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Rate Law And Mechanisms Essay

1312 words - 5 pages

Many reactions occur over a number of steps. The slowest of these steps determines the rate of the reaction and is called the "rate determining step".A. Rate Expression (or Rate Law)For a given reaction:aA + bB cC + dD (where a, b, c & d are the stoichiometricnumber - for balancing)Rate = k [A]x [B]y (x & y have nothing to do with a, b, c or d)(k is called the "rate constant"){using square brackets around a substance, [A], means the concentration of "A" in mol dm-3}The rate is always the initial rate and is found by monitoring the concentration of one of the species. If [C] is being monitored, the rate is equal to .This means we can rewrite the rate expression as:= k [A]x [B]yWe can only find the values of "x" and "y" by doing experiments. In these experiments we first measure the rate with certain known concentrations. Then we vary one concentration while keeping the other constant and measure the rate again.If we double [A] and the rate doesn't change x = 0If we double [A] and the rate also doubles x = 1 (because 2 = 21)If we double [A] and the rate increases 4 times x = 2 (because 4 = 22)We then keep [A] constant while varying the concentration of the other species and use the same logic.EgExperiment [A] [B] Rate1 0.1 mol dm-3 0.1 mol dm-3 0.02 mol dm-3 s-12 0.2 mol dm-3 0.1 mol dm-3 0.04 mol dm-3 s-13 0.1 mol dm-3 0.2 mol dm-3 0.08 mol dm-3 s-1Analysis:Using Exp 1 & 2, [A] doubles while [B] is constant. The rate also doubles. Therefore x = 1.Using Exp 1 & 3, [B] doubles while [A] is constant. The rate goes up 4 x (22). Therefore y = 2.This gives the following final rate expression.Rate = k [A]1 [B]2 or = k [A]1 [B]2The order with respect to A is "1" (value of "x")The order with respect to B is "2" (value of "y"The overall order is "3" (2 + 1 = 3)Calculating the Rate ConstantWe use the data for one of the experiments to calculate "k". (All experiments will give us the same value.)We can also calculate the "unit" for k.Therefore: k = 20 s-1 mol-2 dm6Note: The units of k will vary for different reactions according to the overall order.GraphsGraphs can also be used to work out the order for a reaction. There are two types of graphs which can be used according to which type of data is supplied.1. Effect of Concentration on Initial RateYou need to be supplied with data showing both the initial concentration and the initial rate.Zero order is a horizontal line.First order is a straight sloping line.Second order is a line curving upwards.2. Variation of Concentration with TimeThis is used more commonly.(i) Zero Order (Straight line)(ii) First Order (Curved line)(iii) Second Order (Also a curved line)You will need to use the half lives to distinguish between First Order and Second Order.MechanismsA mechanism is a suggested series of steps that a reaction undergoes to finally form the products. One of these steps will be slow (the rate-determining step) and the others fast. We have to be able to determine if a suggested mechanism is...

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