Fuels and Thermochemistry

The various types of fuel mentioned on the syllabus are the fossil fuels – coal, gas, oil, and hydrogen. Oil chemistry involves the main processes that occur in an oil refinery – fractional distillation, cracking and reforming. There is also a new section on the syllabus pertaining to the car – petrol, auto-ignition, octane number, oxygenates and lead in petrol. One important thing to know about oil chemistry is that the products of an oil refinery are not solely fuels – much of the output is feedstock for the manufacture of hundreds of everyday products.

Definitions and Laws

The main types of heat of reaction are:


Hess’s Law Calculations

Thermochemistry is a frequently appearing topic on examination papers. It poses no great difficulties, but the relevant definitions and laws must be known and understood. The one experiment is the measurement of heat of neutralisation.

Thermochemistry questions always involve Hess’s law calculations. The most reliable method of solving them is as follows:

Stage 1 – Write out the equation for which the heat change is required (if not given in equation form)
Stage 2 – Write out the given information in equation form (if not already given)
Stage 3 – Rearrange the given equations as necessary, so that when all are added together, the equation for which the heat change is required results


Fossil Fuels

Fossil fuels (coal, oil, gas) are sources of hydrocarbons. Coal is used to make coke, needed for metal extraction; gas is mainly used as a fuel, but oil, as well as being a fuel, is the feedstock (the starting material) for the manufacture of thousands of everyday products. In the refining of petroleum (‘crude oil’), the three important processes are:

  • Fractional distillation
  • Cracking
  • Polymerisation


Hydrogen, H2

The few important things known about hydrogen are:

  • How it occurs in nature
  • How it is manufactured industrially
  • Its properties
  • For what it is used
  • The formulae of some of its simple compounds


Petrol and Cars

This is a new section on the course. Important things you should know are:

  • How petrol is made
  • What it consists of (mainly hydrocarbons in the C8 region)
  • Auto ignition
  • Octane number
  • Oxygenates
  • Lead in petrol

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