Question

1. 2A → B + C

Two trials of the above reaction are run. The concentration of A in the second trial is three times that in the first. It is found that the initial rate of the reaction in the second trial is nine times the initial rate in the first trial. This indicates that the reaction is ...

2.

The following reaction is run and the data obtained:

2NO(g) + 2H₂(g) → N₂(g) + 2H₂)(g)

Trial	PNO	PH2	Initial Rate
1	1 atm	1 atm	0.02 atm/s
2	2 atm	1 atm	0.16 atm/s
3	1 atm	2 atm	0.04 atm/s

What is the order of the reaction with respect to NO?

3. 2A → B + C

The above reaction is run and found to follow zero order kinetics with a rate constant of 1.30 x 10^-3 M•sec^-1. If the initial concentration of A is 1.38 M, what is the concentration after 126 seconds?

Answer 1

Q1.

reaction :

2A → B + C

Trial 1:

1x trial rate

Trial 2

3A x Trial 1

it is found that 9x is the effect

that is, 2nd order since doubling will imply 3^2 = 9 x effect

2nd order with respect to A

Q2.

In order to calculate the rate law expression for a A+B reaction, we need to apply Initial Rates Method.

Note that the generic formula goes as follows:

r = k [A]^a [B]^b

Note that if we got at least 3 sets of point, in which we have A and B constant, then we could use:

r1 / r2 = (k1 [A]1^a [B]1^b) / (k2 [A]2^a [B]2^b)

If we assume K1 and K2 are constant, then K1= K2 cancel each other

r1 / r2 = ([A]1^a [B]1^b) / ( [A]2^a [B]2^b)

Then, order according to [A] and [B]

r1 / r2 = ([A]1/[A2])^a * ([B]1/[B]2)^b

If we get two points in which A1 = A2, then we could get B, and vise versa for A...

From the data shown in YOUR table

2NO(g) + 2H2(g) → N2(g) + 2H2)(g)

choose point 1 and so H2 cancels:

trial 1; 1 atm --> 0.02,

Trial 2; 2 atm --> 0.16;

0.02/0.16 = (2/1)^a

a = ln(0.125) / ln(2)

a = 3rd order with respect to NO

C)

2A → B + C

note that if zero order

A = A0 - kt

A = 1.38 - (1.3*10^-3)*126

A = 1.2162 M left