Question

(a) The calorific value of hydrogen is $150000kJ/kg$. What does this mean?
(b) Even though the calorific value of hydrogen is very high, it is not used as a household fuel. Why?
(c) Give a situation in which this fuel is used.

Answer 1

Hint: You could recall what is meant by calorific value of a substance. From that you could get the answer for the first part. You could compare the value with known values of other fuels to determine how high it is. Now you could give reasons as to why this fuel with high calorific value is not used in households and also name a situation where it is used.

Complete step by step answer:
(a) We know that calorific value is the amount of heat released by the unit weight or unit volume of a substance after completion of its combustion. Calorific power is another name of this quantity. The SI unit of calorific value is given by kilo-joule per kilogram, $KJ/kg.$
The calorific value of hydrogen is given and we are asked to find what that means. The value given is $150000KJ/kg$. This means that $150000KJ/kg$ of energy is released by hydrogen on combustion, which is really huge when compared to petrol and diesel with $45000 KJ/kg$ each and LPG that has a calorific value of $55000KJ/kg.$
(b) We know that the efficiency of a fuel is determined by its calorific value, that is, higher the calorific value of the fuel is, the more efficient the fuel is, also, lower value indicates less efficiency. Or simply we could say that the calorific value is proportional to the fuel’s efficiency. But even with this high calorific value hydrogen is not used as a household fuel.
This is because hydrogen reacts explosively when it comes in contact with air. Also, it has to be compressed under high pressure which makes the storage process cumbersome. It is the risk of explosion on exposure to air and storage that further results in difficulty in transportation. Due to these disadvantages we don’t use hydrogen as fuel for household purposes.
(c) Hydrogen fuel can be used as a fuel for spacecraft propulsion. It can also be used in fuel cells and internal combustion engines.

Note:
Knowledge of the calorific value of fuel is important as it helps us determine the amount of energy we transport which helps the gas shippers and suppliers to bill gas consumers. If the water vapour which is the by-product of the combustion process could be recovered then the substance will have higher calorific value. In case if we cannot recover the vapour produced then it results in low calorific value.