AQA GCSE Combined Science: Trilogy · 5.7

Organic chemistry

50 revision questions with answers for 5.7 Organic chemistry in AQA GCSE Combined Science: Trilogy (spec 8464). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below.

All 50 questions and answers

Question 1 · Spec 5.7.1.1
What is crude oil?
Answer:
A finite mixture of many compounds, mainly hydrocarbons, found in rocks.
Question 2 · Spec 5.7.1.1
How did crude oil form?
Answer:
From ancient biomass, mainly plankton, buried in mud and changed over geological time.
Question 3 · Spec 5.7.1.1
Why is crude oil described as finite?
Answer:
It forms far more slowly than humans use it, so accessible reserves can be depleted.
Question 4 · Spec 5.7.1.1
Why does crude oil not have one fixed molecular formula?
Answer:
It is a mixture containing many different compounds, rather than a single pure substance.
Question 5 · Spec 5.7.1.1
What is a hydrocarbon?
Answer:
A compound containing hydrogen and carbon atoms only.
Question 6 · Spec 5.7.1.1
Which is a hydrocarbon: CH₄, CO₂ or C₂H₅OH?
Answer:
CH₄. The other formulae contain oxygen, so they are not hydrocarbons.
Question 7 · Spec 5.7.1.1
Which family contains most hydrocarbons in crude oil?
Answer:
The alkanes.
Question 8 · Spec 5.7.1.1
State the general formula of alkanes.
Answer:
CnH2n+2, where n is the number of carbon atoms.
Question 9 · Spec 5.7.1.1
Give the name and formula of the first alkane.
Answer:
Methane, CH₄.
Question 10 · Spec 5.7.1.1
Give the name and formula of the second alkane.
Answer:
Ethane, C₂H₆.
Question 11 · Spec 5.7.1.1
Give the name and formula of the third alkane.
Answer:
Propane, C₃H₈.
Question 12 · Spec 5.7.1.1
Give the name and formula of the fourth alkane.
Answer:
Butane, C₄H₁₀.
Question 13 · Spec 5.7.1.1
Use the alkane general formula to find the formula when n = 6.
Answer:
H count = 2 × 6 + 2 = 14, so the formula is C₆H₁₄.
Question 14 · Spec 5.7.1.1
How can a displayed formula distinguish ethane from propane?
Answer:
Ethane has a chain of two carbon atoms; propane has three. Each carbon forms four bonds overall.
Question 15 · Spec 5.7.1.1
How many atoms are in one butane molecule, C₄H₁₀?
Answer:
4 + 10 = 14 atoms: four carbon and ten hydrogen.
Question 16 · Spec 5.7.1.1
Does C₅H₁₂ fit the alkane general formula?
Answer:
Yes. For n = 5, 2n + 2 = 12 hydrogen atoms.
Question 17 · Spec 5.7.1.2
What is a crude-oil fraction?
Answer:
A mixture of hydrocarbons with similar boiling points and usually similar numbers of carbon atoms.
Question 18 · Spec 5.7.1.2
Why is a fraction not necessarily a pure substance?
Answer:
Several different hydrocarbons can have similar boiling points and be collected together.
Question 19 · Spec 5.7.1.2
What happens to crude oil before it enters a fractionating column?
Answer:
It is heated so that much of it vaporises.
Question 20 · Spec 5.7.1.2
How does temperature vary up a fractionating column?
Answer:
The column is hottest at the bottom and cooler towards the top.
Question 21 · Spec 5.7.1.2
How does fractional distillation separate hydrocarbons?
Answer:
Vapours rise and condense at different heights according to their boiling points, allowing fractions to be collected.
Question 22 · Spec 5.7.1.2
Where do short-chain, low-boiling hydrocarbons usually leave the column?
Answer:
Near the cooler top; some remain as gases.
Question 23 · Spec 5.7.1.2
Where are larger, higher-boiling hydrocarbons collected?
Answer:
Lower in the column, where temperatures are higher; very heavy material remains near the bottom.
Question 24 · Spec 5.7.1.2
What is a petrochemical feedstock?
Answer:
A raw material used to manufacture other chemicals, rather than simply being burned as fuel.
Question 25 · Spec 5.7.1.2
Name four fuels obtained from crude oil.
Answer:
Petrol, diesel oil, kerosene and liquefied petroleum gases; heavy fuel oil is another example.
Question 26 · Spec 5.7.1.2
Name four useful petrochemical products besides fuels.
