Question

The amount of energy released by burning $1kg$ coal is,
(A) $3MJ$
(B) $30MJ$
(C) $300MJ$
(D) $3000MJ$

Answer 1

Coal is a fossil fuel and like any other fossil fuel it also contains chains and branches of carbon. This allows storage of energy in the form of covalent bonds and that’s why it is considered as an excellent source of energy.

Complete step by step answer:
During reactions that are thermodynamically favourable, that is, the reactions that occur in nature or can be achieved supporting the laws of thermodynamics energy is either released or consumed. This means that the reactions are either exothermic or endothermic respectively.
During these reactions some old bonds are broken in the reactants and some new bonds are formed as products are formed. Various types of covalent bonds contain different amounts of energy. It is listed in the table below.

Bond	Bond Energy($kJmo{l^{ - 1}}$)	Bond	Bond Energy($kJmo{l^{ - 1}}$)
H-H	432	C=O	799
O=O	494	C-C	347
O-H	460	C=C	611
C-H	410	C=C (aromatic)	519
C-O	360	N=O	623

Combustion is a type of oxidation reaction and in the case of the combustion of fossil fuels, the combustion reaction is what we think as a burning process.
In the combustion reaction, the reactants reacting with the oxygen are oxidized because oxygen has high electronegativity.
Fossil fuels are mainly composed of hydrocarbons (molecules containing primarily carbon hydrogen bonds). In these molecules carbon is in a much reduced state. During the combustion reaction of all hydrocarbons the hydrocarbon molecules are converted to carbon dioxide and water.
Well now the reaction mechanisms apart, let’s talk about energy released by burning of coal.
Coal is mainly composed of aromatic hydrocarbons, so we can consider the molecule to consist of multiple $- CH -$ units.
When estimated the average value of energy content of coal it is found that coal has $31kJ.{g^{ - 1}}$ of energy. So when we calculate the amount of energy released by burning $1kg$ of coal it will be roughly around $30000kJ.{g^{ - 1}}$ or $30MJ.{g^{ - 1}}$.

So the correct answer is option B.

Note:
The actual average energy released per each gram of coal from combustion is less than the predicted value since coal contains significant amounts of water and minerals. Hard coals such as bituminous or anthracite have larger energy content ($29 - 33{\text{ }}kJ.{g^{ - 1}}$) than the soft sub-bituminous or lignite coals ($17 - 21{\text{ }}kJ.{g^{ - 1}}$). Ideally, coal has more $C{O_2}$ emissions than that of natural gas and petroleum fuels.