How does hydrogen's energy content per kilogram compare with that of petrol?

Hydrogen releases about 120 MJ per kilogram, the highest of any common fuel. Petrol releases about 44 MJ per kilogram, so hydrogen holds nearly three times as much by mass.

Why is hydrogen's very low density a problem for its use as a fuel?

Hydrogen is the least dense gas, so a kilogram fills a huge volume at normal pressure. The gas must be compressed, liquefied or bound into another material before a useful amount fits in a tank.

The density of hydrogen is 0.0899 kg/m³ at 0°C and atmospheric pressure. What volume does 1 kg of hydrogen occupy?

One kilogram of hydrogen occupies about 11.1 m³. The working is 1 ÷ 0.0899 = 11.1 m³.

CCEA A-Level Environmental Technology 3930

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Hydrogen Fuel Cells

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Transport System Challenges

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Hydrogen Fuel Cells

Hydrogen as a fuel: properties and storage

3 things to know
A2 1

Overview

Hydrogen as an energy carrier, its reaction with oxygen, energy per kilogram against petrol, low density with worked volume and energy calculations, leaks and embrittlement, fire hazards, and compressed, liquid, hydride and bulk storage.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why is hydrogen described as an energy carrier rather than an energy source?

Key term

How does hydrogen react with oxygen, and what is the only product?

Key term

Why does a mixture of hydrogen and air not react at room temperature until it is ignited?

Questions from this subtopic

Each one with the model answer underneath.

1
Why is hydrogen described as an energy carrier rather than an energy source?
Model answer
Hydrogen is almost never found free on Earth, because it is locked up in water and hydrocarbons. Energy must be spent to release it, and the hydrogen then carries that energy to where it is used.
2
How does hydrogen react with oxygen, and what is the only product?
Model answer
Hydrogen reacts with oxygen to form water and nothing else: 2H₂ + O₂ → 2H₂O. The reaction is strongly exothermic, whether the hydrogen is burned or used in a fuel cell.
3
Why does a mixture of hydrogen and air not react at room temperature until it is ignited?
Model answer
The H–H bond is strong, so the reaction needs activation energy to start. A spark, a flame or a catalyst supplies that energy, and the reaction is then rapid.
4
How does hydrogen's energy content per kilogram compare with that of petrol?
Model answer
Hydrogen releases about 120 MJ per kilogram, the highest of any common fuel. Petrol releases about 44 MJ per kilogram, so hydrogen holds nearly three times as much by mass.
5
Why is hydrogen's very low density a problem for its use as a fuel?
Model answer
Hydrogen is the least dense gas, so a kilogram fills a huge volume at normal pressure. The gas must be compressed, liquefied or bound into another material before a useful amount fits in a tank.
6
The density of hydrogen is 0.0899 kg/m³ at 0°C and atmospheric pressure. What volume does 1 kg of hydrogen occupy?
Model answer
One kilogram of hydrogen occupies about 11.1 m³. The working is 1 ÷ 0.0899 = 11.1 m³.

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