7.1.1 Crude oil, hydrocarbons and alkanes
Define crude oil. (2)
- Finite resource found in rocks
- Remains of ancient biomass consisting mainly of plankton that was buried in mud
- Mixture of a very large no. of compound (hydrocarbons)
Define hydrocarbons (2)
- Molecules made up of hydrogen & carbon only
Alkanes
- Hydrocarbons with single C-C bond
General formula: CnH2n+2
What are the first 4 members of alkanes? (Monkey eat proper big peanut)
| Methane (CH₄) | Ethane (C2H6) |
 |
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| Propane (C3H8) | Butane (C4H10) |
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Properties
- Higher boiling point
- Less flammable
- More viscous
Use of alkanes as fuels
- Coz they burn well & release a lot of heat energy
- Complete combustion takes place in good supply of O2 → forms CO2 and H2O
Cracking alkanes
- Shorter alkanes are in very high demand as fuels & it’s hard to meet this demand
- Larger alkanes are in less demand as fuels as they’re harder to ignite & are more viscous
- Larger alkanes can be broken down by cracking to form small alkanes (used as fuels) and alkenes (used to make polymers and other chemicals)
7.1.2 Fractional distillation and petrochemicals
How to separate mixture of hydrocarbons in crude oil into fractions? (1)
- Fractional distillation
When do we use fractional distillation? (2)
- Separate mixture of different liquids
- with different boiling pt
What can the fractions produce for petrochemical industry? (2)
- Fuels & feedstock
Why is crude oil important?
- We depend on fuels produced from crude oil for our modern lifestyle
- Eg petrol, diesel oil, kerosene, heavy fuel oil & liquefied petroleum gases
Why is petrochemical industry important?
- Many useful materials which we depend are produced
- Eg solvents, lubricants, polymers & detergents
Why does a vast array of natural & synthetic carbon compounds occur?
Due to ability of carbon to form families of similar compounds
Explain how fractional distillation works. (6)
- Crude oil is heated & vaporised as it enters fractional distillation column
- Shorter chain lengths have weaker intermolecular forces & lower boiling pt
- This means they will condense at top of column where it’s cooler
- Longer chain lengths have stronger intermolecular forces & higher boiling pt
- This means they will condense at bottom of column where it’s hotter
- This is due to different chains length of hydrocarbons having different boiling pt so they can be separated & tapped off at different levels as fractions
Why do fractions separate at different temp (1)
Hydrocarbons have different boiling pt so they can be separated & tapped off at different levels as fractions
7.1.3Properties of hydrocarbons
Boiling point – temp which liquid turns to gas
Viscosity – how easily it flows
Flammability – how easily it burns
Increase no of carbon / longer chain of hydrocarbons
- ↑ boiling point
- ↑ viscosity
- ↓ flammability
Combustion
| Complete combustion | Incomplete combustion |
|
Eg 2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g) CH4(g) + O2(g) → C(s) + 2H2O(g) |
Explain why incomplete combustion is dangerous (2)
- CO is made which combines with haemoglobin
- Prevents O2 being carried in blood
- 7.1.4 Cracking and alkenesDefine cracking
- Thermal decomposition
- Break down hydrocarbons to produce smaller & more useful molecules
What are the methods of cracking?
- Catalytic cracking – vaporise alkanes & passed over hot catalyst
- Steam cracking – vaporise alkanes, mixed with steam & heated to high temp
Why is cracking useful?
- High demand for fuels with small molecules (so some products of cracking are useful as fuels)
- Another useful product – alkenes (used to make polymers)
Conditions needed for cracking
- High temp
- Catalyst
Eg Alkane → Alkene + Alkane
C10H22 → C2H4 + C8H18