Question

What is the unit of calorific value?
${\text{A}}{\text{.}}$ kJ/kg
${\text{B}}{\text{.}}$ kJ/lit
${\text{C}}{\text{.}}$ km/hr
${\text{D}}{\text{.}}$ kcal

Answer 1

Hint- Here, we will proceed by defining the term calorific value of any substance. Then, we will write down in terms it is represented. Finally, we will use the general formula of heat generated to find the units of the calorific value.

Complete step-by-step answer:
The calorific value of a substance is the amount of energy emitted when the substance is totally burned or is completely burned. This means that the complete combustion of the fuel must occur. Combustion is one of the five basic forms of chemical reactions, which often occurs in the presence of oxygen gas, with the other reactant being some kind of fuel which is mostly carbon based.
A substance's calorific value is typically given on the basis of per amount i.e., fuel burnt is represented in terms of per mole (m).
A calorie is the amount of heat necessary to lift the temperature of 1 gram of water to 1 degree Celsius.
Heat Generated H = mc$\Delta {\text{T}}$ where m is the mass, c is the specific heat and $\Delta {\text{T}}$ is the temperature difference
$\Rightarrow \dfrac{{\text{H}}}{{\text{m}}} = {\text{c}}\Delta {\text{T}}$
As we know that the calorific value of any substance is defined as the heat generated while burning per unit mass of the substance.
i.e., Calorific value = $\dfrac{{\text{H}}}{{\text{m}}} = {\text{c}}\Delta {\text{T}}$
The calorific value of any substance having m as the mass of the substance, $\Delta {\text{T}}$ as the difference in the temperature, c as the specific heat of the substance is given by
Calorific value of the substance = c$\Delta {\text{T}}$
In the above formula, the units of specific heat of the substance c is Kilojoule per kilogram per kelvin (KJ/kgK) and the units of temperature difference $\Delta {\text{T}}$ is same as the units of the temperature i.e., Kelvin (K).
So, the units of calorific value of the substance is given by
Units of calorific value = $\dfrac{{{\text{KJ}}}}{{{\text{kgK}}}} \times {\text{K}} = \dfrac{{{\text{KJ}}}}{{{\text{kg}}}}$
The units of calorific value of the substance is KJ/kg
Therefore, option A is correct.

Note- Gross calorific value (GCV) which is Higher Heating Value which further can be defined as the amount of heat that is generated by burning a fuel unit to complete the combustion. It assumes full condensation of all of the water vapour produced during the combustion process. Net calorific value (NCV) means lower heating value (LHV), i.e. lower calorific value (LCV) is calculated by extracting heat of vaporization from the higher heating value of the water vapour.