Determining Order given initial rates

Question

Dibromoethane and potassium iodide react in methanol (the solvent) according to the equation:

 

C₂H₄Br₂ + 3KI → C₂H₄ + 2KBr + KI₃

 

The initial rate of this reaction was determined in a series of experiments at 65 °C, giving the data below:

Experiment	[C2H4Br2] /mol L–1	[KI] /mol L–1	+(d[KI3]/dt) /mol L–1s–1
1	0.484	1.735	0.00646
2	0.484	2.603	0.00969
3	1.207	1.735	0.0161

 

a) Determine the order of the reaction with respect to C₂H₄Br₂

0

1

2

3

b) Determine the order of the reaction with respect to KI

0

1

2

3

c) Determine the value of the rate constant and select the appropriate units for it.

Answer:

a) s–1

b) L mol–1 s–1

c) L2 mol–2 s–1

Is Dibromoethane first order and KI third order because of number of moles? Explanation for all three questions would be great. Thanks!

Answer 1

To find the order of reaction with respect to C₂H₄Br₂, compare experiments #1 and #3 where the [C₂H₄Br₂] is increased by 2.5 times while the [KI] remains constant. What happens to the rate? It increases by 2.5 times also, showing that the reaction is FIRST ORDER in C₂H₄Br₂.

To find the order of reaction with respect to KI, compare experiments #1 and #2 where [KI] is increased by 1.5 times while the [C₂H₄Br₂] remains constant. What happens to the rate? It increases by 1.5 times also, showing that the reaction is FIRST ORDER in KI.

To determine the rate constant, write the rate equation and substitute in values from the table:

rate = k[C₂H₄Br₂][KI] ... first order in both C₂H₄Br₂ and KI

Choosing values from experiment #1 (you could choose any experiment), we have...

0.00646 M/s = k(0.484 M)(1.735 M)

k = 0.00646 Ms-1 / (0.484 M)(1.735 M)

k = 0.00769 M-1s-1 = 0.00769 Lmol-1s-1