For questions 3, 4, 5, 6 refer the equation and data given below. The following

Question

For questions 3, 4, 5, 6 refer the equation and data given below. The following questions refer to the reaction shown below: A + 2B rightarrow 2AB What is the rate law for this reaction? [A] Rate = k[A]2[B] [B] Rate = k[A][B]2 [C] Rate = k[A]2[B]2 [D] Rate = k[A][B] [E] Rate = k[B] What is the magnitude of the rate constant for the reaction? [A] 140 [B] 21 [C] 119 [D] 164 [E] 79 5. What are the units for the rate constant for this reaction? [A] L3/mol3/s [B] mol/L/s [C] L/mol/s [D] mol3/L [E] L2/mol2/s What is the overall order of this reaction? [A] 2 [B] 4 [C] 3 [D] 1 [E] 0

Explanation / Answer

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

Choose point 1 and 2...

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

substitute

(0.08) / (0.30) = (0.16/0.16)^a * (0.15/0.30)^b

Cleary, the coefficient cancels:

0.2666 = 1 * (0.5)^b

solve,

ln(0.2666 ) / ln(0.5) = b

b = 1.91 .. almost 2

Choose now points 2 and 3:

r2 / r3 = ([A]2/[A]3)^a * ([B]2/[B]3)^b

substitute

(0.30)/(0.08) = (0.16/0.08)^a * (0.30/0.30)^b

Cleary, the coefficient cancels:

3.75 = (2)^a * 1

solve,

ln(3.75 ) / ln(2) = a

a = 1.90

so...

a = 2, b = 2

then

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

so

r = k [A]^2 [B]^2

Q3:

choose the rat to be [C] which is Rate = k*[A] ^2[B]^2 which is the neraest

Q4.

For "k" value... choose any point in your set of data, I will choose 1 for simplicity

substitute data

r = k*A[]^2[B]^2

0.08 = k*(0.16^2)(0.15^2)

K = 0.08/((0.16^2)(0.15^2)) = 138.888 ---> 140 is the best answer

Q5.

units are according to order.. that is, fourth order, so choose

1/(M^3-s) --> L^3 / (mol^3-s)

A

Q6.

the overall order is then a+ b = 2+2 = 4 foruth

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