Summary of reaction rates For a reaction X

Summary of reaction rates

For a reaction X + Y → Z � A series of experiments can be carried out using different initial concentrations of the reactants X and Y. � It is important to change only one variable at a time, so two series of experiments will be needed. 1. Series 1 - the conc. of X is changed, Y is constant 2. Series 2 - the conc. of Y is changed, X is constant � For each experiment we need to; � Plot concentration/time graph � Measure the initial rate from the graph as the tangent drawn at time = 0

1) Perform a reaction several times, changing the concentration of all reactants. 2) Calculate the initial rate (gradient when the time is zero – t=0) 3) Compare the effect of the concentration of each reactant on the rate 4) Does it: - Stay the same (zero order) - Double (1 st order) - Quadruple (2 nd order)

Experiment [X]/mol dm-3 [Y] / mol dm-3 Initial rate / mol dm-3 1 1. 0 x 10 -2 0. 5 x 10 -2 2 2. 0 x 10 -2 1. 0 x 10 -2 2. 0 x 10 -2 3 2. 0 x 10 -2 4. 0 x 10 -2

Experiment [X]/mol dm-3 [Y] / mol dm-3 Initial rate / mol dm-3 1 1. 0 x 10 -2 0. 5 x 10 -2 2 2. 0 x 10 -2 1. 0 x 10 -2 2. 0 x 10 -2 3 2. 0 x 10 -2 4. 0 x 10 -2 Comparing experiments 1 and 2: [Y] has been kept constant • [X] has been doubled, and the rate quadruples therefore the order with respect to X =2 Comparing experiments 2 and 3: [X] has been kept constant • [Y] has been doubled, and the rate doubles therefore the order with respect to Y =1

Method 1: Analyse the shape of rate/concentration graph. Method 2: Initial rates method – using the initial rate method. http: //www. chm. davidson. edu/vce/kinetics/Half-life. html

Iodine clock practical: You will perform an easier version of method two, by studying the colour change of the reaction.

The half life of a reaction is the time it takes for half of the reactant to be used up. The half life of first order reactions are independent of the concentration.