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Module 1: Chemical Kinetics

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This video describes about rate law and how to calculate the order of a chemical reaction.Rate law expresses the mathematical relationship between the rate of a reaction and the concentration of reactants.The rate of a reaction is directly proportional to the concentration of reactants.The proportionality constant is nothing but the rate constant, which is represented by lower case k. The rate constant is different from capital K value that we calculate during equilibrium or acid-base titration.Consider a chemical reaction where a moles of A reacts with b moles of B to form c moles of product C.The rate law for this reaction is expressed as Rate = k [A]m [B]n.In this equation, m and n represent the order of the reaction with respect to reactants A and B respectively. A very important point to note here is that m and n are not the same as the coefficients or number of moles of reactants A and B.In order to calculate the rate law and order of the reaction, we need experimental data for change in concentration and rates of reaction.For this reaction, we have a set of experimental data with four different experimental trials.The next step is to determine how dependent A, B and C are on the rate of reaction.In order to do this, we first have to write the rate law.The Rate = k [A]x [B]y [C]z.In order to determine the order of the reaction of reactant A or how dependent the rate law is with respect to reactant A, we have to find two different experimental trials where the concentrations of A varies but the concentrations of B and C remains constant.So, in trials 3 and 4 the concentration of B and C are the same, but the initial concentration of A is changing.With the data provided in the table, we can write a rate law for each of these trials as Rate3 = k [0.10]x [0.20]y [0.10]z. Similarly for trial 4, Rate4 = k [0.20]x [0.20]y [0.10]z.Dividing the rate law of trial 4 by the rate law of trial 3 allows us to solve for the order of reaction with respect to A. Since the initial concentration of B and C are not changing in these two trials they will simply cancel out.Solving this equation, we get 2 = 2x and x = 1. So, the reaction is first order with respect to reactant A.What this means is that doubling the concentration of A will double the rate of the reaction.We can follow the same procedure to solve for the order of reaction with respect to B and C.Selecting trials 1 and 3 will allow us to solve for the order of B because the initial concentration of B is changing while the initial concentration of A and C remain the same.Solving this equation would give us the value of y = 2. So, the reaction is second order with respect to B.What this means is that doubling the concentration of A will quadruple the rate of the reaction.Selecting trials 1 and 2 will allow us to solve for the order of C because the initial concentration of C is changing while the initial concentration of A and B remain the same.The order of reaction with respect to C is 0 or the reaction is zero order with respect to C.What this means is that changing the concentration of C will have no effect on the rate of reaction.We can rewrite the rate law with the known orders of reaction of each reactants as Rate = k [A]1 [B]2 [C]0.The overall order of the reaction is the sum of order of all the reactants involved in the reaction. In this case, the order of the reaction is 1 + 2 = 3. So, this reaction is a third order reaction.