1. Tutorial 10 Chemical kinetics

2. LEARNING OUTCOMES By the end of this session the student should be able to: 1.Define reaction rate. 2.Identify the factors affecting reaction rate. 3.Calculate the rate of a reaction. 4.Apply the rate laws. 5.Apply the initial concentration method. 6.Determine the order of a reaction. 7.Catalysis Dr. Rasha HanafiPHCM101-WS15-Lecture 102

3. 3 Factors that affect Reaction Rates Concentrations of reactants: Reaction rates increase as the concentrations of the reactants are increased. Temperature: Reaction rates increase rapidly as the temperature is increased. Surface area: For reactions that occur on a surface rather than in solution, the rate increases as the surface area is increased. Catalysts: Catalysts speed up reactions and inhibitors slow them down.

4. 4 How to determine reaction rate? The method of initial rates involves a series of experiments in which the initial concentrations of some reactants are held constant and others are varied in convenient multiples in order to determine the rate law for that reaction.

5. 5 Method Of Initial Rates-Summary The effects of doubling one initial concentration: For zero-order reactions, no effect on rate. Rate = k[A] 0 = k For first-order reactions, the rate doubles. Rate= k[A] For second-order reactions, the rate quadruples. Rate= k[A] 2 For third-order reactions, the rate increases eightfold. Rate= k[A] 3 The value of k for the reaction can be calculated.

6. From the rate expression for the following reactions, determine the order of reaction and the units of the rate constant (k). (i)3NO  N 2 ORate = k[NO] 2 (ii) H 2 O 2 +3I - + 2H+  2H 2 O + 3I - Rate = k[H 2 O 2 ][I - ] Answers: (i) Rate= k[NO] 2 ;Order of reaction is 2. Rate constant unit: k = Rate/[NO] 2 k=(M/s)/M 2 = M -1 s -1 (ii) Rate= k[H 2 O 2 ][I - ]; Order of reaction is 2. Rate constant unit: k=rate/[H 2 O 2 ][I - ] k=(M/s)/M.M = M -1 s -1 Question 1

7. - Calculate the rate constant (K) Question 2

8. With respect to A: Consider experiment 2/3 where A is doubled and B is constant The rate is doubled 1 st order with respect to A With respect to B Consider experiment 1/2 where A is constant and B is variable The rate is the same Zero order with respect to B Overall order: 1+0=1 1 st order Reaction rate law: R=K[A] 1 [B] 0 =K[A] Rate constant 3x10 -2 = K [0.1] K=0.3 s -1 K is the same in all 3 experiments

9. Question 3

10. Start with B Consider 1/2 where B is doubled, A and C are constant Rate is the same: Order is zero *Any change in B conc has no effect on the rate With respect to A Consider 3/4 where A is variable and C is constant (1.2/0.4) x = (0.11/1.2x10 -2 ) 3 x = 9 x=2 2 nd order With respect to C Consider 2/3 where C is doubled and A is const Rate is doubled 1 st order Overall order 2+1+0= 3 3 rd order Reaction rate law: K=[A] 2 [B] 0 [C] 1 = [A] 2 [C]

11. Question 4 From the graph given below, Calculate the instantaneous rate at 600 sec. R instantaneous at 600 sec = - (0.015-0.037)/(800-400) = - (-0.022)/400 = 5.5*10 -5

12. Catalysis Question 5 Which of the following is NOT true about a catalyst? a)It speeds up a reaction b)It can act as an inhibitor c)It may be homogenous d)It may be heterogenous Which of the following is NOT true about a catalyst? a)It speeds up a reaction b)It can act as an inhibitor c)It may be homogenous d)It may be heterogenous