Define : (a) Average rate of reaction
(b) Instantaneous rate of reaction.

Explanation

(a) Average rate of reaction : In this method the change in concentration of reactant or product are measured in an appreciable time internal. Symbol `'Delta'` is used for larger change i.e., for average rate.
(b) Instantaneous rate of reaction : In this methos the change in concentration of reactant and product are measured in very short time interval. Symbol 'd' is used for small change. i.e., for instantaneous rate of reaction.
For example : Let us consider a reaction
`N_(2)+3H_(2) hArr 2NH_(3)`
Instantaneous rate of reaction :
`(-d[N_(2)])/(dt)=-1/3 (d[H_(2)])/(dt)=1/2 (d[NH_(3)])/(dt)`
Average rate of reaction :
`(-Delta [N_(2)])/(Delta t)=-1/3 (Delta [H_(2)])/(Delta t)=1/2 (Delta [NH_(3)])/(Delta t)`