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Chemistry Lab II - Exam 2 Study GuideExperiment 4: KineticsAqueous solutions containing the thiosulfate ion, S2O32-, are stable if neutral or basic but they decompose rapidly in aqueous acidic solutions to form elemental sulfur. oIn this reaction, the actual reactants are Na2S2O3and 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.ok 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. oThey cannotbe determined by the stoichiometry of the overall equation. There are two criteriawhich must be met for a reaction mechanism to be valid:oThe rate-determining step must be consistent with the experimental reaction order.oThe sum of the steps in the mechanism must add up to give the overall reaction. If you plot the dependence of the rate on [S2O32-], differentvalues for the reaction order, m, will produce difference curves characteristic of the partial order for this reactant. oIf 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. oIf 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. oIf 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.