Chemistry Lab II Exam 2 Study Guide - Chemistry Lab II Exam...

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Chemistry Lab II - Exam 2 Study Guide Experiment 4: Kinetics Aqueous solutions containing the thiosulfate ion, S 2 O 3 2- , are stable if neutral or basic but they decompose rapidly in aqueous acidic solutions to form elemental sulfur. o In this reaction, the actual reactants are Na 2 S 2 O 3 and HCl but the sodium and chloride ions are spectator ions. The sulfur is insoluble and forms a colloidal suspension and the solution becomes cloudy and then opaque. The rate expression for the forward reaction of aA + bB  cC + dD is written as: rate = k[A] m [B] n . o k is the rate constant, and m and n are integers that give the order of the reaction with respect to A and B. Rate expressions and reaction orders depend on the reaction mechanism. They are particularly dependent on the slowest step of the reaction mechanism. o They cannot be determined by the stoichiometry of the overall equation. There are two criteria which must be met for a reaction mechanism to be valid: o The rate-determining step must be consistent with the experimental reaction order. o The sum of the steps in the mechanism must add up to give the overall reaction. If you plot the dependence of the rate on [S 2 O 3 2- ], different values for the reaction order, m, will produce difference curves characteristic of the partial order for this reactant. o If m = 0 , the plotted data will be a straight line parallel to the x-axis (0 slope). Doubling the concentration will have little effect on the time of the reaction . o If m = 1 , the curve will be a straight line with a positive slope proportional to the rate constant. Doubling the concentration will approximately
double the rate , so the reaction will take place in half the time. o If m = 2 , the curve will be that of the positive half of a parabola. Doubling the concentration will make the reaction go about 4 times faster .

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