<P> According to IUPAC's Gold Book definition the reaction rate r for a chemical reaction occurring in a closed system under isochoric conditions, without a build - up of reaction intermediates, is defined as: </P> <Dl> <Dd> r = − 1 a d (A) d t = − 1 b d (B) d t = 1 p d (P) d t = 1 q d (Q) d t (\ displaystyle r = - (\ frac (1) (a)) (\ frac (d (\ mathrm (A))) (dt)) = - (\ frac (1) (b)) (\ frac (d (\ mathrm (B))) (dt)) = (\ frac (1) (p)) (\ frac (d (\ mathrm (P))) (dt)) = (\ frac (1) (q)) (\ frac (d (\ mathrm (Q))) (dt))) </Dd> </Dl> <Dd> r = − 1 a d (A) d t = − 1 b d (B) d t = 1 p d (P) d t = 1 q d (Q) d t (\ displaystyle r = - (\ frac (1) (a)) (\ frac (d (\ mathrm (A))) (dt)) = - (\ frac (1) (b)) (\ frac (d (\ mathrm (B))) (dt)) = (\ frac (1) (p)) (\ frac (d (\ mathrm (P))) (dt)) = (\ frac (1) (q)) (\ frac (d (\ mathrm (Q))) (dt))) </Dd> <P> where (X) denotes the concentration of the substance X (= A, B, P or Q). Reaction rate usually has the units of mol / L / s </P>

For a gaseous reaction the unit of rate of reaction is