What is the basic difference between enthalpy of formation and enthalpy of reaction? Illustrate with suitable examples.

Explanation

Basic difference between heat of formation and heat of reaction: The enthalpy change accompanying the reaction representing the formation of a compound from its elements is called heat of formation `(Delta_(f)H^(Θ))`. On the other hand, the enthalpy change in any type of chemical reaction namely combination, combustion, hydrogenation, neutralization, atc. is called as heat of reaction `(Delta_(r)H^(Θ))`.
Example of the difference between `Delta_(f)H^(Θ)` and `Delta_(r)H^(Θ)`
i. The enthalpy change for the reaction.
`H_(2)(g)+1/2O_(2)(g) rarr H_(2)O(l), Delta_(r)H^(Θ)=-286 kJ mol^(-1)`
may be represented as
a. The enthalpy of formation of water is `286 kJ mol^(-1)`.
`Delta_(f)H^(Θ)(H_(2)O, l)=-286 kJ mol^(-1)`
b. The enthalpy of combustion of hydrogen is `-286 kJ mol^(-1)`
`Delta_(f)H^(Θ)(H_(2)O)=-286 kJ mol^(-1)`
or heat of combustion of hydrogen
ii. The enthalpy change for the reaction
`CH_(4)(g)+2O_(2)(g) rarr CO_(2)(g)+2 H_(2)O(l), Delta_(r)H^(Θ)=-890 kJ mol^(-1)`
does not represent the heats of formation of `CO_(2)` and `H_(2)O`. The reaction represents combustion of methane.