The instantaneous rate of an elementary chemical reaction aA+bB (hArr)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 reversible reaction if the reaction is an irreversible one , the rate is expressed as rate =k[A]^(a)[B]^(b) where k is rate constant for the given irreversible reaction and (a+b) is the order of reaction it is also evident from the stoichiometry of reaction that rates 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 Arrhenius equation:k=Ae^-(E_(a)//RT) whereas the 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 enthalpy of reaction respectively The variation of K and K_(f)/K_(b) with increase in temperature shows the following effects: (P) For endothermic reaction K increases, K_(f)/K_(b) also increases (Q) For exothermic reaction K decreases K_(f)/K_(b) also decreases (R) For exothermic reaction K and K_(f)/K_(b) both increases (S) For exothermic reaction K increases and K_(f)/K_(b) decreases (T) For exothermic reaction K and K_(f)/K_(b) both decreases.