4.2 Crude Oil

REVISION NOTES

IGCSE Edexcel Chemistry

4.2 Crude Oil

4.2.1 Know that crude oil is a mixture of hydrocarbons

CRUDE OIL

Mixture of hydrocarbons
Contains molecules in which carbon atoms are in chains or in rings
Important source of useful substances
Finite source

4.2.2 Describe how the industrial process of fractional distillation separates crude oil into fractions

FRACTIONAL DISTILLATION

Crude oil is separated by fractional distillation
Fractional distillation is based on different boiling points of hydrocarbons
- The longer the carbon chain, the higher the boiling point

PROCESS IN SEPARATING CRUE OILS TO FRACTIONS

Crude oil is heated
Vaporised oil rises up the fractionating column
Fractionating column is hotter at the bottom than at top
Shorter hydrocarbons have lower boiling point and rise towards the top

4.2.3 Know the names and uses of the main fractions obtained from crude oil: refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen

4.2.4 Know the trend in colour, boiling point and viscosity of the main fractions

The sizes of hydrocarbon molecules affect some of their properties
If a molecule is longer, then:
- Higher boiling point
- Higher temperature at which the fraction vaporises or condenses
- More viscous (less runny)
- Darker in colour
This is due to stronger intermolecular forces attraction between larger molecules

4.2.5 Know that a fuel is a substance that, when burned, releases heat energy

FUEL

A substance that releases energy as heat when burned

4.2.6 Know the possible products of complete and incomplete combustion of hydrocarbons with oxygen in the air

COMPLETE COMBUSTION OF HYDROCARBONS

hydrocarbon + oxygen [sufficient] → carbon dioxide + water

INCOMPLETE COMBUSTION OF HYDROCARBONS

hydrocarbon + oxygen [insufficient] → carbon / carbon monoxide + water

4.2.7 understand why carbon monoxide is poisonous, in terms of its effect on the capacity of blood to transport oxygen

references to haemoglobin are not required

CARBON MONOXIDE

Red blood cells in the blood carry oxygen around the body
When carbon monoxide and oxygen are present at the same time, red blood cells tend to be more attracted to carbon monoxide
Carbon monoxide thus prevents red blood cells from carrying oxygen around the body

4.2.8 Know that, in car engines, the temperature reached is high enough to allow nitrogen and oxygen from air to react, forming oxides of nitrogen

FORMATION OF OXIDES OF NITROGEN [TITLE]

Temperature in car engines can be very high
This allows nitrogen (N2) and oxygen (O2) from the air to react
It forms oxides of nitrogen, for example:
- Nitrogen monoxide NO
- Nitrogen dioxide NO2

nitrogen + oxygen → nitrogen monoxide / nitrogen dioxide

N₂ + O₂ → NO / NO₂

4.2.9 Explain how the combustion of some impurities in hydrocarbon fuels results in the formation of sulfur dioxide

FORMATION OF SULPHUR DIOXIDE

Besides carbon and/or hydrogen, most fuels might contain sulphur as impurities
Therefore, when fuels are burned, sulphur can be oxidised to produce sulphur dioxide SO2

sulphur + oxygen → sulphur dioxide

S + O₂ → SO₂

4.2.10 Understand how sulfur dioxide and oxides of nitrogen oxides contribute to acid rain

When sulphur dioxide and oxides of nitrogen are emitted into the atmosphere, they react with water (rainwater) to form acids

2SO₂ + 2H₂O + O₂ → 2H₂SO₄

4NO₂ + 2H₂O + O₂ → 4HNO₃

This makes rainwater even more acidic
When acid rain falls, it:
- Alter the pH in soil and rivers, which then affects the ecosystem
- Corrodes limestone, which damages buildings and statues

4.2.11 Describe 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 C)

LONG-CHAIN ALKANES

Crude oils contain a larger proportion of long-chain alkanes
They are less useful compared to short-chain alkanes
Therefore, it is more beneficial and profitable to convert long-chain alkanes to short-chain alkanes through the process of catalytic cracking

CATALYTIC CRACKING

The conversion of a longer chain alkane into a shorter chain alkane and alkenes
It involves heating the hydrocarbons to vaporise them (600-700oC)
Silica or alumina is used as catalyst (to speed up the process)

4.2.12 explain why cracking is necessary, in terms of the balance between supply and demand for different fractions

Demand for shorter-chain alkanes is greater than that for longer-chain alkanes
However, supply for longer-chain alkanes is greater, therefore, cracking is required to produce smaller-chain alkanes from longer-chain alkanes
Shorter chain alkanes burn more cleanly
Cracking produces alkenes which are used to make polymers