Carbon compounds as fuels and feedstock

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


  • 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)
Propane (C3H8) Butane (C4H10)


  • 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


Complete combustion Incomplete combustion
  • Blue flame
  • Plenty of O2
  • Eg CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
  • Orange flame
  • Limited by O2

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