Experiment 2: Factors Affecting Reaction Rates

Transcription

1 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. Part B To study the effect of temperature on the rate of a reaction. Part C To study the effect of a catalyst on the rate of a reaction.

2 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) Reactants: Ammonium persulfate (NH 4 ) 2 S 2 O 8 Potassium iodide KI CHEM 0012 Lecture Notes 2

3 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. How do we do this? Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) Observe a COLOUR CHANGE! Problem: The pale yellow solution is visually very difficult to judge. CHEM 0012 Lecture Notes 3

4 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. How do we do this? Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) Introduce two substances to help us observe the colour change more accurately: 1. Starch indicator 2. Sodium thiosulfate, Na 2 S 2 O 3, of a fixed quantity CHEM 0012 Lecture Notes 4

5 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) (1) ( clear solution ) I 2 (aq) + 2 S 2 O 3 (aq) S 4 O 6 (aq) + 2 I (aq) (2) thiosulfate ion We time how long it takes for the solution to turn colour. I 2 (aq) + starch iodine starch complex (3) ( coloured solution ) As the I 2 is formed (1), it reacts with the fixed amount of sodium thiosulfate that has been added to the reaction mixture (2). When the sodium thiosulfate is used up, the next quantity of I 2 that is formed reacts with the starch indicator to form a colour complex, and turns the clear solution into a coloured solution (3). CHEM 0012 Lecture Notes 5

6 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Make solutions of different concentrations. Prepare 14 solutions in 14 Erlenmeyer flasks with different concentrations of S 2 O 8 and I solutions! Label them: A1, A2, A3, A4, A5, A6, A7. B1, B2, B3, B4, B5, B6, B7. SEPARATE the reactants until we are ready to mix them! KI (I ) in A solutions; (NH 4 ) 2 S 2 O 8 (S 2 O 8 ) in B solutions CHEM 0012 Lecture Notes 6

7 Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Total volume of A solutions = 30.0 ml. CHEM 0012 Lecture Notes 7

8 Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Total volume of B solutions = 20.0 ml. CHEM 0012 Lecture Notes 8

9 Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Total volume of the combined solution = 50.0 ml. MIX and TIME: A1 + B1 A2 + B2 A3 + B3 A4 + B4 A5 + B5 A6 + B6 A7 + B7 (expt 1) (expt 2) (expt 3) (expt 4) (expt 5) (expt 6) (expt 7) CHEM 0012 Lecture Notes 9

10 When solutions A1 and B1 are combined, what is the concentration of (NH 4 ) 2 S 2 O 8 in moles/l? ( L) x (0.100 moles/l) = moles (NH 4 ) 2 S 2 O 8 Total volume of the combined solution = 50.0 ml moles (NH 4 ) 2 S 2 O 8 = M (NH 4 ) 2 S 2 O L CHEM 0012 Lecture Notes in the combined A1 + B1 solution 10

11 Follow the example on the previous slide and calculate the concentrations of (NH 4 ) 2 S 2 O 8 in moles/l when A2+B2, A3+B3, A4+B4, A5+B5, A6+B6 and A7+B7 are combined? ( L) x (0.100 moles/l) = moles (NH 4 ) 2 S 2 O moles (NH 4 ) 2 S 2 O 8 = M (NH 4 ) 2 S 2 O L Note: The concentrations of (NH 4 ) 2 S 2 O 8 should be the same for the A4/B4, A5/B5, A6/B6 and A7/B7 solutions because the B4, B5, B6, and B7 solutions each contains 20.0 ml of (NH 4 ) 2 S 2 O 8. CHEM 0012 Lecture Notes 11

12 Follow the example on the previous slide and calculate the concentrations of (NH 4 ) 2 S 2 O 8 in moles/l when A2+B2, A3+B3, A4+B4, A5+B5, A6+B6 and A7+B7 are combined? Enter concentrations in Table 2-4 and Table 2-6 in the post-lab questions of Experiment 2. Note: The concentrations of (NH 4 ) 2 S 2 O 8 should be the same for the A4/B4, A5/B5, A6/B6 and A7/B7 solutions. CHEM 0012 Lecture Notes 12

13 When solutions A1 and B1 are combined, what is the concentration of KI in moles/l? ( L) x (0.200 moles/l) = moles KI Total volume of the combined solution = 50.0 ml moles KI = M KI L CHEM 0012 Lecture Notes in the combined A1 + B1 solution 13

14 Follow the example on the previous slide and calculate the concentrations of KI in moles/l when A2+B2, A3+B3, A4+B4, A5+B5, A6+B6 and A7+B7 are combined? ( L) x (0.200 moles/l) = moles KI moles KI = M KI L Note: The concentrations of KI should be the same for the A1/B1, A2/B2, A3/B3 and A4/B4 solutions because the A1, A2, A3, and A4 solutions each contains 20.0 ml of KI. CHEM 0012 Lecture Notes 14

15 Follow the example on the previous slide and calculate the concentrations of KI in moles/l when A2+B2, A3+B3, A4+B4, A5+B5, A6+B6 and A7+B7 are combined? Enter concentrations in Table 2-4 and Table 2-6 in the post-lab questions of Experiment 2. Note: The concentrations of KI should be the same for the A1/B1, A2/B2, A3/B3 and A4/B4 solutions. CHEM 0012 Lecture Notes 15

