Question

A particular redox reaction, provided below, was analyzed by the method of initial rates.

8 ClO3- + 2 MnO4- + 6 H+ --> 8 ClO4- + 2 Mn2+ + 3 H2

The following concentration and initial rate data was obtained:

Exp #	[MnO4-]	[ClO3-]	[H+]	Initial Rate (M/s)
1	0.100	0.100	0.100	1.20
2	0.200	0.100	0.100	4.80
3	0.100	0.200	0.100	2.40
4	0.100	0.100	0.200	1.20

Part #1:

If you desire to determine the order for ClO3-, which TWO experiments are best?

  A) 1 and 2

B) 1 and 3

C) 1 and 4

D) 2 and 3

E) 2 and 4

F) 3 and 4

Part #2:

What is the reaction order for ClO3-?

Part #3:

What is the reaction order for MnO4-?

Part #4:

What is the reaction order for H+?

  A) 0

B) 1

C) 2

Part #5:

What is the overall reaction order?

Part #6:

What are the appropriate units of the rate constant k?

A) M/s

B) M4/s

C) 1/s

D) 1/(M2*s)

Part #7:

What is the value of the rate constant k?

Answer 1

Part #1:  If you desire to determine the order for ClO3-, experiments 1 and 3 are best because in experiments 1 & 3 the concentration of MnO4- and H+ is constant while the concentration of ClO3- is changing.

So, B) 1 and 3 is the correct choice.

Part #2: Suppose the order of the reaction is a, b and c with respect to MnO4-, ClO3- and H+ respectively.

So the rate law of the reaction is given as:

rate = k [MnO4-]^a[ClO3-]^b[H+]^c

Taking experiment 1 & 2:

1.20 =  k [0.100]^a[0.100]^b[0.100]^c .............(1)

4.80 = k [0.200]^a[0.100]^b[0.100]^c .............(2)

Dividing equation 2 by 1: we get:

4.80 / 1.20 = [0.200]^a / [0.100]^a

4 = 2^a

a = 2

Again taking experiments 1 and 3:

1.20 =  k [0.100]^a[0.100]^b[0.100]^c .............(3)

2.40 =  k [0.100]^a[0.200]^b[0.100]^c .............(4)

Dividing equation 4 by 3: we get:

2.40 / 1.20 = [0.200]^b / [0.100]^b

2 = 2^b

b = 1

Again taking experiments 1 and 4:

1.20 =  k [0.100]^a[0.100]^b[0.100]^c .............(5)

1.20 =  k [0.100]^a[0.100]^b[0.200]^c .............(6)

Dividing equation 6 by 5: we get:

1.20 / 1.20 = [0.200]^c / [0.100]^c

1 = 2^c

c = 0

Hence, the reaction order for ClO3- = b = 1

Part #3:

The reaction order for MnO4- = a = 2

Part #4:

The reaction order for H+ = c = 0

Part #5:

The overall reaction order = a + b + c

= 2 + 1 + 0

= 3

Part #6:

rate = k [MnO4-]² [ClO3-]

M/s = k (M)³

k = (M/s) / (M)³

k = 1 / (M²s)

So, D) 1/(M2*s) is the correct choice.

Part #7: Putting values of a, b and c in equation 1:

1.20 =  k [0.100]² [0.100]¹ [0.100]⁰

1.20 = k (0.01)(0.10)

k = 1.20 / 0.001

k = 1200 1 / (M²s)