Answer:
Solvents, lubricants, polymers and detergents.
Question 27 · Spec 5.7.1.2
Why can carbon form such a wide range of useful compounds?
Answer:
Carbon atoms can bond to one another in many different chains and ring structures.
Question 28 · Spec 5.7.1.3
How does boiling point change as hydrocarbon molecules get larger?
Answer:
Boiling point generally increases.
Question 29 · Spec 5.7.1.3
How does viscosity change as hydrocarbon molecules get larger?
Answer:
Viscosity generally increases: the liquids flow less easily.
Question 30 · Spec 5.7.1.3
How does flammability change as hydrocarbon molecules get larger?
Answer:
They generally become less easy to ignite.
Question 31 · Spec 5.7.1.3
Why are smaller hydrocarbons useful as fuels?
Answer:
They are generally more flammable and flow more easily, making them suitable for many fuel applications.
Question 32 · Spec 5.7.1.3
What happens to carbon and hydrogen during complete combustion?
Answer:
They are oxidised: carbon forms carbon dioxide and hydrogen forms water.
Question 33 · Spec 5.7.1.3
What oxygen condition is needed for complete hydrocarbon combustion?
Answer:
A sufficient oxygen supply, allowing carbon to be fully oxidised to carbon dioxide.
Question 34 · Spec 5.7.1.3
Write the balanced equation for complete combustion of methane.
Answer:
CH₄ + 2O₂ → CO₂ + 2H₂O.
Question 35 · Spec 5.7.1.3
Write the balanced equation for complete combustion of ethane.
Answer:
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Question 36 · Spec 5.7.1.3
Write the balanced equation for complete combustion of propane.
Answer:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Question 37 · Spec 5.7.1.3
Write the balanced equation for complete combustion of butane.
Answer:
2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.
Question 38 · Spec 5.7.1.3
Why does burning a hydrocarbon warm the surroundings?
Answer:
Combustion is exothermic, transferring energy to the surroundings.
Question 39 · Spec 5.7.1.4
What is cracking?
Answer:
Breaking larger hydrocarbon molecules into smaller, more useful molecules.
Question 40 · Spec 5.7.1.4
Why is cracking economically useful?
Answer:
It converts less-demanded large hydrocarbons into smaller fuel molecules and useful chemical feedstocks.
Question 41 · Spec 5.7.1.4
What are the general conditions for catalytic cracking?
Answer:
Vaporise the hydrocarbons and pass the vapour over a hot catalyst.
Question 42 · Spec 5.7.1.4
What are the general conditions for steam cracking?
Answer:
Mix hydrocarbon vapour with steam and heat to a very high temperature.
Question 43 · Spec 5.7.1.4
Which two hydrocarbon families are commonly produced by cracking?
Answer:
Alkanes and alkenes.
Question 44 · Spec 5.7.1.4
How do alkenes react with bromine water?
Answer:
They decolourise it from orange to colourless, providing a test for alkenes.
Question 45 · Spec 5.7.1.4
Complete C₁₀H₂₂ → C₈H₁₈ + ___.
Answer:
C₂H₄, because both sides must contain ten carbon and twenty-two hydrogen atoms overall.
Question 46 · Spec 5.7.1.4
Complete C₁₂H₂₆ → C₉H₂₀ + ___.
Answer:
C₃H₆, balancing the remaining three carbon and six hydrogen atoms.
Question 47 · Spec 5.7.1.4
Balance C₁₄H₃₀ → C₁₀H₂₂ + __ C₂H₄.
Answer:
The coefficient is 2: C₁₄H₃₀ → C₁₀H₂₂ + 2C₂H₄.
Question 48 · Spec 5.7.1.4
Demand for small fuel molecules exceeds their natural fraction supply. How can cracking help?
Answer:
It breaks surplus larger hydrocarbons into smaller molecules, increasing useful fuel supply.
Question 49 · Spec 5.7.1.4
What are alkenes used for after cracking?
Answer:
Making polymers and serving as starting materials for many other chemicals.
Question 50 · Spec 5.7.1.4
Compare fractional distillation with cracking.
Answer:
Fractional distillation physically separates existing molecules; cracking chemically changes large molecules into smaller ones.

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