Question

How do you calculate the enthalpy change of combustion?

Answer 1

Hint The equation needs to be formed in such a way that 1 mole of the reactant is used with the required number of oxygen to produce desired products. The heat of this reaction is the heat of combustion.

Complete step by step solution:
In order to answer our question, we need to know what enthalpy is. Enthalpy shows us the heat change that happens during a reaction, this may be heat gained or heat lost. Now, let us talk about standard enthalpy of combustion, which is denoted as ${{\Delta }_{c}}H$. As combustion reactions are exothermic, standard enthalpy of combustion is the amount of heat evolved when one mole of that particular substance under standard conditions (298K, 1 bar pressure) is completely burnt into products also under standard conditions.
For example, butane(${{C}_{4}}{{H}_{10}}$) which is the chief constituent in the cooking gas cylinders release 2658kJ of heat on combustion of one mole. The thermochemical equation can be written as:
\[{{C}_{4}}{{H}_{10}}(g)+\dfrac{13}{2}{{O}_{2}}(g)\to 4C{{O}_{2}}(g)+5{{H}_{2}}O(l)\,{{\Delta }_{c}}H=-2658kJ\,mo{{l}^{-1}}\]
Just like fuels which generate energy on combustion, food we eat also generates energy to drive the human body for day to day activities. The energy obtained from the food is similar as in combustion even though it is obtained through a series of complex biochemical reactions involving enzymes.
The energy produced by the combustion of fuels or foods is usually compared in terms of their combustion energies per gram. It is known as calorific value. So, the calorific value can be defined as the amount of heat produced in calorie or joule when one gram of a substance (food or fuel) is completely burnt or oxidised. For example, combustion of glucose gives out $2802kJ\,mo{{l}^{-1}}$ of heat for which the overall equation is:
\[{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{O}_{2}}\to 6C{{O}_{2}}+6{{H}_{2}}O\,\,\,;{{\Delta }_{c}}H=-2802kJ\,mo{{l}^{-1}}\]

NOTE: It is to be noted that there is no value of absolute enthalpy. Enthalpy is always given as enthalpy change or enthalpy difference. It is so because the origin or zero point of enthalpy is not determined as we cannot reach absolute zero Kelvin in practical life.