chemistry experiment

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	0.468	1.778	0.00655
2	0.468	2.665	0.00982
3	1.165	1.778	0.0163

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

s–1

L mol–1 s–1

L2 mol–2 s–1

NOTE: You can enter your answer either as a decimal or in scientific notation.

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For example: enter 1.24 x 10^-3 as 1.24e-3

or simply as 0.00124

Answer 1

Kinetics

Order with respect to C₂H₄Br₂:

compare experiment 1 to 3. [C₂H₄Br₂] increases by 2.5 x while [KI] is constant, and the rate increases by 2.5 x. So it is FIRST ORDER in C₂H₄Br₂

Order with respect to :

compare experiment 1 to 2. [KI] increases by 1.5 x while [C₂H₄Br₂] is constant, and the rate increases by 1.5x. So it is FIRST ORDER IN KI

To determine the rate constant, we first write the rate law, and then substitute values and solve.

Rate = k[C₂H₄Br₂][KI]

0.00655 Ms^-1 = k (0.468 M)(1.778 M)

k = 7.87x10^-3 M^-1s^-1 = 0.00787 M^-1s^-1 or 7.87e^-3 M^-1s^-1 (note: this is the same unit as L mol^–1 s^–1)