concepts of vaporisation, temperature gradient, and condensation linked to different boiling points.

Level 2 (3-4 marks): Descriptive account including heating and condensing, with some reference to boiling points, but steps may not be perfectly sequenced or the temperature gradient may be omitted.

Level 1 (1-2 marks): Simple statements identifying that crude oil is heated and separates, but lacking scientific explanation of state changes.

Indicative content:

Crude oil is heated until most hydrocarbons evaporate/vaporise
Vapours are fed into a fractionating column
There is a temperature gradient. Hot at the bottom, cooler at the top
As vapours rise and cool, different hydrocarbons condense at different temperatures because they have different boiling points
Long-chain molecules (high boiling points) condense near the bottom; short-chain molecules (low boiling points) condense near the top

Examiner tip: Clearly state that vapours cool and condense. Do not confuse the physical separation process of fractional distillation with the chemical process of cracking. Mentioning catalysts or breaking bonds here will lose you marks.

A student carries out a laboratory fractional distillation to separate a mixture of liquids with different boiling points.

(a) Explain how the process of fractional distillation separates the different fractions. [4]

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Fractions evaporate / vaporise [1]
At different boiling points [1]
These vapours then condense [1]
At different points / heights in the fractionating column [1]

Examiner tip: Many students only mention boiling or evaporation. You must clearly state that separation relies on the different boiling points of the fractions, and ensure you mention both the evaporation and condensation stages.

Links to: Topic 1. Mixtures & Separation (distillation is a key separation technique)

Crude oil is a complex mixture of hydrocarbons. In industry, it is continuously fed into a fractionating column to be separated into useful fractions.

(a) Describe the process of fractional distillation. In your answer, explain how the size of the hydrocarbon molecules affects their properties and dictates where they are collected from the fractionating column. [6]

Show Mark Scheme

Level 3 (5-6 marks): Detailed, logically sequenced description of fractional distillation. Student clearly links heating and condensing steps to properties of hydrocarbons (molecular size and boiling point) and explains where they are collected.

Level 2 (3-4 marks): Mostly correct description with some explanation of how molecular size relates to boiling points or collection points, but may lack some detail or clarity.

Level 1 (1-2 marks): Simple, fragmented statements about heating, condensing, or hydrocarbon sizes, lacking a clear logical structure.

Indicative content:

Process: crude oil is heated until it evaporates/vaporises
The fractionating column has a temperature gradient. Hotter at the bottom, cooler at the top
Hydrocarbon vapours rise up the column and condense when they reach a region at their boiling point
Molecule size link: smaller/shorter hydrocarbons have lower boiling points → rise further → collected at the top (e.g. Petroleum gases)
Larger/longer hydrocarbons have higher boiling points → condense lower down → collected at the bottom (e.g. Heavy fuel oil)

Examiner tip: Don't just list facts. Use connecting words to show cause and effect. Link the size of the molecule directly to its boiling point and, consequently, where it condenses in the temperature gradient.

Ethanol (C₂H₅OH) is a member of the alcohol homologous series.

(a) Give the name of the functional group present in all alcohols. [1]

(b) Ethanol dissolves in water to form a neutral solution. Suggest what this tells you about ethanol. [1]

(c) Ethanol reacts with sodium metal. State what you would observe and name the gas produced. [2]

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(a) Hydroxyl group / -OH [1]
(b) Ethanol is not acidic or alkaline / it does not produce H⁺ or OH⁻ ions in excess [1]
(c) Effervescence / fizzing / bubbling observed [1]
Hydrogen (gas) is produced [1]

Examiner tip: Students often confuse the reaction of alcohols with sodium and the reaction of carboxylic acids with sodium carbonate. Alcohols react with sodium metal to produce hydrogen. Carboxylic acids react with carbonates to produce carbon dioxide.

Ethanoic acid (CH₃COOH) is a carboxylic acid found in vinegar.

(a) Give the name of the functional group present in all carboxylic acids. [1]

(b) A student adds sodium carbonate to ethanoic acid. Describe what would be observed and write a word equation for the reaction. [3]

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(a) Carboxyl group / -COOH [1]
(b) Effervescence / fizzing / bubbling observed [1]
Ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide [2]

Examiner tip: Remember, carboxylic acids are weak acids. They react with carbonates to produce a salt, water, and carbon dioxide. The observation is fizzing or effervescence, and the gas produced is CO₂. Do not confuse this with the reaction of alcohols with sodium, which produces hydrogen.

Propene (C₃H₆) is an alkene that can be used to make poly(propene).

