The instantaneous rate of an elementary chamical reactkon aA+bBhArr cC+dD can be given by rate =K_(f)[A]^(a)[B]^(b)-K_(b)[C]^(c)[D]^(d) where K_(f) and K_(b) are rate constants for forward and backward reactions respectively for the reversiblereaction. If the reaction is an irreversible one, the rate is expressed as, rate =K[A]^(a)[B]^(b) where K is rate contant for the given irreversible rate of disappearance of A is a/b times the rate of disappearance of B. The variation of rate constant K with temperature is expressed in terms of Arrheniusequation: K=Ae^(-E_(a)//RT) whereasthe ratio (K_(f))/(K_(b)) is expressed in terms of van't Hoff isochore: (K_(f))/(K_(b))=Ae^(-DeltaH//RT), where E_(a) and DeltaH are energy of activation and heat of reaction respectively. The variation of rate constant K and (K_(f))/(K_(b)) with temperature shows the following effects: For endothrmic reaction when T increases then K increases and (K_(f))/(K_(b)) also increases. (ii) For endothemic reaction when T decreases then K decreases and (K_(f))/(K_(b)) also decreases. (iii) For exothermic when T increases then K and (K_(f))/(K_(b)) both increases. (iv) For exothermic reaction when T decreases then K increases and (K_(f))/(K_(b)) decrease. (v) For exothermic reaction when T increases thenK and (K_(f))/(K_(b)) both decrease.