1. Petroleum Fractional Distillation
Alkanes are saturated hydrocarbons with the general formula \(\text{C}_n\text{H}_{2n+2}\). Petroleum (crude oil) is a mixture consisting mainly of alkane hydrocarbons, both branched and unbranched. The fractions have different boiling points, so they can be separated by fractional distillation:
Operation of the Fractionating Tower
- Crude oil is vaporised in a furnace at approximately 350°C and pumped into the bottom of the fractionating column.
- The column has a temperature gradient: hot at the bottom and cooler at the top.
- Vapour rises up through bubble caps. As fractions reach a tray where the temperature is just below their boiling point, they condense into liquid and are drawn off.
- Smaller molecules with lower boiling points rise to the top; larger molecules with higher boiling points condense near the base.
2. Industrial Cracking of Hydrocarbons
Cracking breaks long-chain alkanes into shorter alkanes and alkenes. For example:
\[ \text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4 \]
There is more demand for shorter-chain fractions (such as petrol) and for alkenes than crude oil supplies. Cracking turns low-demand long-chain fractions into these more valuable products. Alkenes are used to make polymers and other chemicals.
Thermal Cracking
- Conditions: High temperature (700-1200 K) and high pressure (up to 70 atm).
- Beyond the OxfordAQA specification (mechanism not required): homolytic C-C bond fission forming free radicals.
- Major Products: High percentage of alkenes (e.g. ethene, propene) used as feedstocks for addition polymerisation.
Catalytic Cracking
- Conditions: High temperature (about 720 K), slight pressure (just above 1 atm), and a zeolite catalyst (aluminosilicate with honeycombed acidic active sites).
- Beyond the OxfordAQA specification (mechanism not required): carbocation intermediate route.
- Major Products: Branched alkanes, cycloalkanes, and aromatic hydrocarbons with high octane numbers for motor fuels.
3. Combustion and Environmental Pollutants
- Complete Combustion: Excess oxygen produces CO₂ and H₂O: \[ \text{C}_8\text{H}_{18} + 12.5\text{O}_2 \rightarrow 8\text{CO}_2 + 9\text{H}_2\text{O} \]
- Incomplete Combustion: Limited oxygen produces toxic carbon monoxide (CO) and carbon particulates (soot). CO binds with very high affinity to haemoglobin in red blood cells, displacing oxygen and preventing oxygen transport. \[ \text{C}_8\text{H}_{18} + 8.5\text{O}_2 \rightarrow 8\text{CO} + 9\text{H}_2\text{O} \] \[ \text{C}_8\text{H}_{18} + 4.5\text{O}_2 \rightarrow 8\text{C} + 9\text{H}_2\text{O} \]
Pollutants from Engines
| Pollutant | How it forms | Problem |
|---|---|---|
| Nitrogen oxides (NOx) | The high temperature in the engine makes nitrogen and oxygen from the air react: N₂ + O₂ → 2NO. | Acid rain and photochemical smog. |
| Carbon monoxide (CO) | Incomplete combustion. | Toxic. |
| Carbon (soot) | Incomplete combustion. | Breathing problems; blackens buildings. |
| Unburned hydrocarbons | Some fuel passes through the engine without burning. | Help to form photochemical smog. |
Catalytic Converters
Exhaust gases pass through a ceramic honeycomb coated with a thin layer of platinum and rhodium (Pt/Rh). The honeycomb gives a large surface area, so little of the expensive metal is needed.
\[ 2\text{CO}(g) + 2\text{NO}(g) \rightarrow 2\text{CO}_2(g) + \text{N}_2(g) \]
Unburned hydrocarbons are also oxidised to carbon dioxide and water.
Removing Sulfur Dioxide
Fossil fuels contain sulfur impurities. When the fuel burns, the sulfur forms sulfur dioxide, which causes acid rain:
\[ \text{S} + \text{O}_2 \rightarrow \text{SO}_2 \]
- SO₂ is an acidic oxide, so it can be removed by reacting it with a base.
- In power stations, the waste gases are passed through calcium oxide or calcium carbonate (flue gas desulfurisation): \[ \text{CaO} + \text{SO}_2 \rightarrow \text{CaSO}_3 \] \[ \text{CaCO}_3 + \text{SO}_2 \rightarrow \text{CaSO}_3 + \text{CO}_2 \]
- The calcium sulfite can be oxidised to calcium sulfate (gypsum), which has uses, for example in plaster and plasterboard: \[ 2\text{CaSO}_3 + \text{O}_2 \rightarrow 2\text{CaSO}_4 \]
4. Free-Radical Substitution Mechanism
A chemical species containing an unpaired valence electron, represented with a dot (\(\text{Cl}^\bullet\)). Highly reactive.
Alkanes react with halogens in the presence of ultraviolet (UV) light via a 3-stage chain reaction:
Stage 1: Initiation
UV light provides the energy for homolytic fission of the halogen-halogen bond. Each halogen atom retains one electron from the shared pair:
\[ \text{Cl}_2 \xrightarrow{UV} 2\text{Cl}^\bullet \]
Stage 2: Propagation (Chain Reaction)
Radicals attack molecules to form new radicals in a continuous cycle:
\[ \text{CH}_4 + \text{Cl}^\bullet \rightarrow \text{CH}_3^\bullet + \text{HCl} \]
\[ \text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet \]
Stage 3: Termination
Two free radicals collide and combine their unpaired electrons, removing radicals from the system:
- \(\text{CH}_3^\bullet + \text{Cl}^\bullet \rightarrow \text{CH}_3\text{Cl}\)
- \(\text{CH}_3^\bullet + \text{CH}_3^\bullet \rightarrow \text{C}_2\text{H}_6\) (Produces trace amounts of ethane!)
- \(\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2\)
Limiting Polysubstitution: Excess methane is used to ensure a high yield of chloromethane and minimise further substitution to CH₂Cl₂,CHCl₃, and CCl₄.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
5. Practice Questions
Show Answer & Mark Scheme
Initiation: \[ \text{Br}_2 \xrightarrow{UV} 2\text{Br}^\bullet \] (1 mark)
Propagation Step 1: \[ \text{C}_2\text{H}_6 + \text{Br}^\bullet \rightarrow \text{C}_2\text{H}_5^\bullet + \text{HBr} \] (1 mark)
Propagation Step 2: \[ \text{C}_2\text{H}_5^\bullet + \text{Br}_2 \rightarrow \text{C}_2\text{H}_5\text{Br} + \text{Br}^\bullet \] (1 mark)
Show Answer & Mark Scheme
Answer:
- Slight pressure, high temperature and a zeolite catalyst. (1 mark)
- Motor fuels and aromatic hydrocarbons. (1 mark)
- There is more demand for shorter-chain fractions (petrol) and alkenes than crude oil supplies. (1 mark)
Show Answer & Mark Scheme
Answer:
- The high temperature in the engine makes nitrogen and oxygen from the air react. (1 mark)
- 2CO + 2NO → 2CO₂ + N₂. (1 mark)
- Platinum and rhodium (on a ceramic honeycomb). (1 mark)
Show Answer & Mark Scheme
Answer:
- SO₂ is an acidic gas. (1 mark)
- Calcium carbonate is a base, so it neutralises the SO₂. (1 mark)
- CaCO₃ + SO₂ → CaSO₃ + CO₂. (1 mark)
Show Answer & Mark Scheme
Answer: \[ \text{C}_3\text{H}_8 + 3.5\text{O}_2 \rightarrow 3\text{CO} + 4\text{H}_2\text{O} \] (1 mark)