Hydrogen versus petrol

For more than a century most cars have been fuelled by petrol or diesel. Hydrogen is another fuel that can be used for cars. It has advantages over petrol but it has disadvantages too.

Production of hydrogen and petrol

Petrol

Petrol is a non-renewable fossil fuel. It is made from crude oil, a finite resource. Some of the substances found in petrol are made by cracking crude oil fractions, a process which uses large amounts of energy.

Hydrogen

Hydrogen is manufactured in several ways. These include:

  • reaction of methane from natural gas with steam
  • cracking of crude oil fractions (hydrogen is a by-product of this process)
  • electrolysis of water

Methane is a non-renewable fossil fuel made from natural gas, a finite resource. The production of hydrogen from natural gas or from crude oil offers no advantage over petrol.

Question

The electrolysis of water needs large amounts of electrical energy. Explain an advantage and a disadvantage of producing hydrogen by the electrolysis of water.

Hydrogen can be renewable if the electricity is generated from renewable resources, such as sunlight and wind. However, most electricity worldwide is generated from coal, oil or natural gas. Hydrogen produced this way will be non-renewable.

Combustion products

Petrol

Incomplete combustion happens in cars fuelled with petrol. This produces:

Question

Carbon dioxide is produced during the combustion of petrol. Describe a problem this causes to the environment.

Carbon dioxide is a greenhouse gas. Increasing amounts of this gas are linked to global warming and climate change.

Hydrogen

Hydrogen usually provides the fuel for hydrogen-oxygen fuel cells in cars. However, it can also be used in some internal combustion engines (the type of engines found in petrol cars). However, water is the only product of combustion when hydrogen is used.

There are far fewer hydrogen filling stations than petrol filling stations. Depending on where you live, this could make it more difficult to run a car fuelled by hydrogen.

Filling a car at a filling station
Filling a car at a filling station

Other features

The table shows some other features of hydrogen and petrol as fuel for cars.

HydrogenPetrol
Ease of ignitionVery flammable - ignites easilyVery flammable - ignites easily
Energy released per kg of fuel (MJ)14246
State at room temperature and pressureGasLiquid
Question

Suggest an explanation for why hydrogen is usually liquefied by cooling or stored as a gas under pressure.

Hydrogen is a gas at room temperature and pressure. Its molecules are spread far apart so not very much can be stored in a given volume. If it is liquefied or pressurised, more can be stored.

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