Combustion (Edexcel IGCSE Chemistry)

Fuels & Combustion

Spec point — know that a fuel is a substance that, when burned, releases heat energy know the possible products of complete and incomplete combustion of hydrocarbons with oxygen in the air

Combustion of fuels

The combustion of fossil fuels

• A fuel is a substance that, when burned, releases heat energy (exothermic reaction)

• The combustion of fossil fuels is the major source of atmospheric pollution

  • Fossil fuels include: coal, oil, natural gas, oil shales and tar sands

• Non-renewable fossil fuels are obtained from crude oil by fractional distillation

• Petrol is used as a fuel in cars, kerosene is used to fuel aircraft and diesel oil is used as a fuel in some cars, trucks and heavy vehicles such as tanks and trains

• Coal is used in power stations and also steel production

• Natural gas consists mainly of methane, CH₄

• There are finite amounts of fossil fuels and they all contribute to pollution and global warming

• All these fuels contain carbon, hydrogen and small quantities of sulfur

Combustion products

• The burning of fossil fuels releases the gases carbon dioxide, carbon monoxide, oxides of nitrogen and oxides of sulfur
• In addition incomplete combustion of the fuels gives rise to unburned hydrocarbons and carbon particulates
• When the fuel is a hydrocarbon then water and carbon dioxide are the products formed
• Hydrocarbon compounds undergo complete and incomplete combustion

Complete combustion

• Complete combustion occurs when there is excess oxygen
• For example, the combustion equation for propane is:

C₃H₈ + 5O₂→ 3CO₂+ 4H₂O

Incomplete combustion

• Incomplete combustion occurs when there is insufficient oxygen to burn
• It occurs in some appliances such as boilers and stoves as well as in internal combustion engines
• The products of these reactions are unburnt fuel (soot), carbon monoxide and water
• Methane for example undergoes incomplete combustion in an oxygen-poor environment:

2CH₄ + 3O₂→ 2CO + 4H₂O

CH₄ + O₂→ C + 2H₂O

Exam Hint You don't need to learn these equations, but you do need to be able to predict the products of combustion given the composition of the fuel and the conditions.

Dangers of Carbon Monoxide

Spec point — 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

Why is carbon monoxide dangerous?

• Carbon monoxide (CO) is a highly poisonous gas produced during the incomplete combustion of carbon-containing fuels

• It is particularly dangerous because it is:

  • Odourless
  • Colorless
  • Tasteless

• This means it can be inhaled without a person realising it is present

How carbon monoxide affects the body

The basic explanation

• When inhaled, carbon monoxide is absorbed from the lungs into the red blood cells

• It binds tightly to the molecules that normally transport oxygen

• This means the blood's capacity to carry oxygen around the body is significantly reduced

• This lack of oxygen supply to tissues and organs leads to symptoms such as:

  • Headaches and dizziness
  • Nausea and vomiting
  • Shortness of breath
  • Loss of consciousness and, in high concentrations, death

The detailed explanation

• Inside red blood cells, haemoglobin is responsible for binding to oxygen in the lungs and transporting it to the body's tissues

• Carbon monoxide can also bind to haemoglobin

• However, haemoglobin has a much higher affinity for carbon monoxide than for oxygen

  • It binds around 200-250 times more strongly

• This means CO binds preferentially and irreversibly to haemoglobin

  • This forms a stable compound called carboxyhaemoglobin

• This process prevents haemoglobin molecules from binding to oxygen and transporting it around the body

The high affinity of carbon monoxide to haemoglobin prevents it from binding to oxygen

Exam Hint

• The exam-board specification clearly talks about how carbon monoxide reduces the capacity of blood to transport oxygen

  • This is the key marking point that examiners look for

• The specfication also states that "references to haemoglobin are not required"

• However, recent mark schemes show that it is acceptable to refer to haemoglobin

• But, you must be sure to state:

  1. Carbon monoxide binds irreversibly to haemoglobin in red blood cells
  2. This reduces the amount of oxygen being carried around the body