The rate of a reaction, which may also be called its velocity or speed, can be defined with relation to the concentration of any of the reacting substances, or to that of any product of the reaction. If the species chosen is a reactant which has a concentration c at time t the rate is - dc/dt, while the rate with reference to a product having a concentration x at time t is dx/dt. Any concentration units may be used for expressing the rate, thus, if moles per liter are employed for concentration and seconds for the time, the units for the rate are moles "liter"^(-1) sec^(-1) . For gas reactions pressure units are sometimes used in place of concentrations, so that legitimate units for the rate would be (mm. Hg) sec^(-1) and atm. sec^(-1) The order of a reaction concerns the dependence of the rate upon the concentrations of reacting substances, thus, if the rate is found experimentally to be proportional to the alpha^(th) power of the concentration of one of the reactants A, to the beta^(th) power of the concentration of a second reactant B, and so forth, via., rate kC_(A)^(alpha) C_(B)^(beta)" " (1) the over-all order of the reaction is simply n = alpha +beta + ---------(2) Such a reaction is said to be of the alpha^(th) order with respect to the substance A, the beta^(th) order with respect to B and so on... (Laidler, K. J., & Glasstone, S. (1948). Rate, order and molecularity in chemical kinetics. Journal of Chemical Education, 25(7), 383.) In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage. Assertion: Rate of reaction is a measure of change in concentration of reactant with respect to time. Reason: Rate of reaction is a measure of change in concentration of product with respect to time.
