CIE IGCSE Chemistry

Revision Notes

6.1.2 Energy Transfer

Fuel, Combustion, Hydrogen

Energy from fuels

  • A fuel is a substance which releases energy when burned
  • When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions
  • Propane for example undergoes combustion according to the following equation:
C3H8 + 5O2 → 3CO2 + 4H2O ΔH = -2219 kJ/mol
  • The efficiency of a fuel refers to how much energy is released per unit amount
  • We can measure the efficiency of fuels by calorimetry
  • A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion
  • Different fuels heat the water by different amounts and they can be analysed and compared in this way

 

Calorimetry experiment – combustion

Using calorimetry to investigate energy release in combustion reactions, IGCSE & GCSE Chemistry revision notesDiagram showing the calorimetry experiment for combustion

 

Method:

  • Using a measuring cylinder, place 100 cm3 of water into a copper can
  • Measure and record the initial temperature of the water
  • Fill the spirit burner with test substance and measure and record its mass
  • Place the burner under the copper can and light the wick
  • Stir the water constantly with the thermometer and continue heating until the spirit burner burns out
  • Measure and record the highest temperature of the water

Calculation:

Temperature change of water = final temperature – initial temperature

Number of moles burned = change in mass ÷ molecular mass

Amount of energy = change in temperature x mass of water x specific heat capacity

Amount of energy per mole (J mol-1) = total amount of energy ÷  moles burned

 

Hydrogen as a fuel

  • Hydrogen is used in rocket engines and in fuel cells to power some cars
  • Hydrogen has a series of advantages and disadvantages regarding its use as a fuel
  • Advantages:
    • It releases more energy per kilogram than any other fuel (except for nuclear fuels)
    • It does not pollute as it only produces water on combustion, no other product is formed
  • Disadvantages:
    • Expensive to produce and requires energy for the production process
    • Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers)

Radioactive isotopes as fuels

  • Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness
  • The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity
  • Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur
  • But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released

 

Nuclear-fission, IGCSE & GCSE Chemistry revision notesThe nuclear fission of a large nucleus of uranium-235 into smaller daughter nuclei

Extended Only

Fuel Cells

The hydrogen fuel cell

  • A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
  • These cells are becoming more common in the automotive industry to replace petrol or diesel engines
  • H2 and O2 are pumped through two porous electrodes where the half-reactions occur
  • The following reaction occurs at the cathode:
2H2 → 4H+ + 4e-
  • At the anode the following reaction takes place:
4H+ + O2 + 4e- → 2H2O
  • The overall reaction is:
2H2 + O2 → 2H2O
  • The electrons move around the external circuit from the cathode to the anode
  • This movement of electrons is used to drive an electric motor

 

Hydrogen-oxygen fuel cell, IGCSE & GCSE Chemistry revision notesDiagram showing the movement of hydrogen, oxygen and electrons in a Hydrogen-Oxygen fuel cell

 

Advantages and disadvantages of fuel cells

  • Advantages
    • They do not produce any pollution
    • They produce more energy per kilogram than either petrol or diesel
    • No power is lost on transmission as there are far fewer moving parts than in an internal combustion engine
  • Disadvantages
    • Materials used in producing fuel cells are expensive
    • High-pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts
    • Fuel cells are affected by low temperatures, becoming less efficient
    • Hydrogen is expensive to produce and store

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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