(b) Crude oil
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
4.7Crude oil as a hydrocarbon mixtureKnow that crude oil is a mixture of hydrocarbons4.8Fractional distillation of crude oilDescribe how the industrial process of fractional distillation separates crude oil into fractions4.9Crude-oil fractions and usesKnow the names and uses of the main fractions obtained from crude oil: refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen4.10Trends in fraction propertiesKnow the trend in colour, boiling point and viscosity of the main fractions4.11FuelsKnow that a fuel is a substance that, when burned, releases heat energy4.12Hydrocarbon combustion productsKnow the possible products of complete and incomplete combustion of hydrocarbons with oxygen in the air4.13Carbon monoxide toxicityUnderstand why carbon monoxide is poisonous, in terms of its effect on the capacity of blood to transport oxygen references to haemoglobin are not required4.14Nitrogen oxides in enginesKnow that, in car engines, the temperature reached is high enough to allow nitrogen and oxygen from air to react, forming oxides of nitrogen4.15Sulfur impurities and sulfur dioxideExplain how the combustion of some impurities in hydrocarbon fuels results in the formation of sulfur dioxide4.16Acid rainUnderstand how sulfur dioxide and oxides of nitrogen contribute to acid rain4.17Catalytic crackingDescribe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (using silica or alumina as the catalyst and a temperature in the range of 600-700 ºC)4.18Why cracking is necessaryExplain why cracking is necessary, in terms of the balance between supply and demand for different fractions
Recognise crude oil as a hydrocarbon mixture
Crude oil is a mixture of many hydrocarbons: compounds made from hydrogen and carbon only.
The hydrocarbons have different molecule sizes and structures, so they have different physical properties. Because they are mixed rather than chemically bonded into one substance, physical separation can group them into useful fractions.
| Crude oil is | Crude oil is not |
|---|---|
| a mixture containing many hydrocarbon compounds | one pure hydrocarbon |
| variable in composition | a compound with one fixed formula |
| separable into fractions by physical properties | separated by breaking covalent bonds during distillation |
A fraction obtained from crude oil is still a mixture of hydrocarbons with similar boiling points; it is not normally one pure compound.
Follow fractional distillation from furnace to fraction
Industrial fractional distillation separates crude oil because its hydrocarbons have different boiling-point ranges. It separates physically; no covalent bonds are broken.
| Stage | What happens |
|---|---|
| 1 | crude oil is heated strongly so most of it vaporises |
| 2 | the vapour enters near the bottom of a fractionating column |
| 3 | the column is hot at the bottom and becomes cooler toward the top |
| 4 | vapours rise, cool and condense at different heights according to boiling point |
| 5 | condensed liquids are drawn off as fractions; refinery gases leave at the top and bitumen remains near the bottom |
Large, high-boiling molecules condense low in the hot column. Smaller, lower-boiling molecules rise farther before condensing; the lowest-boiling hydrocarbons remain gases and leave from the top.
A fraction condenses over a boiling range because it contains several hydrocarbons. Do not describe cracking or a laboratory flask: the required process is an industrial fractionating column with a temperature gradient.
Match the six main crude-oil fractions to their uses
The main crude-oil fractions are named by their boiling ranges and chosen for uses that fit their physical properties.
| Fraction, from top toward bottom | Main use |
|---|---|
| refinery gases | bottled gases for heating and cooking |
| gasoline | fuel for cars |
| kerosene | aircraft fuel |
| diesel | fuel for diesel engines |
| fuel oil | fuel for ships and some power stations |
| bitumen | surfacing roads and roofing |
Fractions near the top contain smaller, more easily vaporised molecules and are commonly used as mobile fuels. The very viscous residue at the bottom suits waterproofing and road surfaces rather than vaporising as an engine fuel.
Use the specification names exactly: gasoline is the car-fuel fraction, kerosene is the aircraft-fuel fraction, and fuel oil—not diesel—is the named ship-fuel fraction in this list.
Track property trends down the fractionating column
From the top fractions toward the bottom fractions, boiling point and viscosity increase, and colour becomes darker.
| Direction through main fractions | Molecules | Boiling point | Viscosity | Colour |
|---|---|---|---|---|
| top → bottom | generally larger / longer | increases | increases; flows less easily | becomes darker |
| bottom → top | generally smaller / shorter | decreases | decreases; flows more easily | becomes paler |
Larger hydrocarbon molecules have stronger intermolecular attractions overall, so more energy is needed to separate them during boiling. They also move past one another less easily, producing greater viscosity.
Viscosity means resistance to flow, not density. A more viscous fraction flows more slowly; it is not described as having a lower boiling point.
Define a fuel by what burning releases
A fuel is a substance that releases heat energy when it burns.
Burning is combustion: the fuel reacts with oxygen and transfers chemical energy to the surroundings as heat. A useful fuel must therefore release energy during combustion, rather than merely being flammable in name.
Gasoline, kerosene and diesel are used as fuels because their hydrocarbons combust in oxygen and the released heat can power engines.
A substance is not defined as a fuel simply because it contains stored chemical energy. The definition requires heat energy to be released when the substance is burned.