(a) What feature of the propene molecule allows it to form an addition polymer? [1]

(b) Explain why addition polymers are difficult to dispose of. [2]

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(a) It contains a carbon-carbon double bond (C=C) [1]
(b) They are chemically inert / unreactive [1]
So they are not (easily) broken down by microorganisms / not biodegradable [1]

Examiner tip: Be specific: the C=C double bond opens up during addition polymerisation. The resulting polymer has only single C-C bonds in the backbone. These are very strong and unreactive, which is why they persist in the environment. Just saying "they don't decompose" is not enough for full marks.

There are two main types of polymerisation: addition and condensation.

(a) State one difference between addition polymerisation and condensation polymerisation. [1]

(b) Name the small molecule that is released during condensation polymerisation. [1]

(c) Give two examples of naturally occurring polymers. [2]

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(a) Condensation polymerisation produces a small molecule (e.g. water) as a by-product / addition polymerisation does not [1]
(b) Water [1]
(c) Any two from: proteins, DNA, starch, cellulose (accept amino acids → proteins, nucleotides → DNA) [2]

Examiner tip: You must be precise about the difference. The key point is that condensation polymerisation releases a small molecule (usually water), while addition does not. Do not confuse this with the type of monomer used. Also remember: amino acids form proteins, and nucleotides form DNA. Both are naturally occurring condensation polymers.

Ethanol and ethanoic acid are both commonly found in household products. Ethanol is used in hand sanitiser and ethanoic acid is the main component of vinegar.

(a) Compare the properties and reactions of ethanol and ethanoic acid. In your answer, include the functional group of each, their pH in solution, and one named reaction for each compound. [6]

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Level 3 (5-6 marks): Detailed, logically structured comparison covering both compounds. Functional groups, pH, and at least one specific reaction for each are correctly identified and compared.

Level 2 (3-4 marks): Reasonable comparison with most key information, but some gaps in detail or minor inaccuracies. May discuss one compound better than the other.

Level 1 (1-2 marks): Basic statements about either alcohols or carboxylic acids. Lacks a clear comparative structure.

Indicative content:

Ethanol: functional group is -OH (hydroxyl), dissolves in water to form a neutral solution (pH 7)
Ethanol reacts with sodium to produce hydrogen (OR: ethanol burns to produce CO₂ and H₂O, OR: ethanol can be oxidised to ethanoic acid)
Ethanoic acid: functional group is -COOH (carboxyl), dissolves in water to form a weakly acidic solution (pH 3-6)
Ethanoic acid reacts with sodium carbonate to produce CO₂, water, and a salt (OR: ethanoic acid reacts with alcohols to form esters)
Ethanoic acid is a weak acid because it only partially ionises in solution

Examiner tip: For a 6-mark question, structure matters. Discuss ethanol first, then ethanoic acid, making direct comparisons. Use correct chemical terminology (e.g. "hydroxyl group" not "OH bit"). Remember: ethanol forms a neutral solution, while ethanoic acid forms a weakly acidic solution. Do not say ethanoic acid is a strong acid.

Living organisms contain many naturally occurring polymers that are essential for life, including proteins and deoxyribonucleic acid (DNA).

(a) Amino acids are the building blocks of proteins. State the names of the two different functional groups present in every amino acid molecule. [2]

(b) Describe the structure of a DNA molecule, including the name of the monomers from which it is made. [3]

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(a) Amine group (-NH2) [1]
Carboxyl group (-COOH) (accept carboxylic acid group) [1]
(b) Made of monomers called nucleotides [1]
Consists of two polymer chains [1]
Forming a double helix structure [1]

Examiner tip: Ensure you know the names of the functional groups: amine (or amino) and carboxyl (or carboxylic acid). For DNA, the term nucleotides is required. Make sure you state that there are two polymer chains and that they form a double helix.

Topic 7: Organic Chemistry Exam Practice

📋 Structured Questions

Question 1: Combustion of an Alkane

Question 2: Testing for Alkenes

Question 3: Properties of Hydrocarbon Fractions

Question 4: Fractional Distillation

Question 5: Cracking Conditions

Question 6: Production of Ethanol

Question 7: Fractional Distillation of Crude Oil Extended Response

Question 8: Required Practical. Fractional Distillation

Question 9: Required Practical. Molecule Size & Distillation Extended Response

Question 10: Alcohols HT

Question 11: Carboxylic Acids HT

Question 12: Addition Polymerisation HT

Question 13: Condensation Polymers and Natural Polymers HT

Question 14: Alcohols and Carboxylic Acids Extended Response HT

Question 15: Amino Acids, DNA, and Proteins HT