What is the standard enthalpy of combustion ? Give an example. - Vidyadip

Question

What is the standard enthalpy of combustion ? Give an example.

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Answer

Standard enthalpy of combustion or standard heat of combustion : it is defined as the enthalpy change when one mole of a substance in the standard state undergoes complete combustion in a sufficient amount of oxygen at constant temperature $(298 K$ ) and pressure ( 1 atmosphere or 1 bar). It is denoted by $\Delta_{ C } H ^0$.
E.g. $CH _3 OH _{( l )}+\frac{3}{2} O _{2( g )}= CO _{2( g )}+2 H _2 O$
$\Delta_{ C } H ^0=-726 kJ mol ^{-1}$
( $\Delta_{ c } H ^0$ is always negative.)
[Note: Calorific value : It is the enthalpy change or amount of heat liberated when one gram of a substance undergoes combustion.
$\therefore$ Calorific value $=\frac{\Delta_{ f } H^0}{\text { Molar mass }}$

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