Petrol and hydrogen burn to different products
- Petrol is a hydrocarbon fuel, so burning it gives carbon dioxide and water.
- Hydrogen burns to give water alone: 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}2H2+O2→2H2O
- No carbon is present in hydrogen, so no carbon dioxide can form.
- No carbon monoxide or soot is possible either, whatever the air supply.
- That difference in products is the starting point for comparing the two fuels.
Key Idea
Hydrogen's advantage is the carbon it does not produce, and a fuel cell avoids the nitrogen oxides too.
The advantages of hydrogen
- Burning hydrogen produces no carbon dioxide, so it adds nothing to the greenhouse effect at the point of use.
- It produces none of the carbon-based pollutants, though a hot engine still makes oxides of nitrogen from the air.
- Hydrogen can be made from water, which is not a finite resource in the way crude oil is.
- It releases a large amount of energy per gram compared with petrol.
- A hydrogen vehicle can be refuelled in minutes, unlike a battery that must recharge.
Example
- Petrol exhaust: carbon dioxide, water, and pollutants from impurities and from the air.
- Hydrogen exhaust: water, plus oxides of nitrogen when it is burned in a hot engine.
The disadvantages of hydrogen
- Hydrogen is a gas, so a large volume is needed unless it is compressed or cooled.
- Storing it under high pressure makes the tank heavy and costly.
- It is highly flammable, and a leak mixes with air to form an explosive mixture.
- There are very few filling stations, so a vehicle cannot be refuelled everywhere.
- Most hydrogen is currently made from natural gas, which releases carbon dioxide elsewhere.
Common Mistake
Water is the only product in a fuel cell, where there is no flame hot enough to make nitrogen react.
Petrol, kerosene, diesel and methane are non-renewable
Definition
Fossil fuel
A fuel formed over millions of years from the remains of living organisms, which is used up far faster than it forms.
- Petrol, kerosene and diesel oil are all obtained from crude oil.
- Methane is the main constituent of natural gas.
- All four are fossil fuels, formed from the remains of organisms over millions of years.
- They form far more slowly than they are used, so they are non-renewable.
- A non-renewable fuel cannot be replaced once it has been burned.
Exam technique
- An evaluation gives points on both sides and then reaches a conclusion.
- A judgement is worth tying to a use, since hydrogen suits some vehicles better than others.
- Where the hydrogen came from belongs in the answer, not just what leaves the exhaust.
Self review
- What is the only product of burning hydrogen?
- Give two advantages of hydrogen over petrol as a car fuel.
- Give two disadvantages of using hydrogen in a car.
- Why is it misleading to call a hydrogen car completely clean?
- Name four non-renewable fossil fuels and say where each comes from.
