Hydrocarbons: Fuel & Feedstock (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards
Exam code: 8464
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What is a homologous series?
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What is a homologous series?
A homologous series is a group of organic compounds with the same functional group and the same general formula, with similar chemical properties and gradually changing physical properties. Each member differs from the next by CH2.
What does saturated mean for alkanes, and what is the general formula for the alkane homologous series?
Saturated means the molecule contains only single carbon-carbon bonds (no double bonds).
The general formula for alkanes is CnH2n+2.
True or False?
A hydrocarbon is a compound that contains carbon and hydrogen atoms.
False (incomplete).
A hydrocarbon is a compound that contains carbon and hydrogen atoms only.
Missing the word "only" will lose a mark in an exam, as any compound containing C and H atoms could qualify otherwise.
Complete the molecular formulae of the first four alkanes:
Alkane
Molecuar formula
Methane (1C)
Ethane (2C)
Propane (3C)
Butane (4C)
Alkane
Molecuar formula
Methane (1C)
CH4
Ethane (2C)
C2H6
Propane (3C)
C3H8
Butane (4C)
C4H10
How was crude oil formed, and why is it described as a finite resource?
Crude oil formed over millions of years from the effects of high pressure and temperature on buried plankton (and other biomass).
It is a finite resource because it is used up far faster than it is formed.
A common error is stating it formed from "hydrocarbons" (this confuses decomposition with composition).
Why do the boiling points of alkanes increase as the length of the carbon chain increases?
As chain length increases, the intermolecular forces between molecules get stronger (more surface area for forces to act). More energy is needed to overcome these forces, so the boiling point rises.
Do not say "covalent bonds" are overcome. It is the forces between molecules, not within them.
What is a fraction of crude oil, and how are fractions separated?
A fraction is a group of hydrocarbons with similar chain lengths and similar boiling points.
Fractions are separated from crude oil by fractional distillation, which exploits differences in boiling point to separate the mixture physically.
What are the four stages of fractional distillation of crude oil, in the correct order?
Crude oil is heated (before/outside the column)
The hydrocarbons vaporise
The vapours enter a column with a temperature gradient (hot at bottom, cooler at top)
Fractions condense at different heights in the column because they have different boiling points.
True or False?
During fractional distillation, the crude oil is heated inside the fractionating column.
False.
The crude oil is heated before it enters the column.
Stating that it is heated inside the column does not gain credit. Heating before entry causes the hydrocarbons to vaporise before they rise through the column.
Give a use for each fraction:
Fraction
Use
LPG
Petrol
Kerosene
Diesel
Heavy fuel oil
Fraction
Use
LPG
domestic heating and cooking
Petrol
fuel for cars
Kerosene
jet fuel
Diesel
diesel engines
Heavy fuel oil
ships and power stations
What two things must you include when explaining how fractional distillation separates crude oil fractions?
You must state that:
Fractions condense at different heights in the column
This is because they have different boiling points.
Stating only that fractions have "different sizes and boiling points" does not explain the separation process.
True or False?
Fractional distillation and cracking are both processes that involve a chemical change.
False.
Fractional distillation is a physical separation: no chemical change occurs.
Cracking is a chemical change (thermal decomposition) and involves a catalyst.
These are two distinct processes and must not be confused.
How does molecular size affect the boiling point and viscosity of hydrocarbons?
As molecular size increases: boiling point increases (stronger intermolecular forces require more energy to overcome) and viscosity increases (larger molecules flow less easily).
Both trends have the same cause: stronger intermolecular forces between larger molecules.
True or False?
Larger hydrocarbon molecules are more flammable than smaller ones.
False.
Smaller hydrocarbon molecules are more flammable and ignite more easily.
This is why small-chain fractions (such as petrol) are more useful as fuels, while larger-chain fractions are more useful as lubricants.
What are the products of the complete combustion of a hydrocarbon, and why does complete combustion require sufficient oxygen?
Complete combustion produces carbon dioxide (CO2) and water (H2O).
Sufficient oxygen is needed to fully oxidise both carbon (to CO2) and hydrogen (to H2O). Without enough oxygen, incomplete combustion occurs instead.
Complete the balanced equation for the complete combustion of methane:
CH4 + __O2 → ________ + ________
The balanced equation for the complete combustion of methane is:
CH4 + 2O2 → CO2 + 2H2O
What are the products of incomplete combustion of hydrocarbons, and when does it occur?
Incomplete combustion occurs when there is insufficient oxygen.
Products include carbon monoxide (CO), soot (carbon particles), and unburnt hydrocarbons, as well as water. Carbon monoxide is toxic, colourless and odourless.
Mark schemes explicitly ignore the word "clear", so you must say "colourless" or "odourless"
When balancing a combustion equation, in what order should you balance elements, and what do you do if the number of oxygen atoms is a fraction?
Balance in this order:
Carbon
Hydrogen
Oxygen
If the oxygen count gives a fraction (e.g. 2.5), multiply every coefficient by 2 to obtain whole numbers throughout.
What is cracking and why is it needed?
Cracking breaks large, long-chain hydrocarbon molecules into smaller, more useful ones.
It is needed because demand exceeds supply for short-chain fractions (petrol, kerosene), while long-chain fractions are in excess. Products include smaller alkanes (fuels) and alkenes (for polymers).
True or False?
A valid condition for cracking is to describe the process as requiring heat or hot.
False.
Mark schemes explicitly ignore the words "heat" or "hot".
To gain the mark you must state "high temperature" (or quote a specific temperature: 400–900 °C for catalytic cracking).
Other accepted conditions: catalyst (aluminium oxide/alumina/zeolites), steam, high pressure, low/no oxygen.
Describe the bromine water test for an alkene. What result is seen with an alkene, and what are two common errors?
Add bromine water (orange) to the substance.
With an alkene: the solution turns colourless as bromine adds across the C=C double bond. With an alkane: the solution stays orange.
Common errors include writing:
"clear" instead of "colourless"
"bromide" instead of "bromine water"
Complete the cracking equation by identifying the missing product:
C6H14 → C4H10 + ________
Is the missing product an alkane or an alkene? How do you know?
The cracking equation is:
C6H14 → C4H10 + C2H4
The missing product is an alkene:
C = 6 − 4 = 2; H = 14 − 10 = 4.
C2H4 fits the alkene general formula CnH2n (2×2=4).
Alkane check: 2×2+2=6 ≠ 4, so it is not an alkane.
Why are the two main products of cracking useful? Give a property-linked reason for each.
Smaller alkanes (e.g. ethane, butane): useful as fuels because they are more flammable than large-chain hydrocarbons.
Alkenes (e.g. ethene): used to make polymers (plastics) because they are more reactive than alkanes, due to the C=C double bond.
What is the general formula for alkenes, and how does it differ from the general formula for alkanes?
Alkenes: CnH2n (unsaturated; contains at least one C=C double bond).
Alkanes: CnH2n+2 (saturated; only C-C single bonds).
Alkenes have two fewer hydrogen atoms per molecule than the corresponding alkane, due to the double bond.
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