Predict complete and incomplete combustion products
Hydrocarbon combustion always forms water from hydrogen. The carbon product depends on the oxygen supply.
| Oxygen supply | Type | Possible products |
|---|---|---|
| plentiful | complete combustion | carbon dioxide and water |
| limited | incomplete combustion | carbon monoxide and water, and/or carbon (soot) and water |
\ce{CH4 + 2O2 -> CO2 + 2H2O}
\ce{2CH4 + 3O2 -> 2CO + 4H2O}
When balancing, keep the hydrocarbon formula unchanged: balance carbon first, hydrogen second and oxygen last. Limited oxygen does not mean no oxygen; it means there is insufficient oxygen to oxidise all carbon fully to carbon dioxide.
Carbon monoxide and soot are alternative or simultaneous products of incomplete combustion. Do not list hydrogen gas, and do not omit water simply because the question emphasises the carbon-containing product.
Explain carbon monoxide poisoning through oxygen transport
Carbon monoxide is poisonous because it reduces the capacity of blood to transport oxygen.
Incomplete combustion in a limited oxygen supply can produce colourless carbon monoxide. If it is inhaled, less oxygen is delivered by the blood to body tissues, so aerobic respiration cannot be sustained normally.
A faulty or poorly ventilated fuel-burning appliance is dangerous because carbon monoxide can accumulate without visible soot being a reliable warning.
The required explanation is reduced oxygen-carrying capacity of blood. References to haemoglobin are not required, and saying only that carbon monoxide is ‘toxic’ does not explain why.
Explain why car engines form oxides of nitrogen
Inside a car engine, the temperature becomes high enough for nitrogen and oxygen from the air to react, forming oxides of nitrogen.
\ce{N2(g) + O2(g) ->[high\ temperature] 2NO(g)}
Both reactants come from the air drawn into the engine. The high temperature supplies the energy needed for normally unreactive nitrogen and oxygen molecules to react; further oxidation can form other nitrogen oxides.
The nitrogen does not need to be present in the fuel. Oxides of nitrogen form because air is heated strongly in the engine, not because nitrogen is a normal hydrocarbon impurity.
Trace sulfur impurities to sulfur dioxide
Some hydrocarbon fuels contain sulfur impurities. When the fuel burns, the sulfur also reacts with oxygen and forms sulfur dioxide.
\ce{S(s) + O2(g) -> SO2(g)}
sulfur impurity in fuel → combustion in air → sulfur dioxide released with the exhaust gases
Sulfur dioxide is produced from sulfur impurities, not from the carbon and hydrogen of a pure hydrocarbon. The hydrocarbon’s combustion products and the impurity’s combustion product must be traced separately.
Connect sulfur and nitrogen oxides to acid rain
Sulfur dioxide and oxides of nitrogen contribute to acid rain by entering the atmosphere and forming acidic solutions in cloud and rain water.
| Pollutant | Main source in this Topic | Route to acid rain |
|---|---|---|
| sulfur dioxide, SOX2 | burning sulfur impurities in fuels | dissolves and reacts in atmospheric water, producing acidic rain |
| oxides of nitrogen, NOXx | high-temperature reaction of nitrogen and oxygen in engines | react with oxygen and water in the atmosphere, producing acidic rain |
Acid rain can acidify lakes and soils, harm aquatic life and plants, and react with carbonate stone in buildings and statues.
Acid rain is not the same environmental problem as the greenhouse effect. Carbon dioxide contributes to climate warming, whereas the required pollutants here are sulfur dioxide and oxides of nitrogen.
Describe catalytic cracking with exact conditions and products
Catalytic cracking breaks long-chain alkanes into shorter-chain alkanes and alkenes.
| Stage | Industrial change |
|---|---|
| 1 | vaporise a long-chain alkane fraction |
| 2 | pass the vapour over hot silica or alumina catalyst |
| 3 | maintain a temperature in the range 600–700∘C |
| 4 | collect a mixture containing shorter-chain alkanes and alkenes |
\ce{C10H22 -> C4H10 + C2H4 + C4H8}
A valid cracking equation conserves every carbon and hydrogen atom. At least one product is an alkene, so its formula follows the alkene pattern rather than the alkane pattern.
Cracking is a chemical reaction that breaks C–C bonds; fractional distillation is a physical separation and does not change molecule size. The required catalysts are silica or alumina, not a generic metal catalyst.
Explain cracking through the supply–demand mismatch
Cracking is necessary because fractional distillation produces fractions in proportions that do not match demand.
| Before cracking | What cracking produces | Why this helps |
|---|---|---|
| surplus / lower demand for some long-chain fractions | shorter-chain alkanes | helps meet higher demand for useful fuels such as gasoline components |
| demand for reactive small molecules | alkenes | supplies feedstock for making polymers and other chemicals |
Crude oil supply is fixed by its natural composition, but market demand is different. Converting less-demanded long chains into more-demanded short chains and alkenes improves the balance between what refineries obtain and what users need.
Cracking does not create more total carbon or merely separate an existing fraction. It chemically redistributes atoms into smaller molecules, changing the product mix to address supply and demand.