16 How many moles of Na 2 S 2 O 3 are available? ( L) x ( moles/l) = moles Na 2 S 2 O 3 This is the same amount of Na 2 S 2 O 3 that is added to ALL the A solutions. CHEM 0012 Lecture Notes 16

17 Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) (1) ( clear solution ) 1 I 2 (aq) + 2 S 2 O 3 (aq) S 4 O 6 (aq) + 2 I (aq) (2) thiosulfate ion ( clear solution ) How many moles of I 2 will consume the Na 2 S 2 O 3 added? From the previous slide, we calculated that there is moles Na 2 S 2 O 3 in each A solution. From equation (2), the I 2 and S 2 O 3 is 1:2. It takes /2 = moles of I 2 to consume the S 2 O 3. CHEM 0012 Lecture Notes 17

18 What is the concentration of I 2 when the Na 2 S 2 O 3 is consumed? From the previous slide, we calculated that it takes moles of I 2 to consume the S 2 O 3. Total volume of the combined solution = 50.0 ml moles I 2 = M I L When you time how long it takes for the solution to turn colour, you are actually timing how long it takes to produce M I 2!!! Remember this number. You ll need the [I 2 ] in the rate calculation. CHEM 0012 Lecture Notes 18

19 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) What is a reaction s rate law? CHEM 0012 Lecture Notes 19

20 What is a reaction s rate law? Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) The rate law for a chemical reaction is an experimentally determined mathematical equation that describes the progress of the reaction. For the Iodine clock reaction, the reaction rate of formation of I 2 is proportional to the product of the concentrations each reactant each raised to some power, x and y. Rate [S 2 O 8 ] x [I - ] y CHEM 0012 Lecture Notes 20

21 What is a reaction s rate law? Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Rate [S 2 O 8 ] x [I - ] y We can remove the proportional symbol and introduce a proportionality constant, k. Rate k [S 2 O 8 ] x [I - ] y The proportionality constant, k, is known as the rate constant. We will experimentally determine the values of x and y. CHEM 0012 Lecture Notes 21

22 Objective: Part A To determine the effect of concentration on the rate of formation of Iodine, I 2, and therefore, determine the reaction s rate law. Experiment Solution Time (sec) Table 2-4 Sample Calculation [S 2 O 8 ] (moles/l) [I ] (moles/l) 1 A1/B1 264* Rate of formation of I 2 (moles/l) * Sample data Rate of formation of I [I ] formed time M s 1.89 x 10-6 M/s CHEM 0012 Lecture Notes 22

23 Experiment Solution Time (sec) Table 2-4 Sample Calculation [S 2 O 8 ] (moles/l) 1 A1/B [I ] (moles/l) Rate of formation of I 2 (moles/l) 1.89 x A2/B2 122* x 10 6 * Sample data Similarly, we can calculate the Rate of formation of I 2 for expt 2. CHEM 0012 Lecture Notes 23

24 Table 2-5 Sample Calculation of x Experiment pairs 1 and 2 x, the order of the reaction with respect to [S 2 O 8 ] Average value of x Rate k [S 2 O 8 ] x [I - ] y Expt 1: Expt 2: 1.89 x x k [S k [S 2 2 O O ] ] x x [I [I - - ] ] y y k(0.0100) k(0.0200) CHEM 0012 Lecture Notes 24 x x (0.0800) (0.0800) x = 0.5 x X = y y

25 Similar calculation can be carried out to calculate y for Table 2-7. Rate k [S 2 O 8 ] x [I - ] y x is the order of the reaction with respect to S 2 O 2-8 y is the order of the reaction with respect to I - x + y is the overall order of the reaction CHEM 0012 Lecture Notes 25

26 Once x and y are determined, we can calculate the rate constant, k. Table 2-9 Sample Calculation Experiment Rate constant, k Average value of k x 10 6 = k (0.0100) x (0.0800) y Substitute x and y and solve for k. CHEM 0012 Lecture Notes 26

27 Objective: Part B To study the effect of temperature on the rate of a reaction. How do we do this? Keep the concentrations of the reactants constant, but let the Reaction react at different temperatures: 0 o C, 20 o C, 30 o C, 40 o C Use A4, B4 solutions for all the temperatures!! Part B calculations for Table 2-10 is similar to Part A. CHEM 0012 Lecture Notes 27

28 Objective: Part B To study the effect of temperature on the rate of a reaction. Verify Arrhenius Equation k Ae E act RT A is the pre-exponential or frequency factor, a constant related to the collision frequency R is the gas constant (8.314 J / K mole) T is the absolute temperature (K) k is the rate constant at temperature T E act is the activation energy, the energy required by the reacting species for their collisions to be effective (ie - those that lead to the formation of products) CHEM 0012 Lecture Notes 28

29 Objective: Part B To study the effect of temperature on the rate of a reaction. Rewrite Arrhenius Equation k Ae Eact RT ln k E act R 1 T ln A y = m x + b Plot "ln k versus 1 T " should yield a straight line!! CHEM 0012 Lecture Notes 29

30 Objective: Part B To study the effect of temperature on the rate of a reaction. Ea m R b ln( A) E a A e Rm b CHEM 0012 Lecture Notes 30

31 Objective: Part C To study the effect of a catalyst on the rate of a reaction. Use the A4, B4 solutions and add copper as a catalyst. A catalyst speeds up a reaction. When Cu is added, by how many times does the reaction increase? Rate of formation of Rate of formation of I with Cu added 2 I 2 without Cu 1 Data from Part B. CHEM 0012 Lecture Notes 31