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Module 1: Chemical Kinetics

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The Rate of a Reaction

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This video describes the rate of a reaction and how to calculate the rate of disappearance of reactants or the appearance of products.The rate of a reaction is defined as the ratio of change in concentration of a reactant or product and change in time.In other words, rate of reaction is represented as “(Ci – Ci-1)/(ti – ti-1)”.The units for concentration is in moles per liter or molar.And the time is in seconds, so the units for rate of a reaction is molar per second.For example, consider a chemical reaction where we have 1 mole of A reacts with 1 mole of B to form 1 mole of product C.In this reaction, we have two reactants namely A and B and a product C.Let us assume that at the start of the experiment that the time is t = 0, the concentration of reactant A = 1 molar and the concentration of reactant B is 0.5 molar.At time t = 0 the concentration of product C is 0 as there are no products formed yet.At time, t = 10 s, concentration of reactant A decreased to 0.9 molar and B to 0.45 molar whereas the concentration of product C is 0.01 molar.The average rate of reaction with respect to reactant A at the end of 10 seconds is nothing but the difference in the concentration of reactant A at time t = 10 s and t = 0 s divided by the change in time, which is 10 seconds minus 0 seconds.The rate of reaction with respect to reactant A is represented as “Rate or RA rA” is equal to concentration at time t = 10 sec which is 0.9 molar minus concentration at time t = 0 sec which is 1 molar divided by 10 seconds minus 0 seconds.So, the rate of reaction with respect to A, RA rA = -0.1 M/10 s which is equal to -0.01 M/s.The negative sign associated with the rate of reaction indicates disappearance of a species. Hence, the rate of reaction with respect to the reactant always carries a negative sign whereas products carry a positive sign.Similarly, the rate of reaction with respect to reactant B at the end of 10 seconds, RB rB = (0.45-0.5) M/(10-0) s.Rate with respect to B, RB rB = - 0.05 M/10 s which is - 0.005 M/s. The rate of reaction with respect to product C, RC rC = (0.01-0) M/10 s which is equal to + 0.001 M/s. Since the product is evolving as the reaction proceeds, we have a positive sign associated with the rate of reaction for product C.For a reaction where “a moles of A” reacts with “b moles of B” to form “c moles of C” and “d moles of D”, the relationship between the rate of reaction with respect to the reactants and products is represented as (-1/a) number of moles of reactant A into the rate of reaction with respect to A. Is equal to (-1/b) number of moles of reactant B into the rate of reaction with respect to B.Is equal to (+1/c) number of moles of product C into the rate of reaction with respect to C. We have + sign here because the product C is appearing as the reaction progresses.Is equal to (+1/d) number of moles of product D into the rate of reaction with respect to D.