Please help me with chemistry rates calculation.

Question

The data below were collected for the following reaction.

                    

A (aq) + 2B (aq) → 2C (aq) + D (aq)

Run	A (mol/L)	B (mol/L)	Initial Rate (mol/L.s)
1	0.025	0.050	1.24 x 10^-2
2	0.025	0.025	6.20 x 10^-3
3	0.050	0.025	6.20 x 10^-3

    

a) Determine the rate law reaction for the reaction.   

b) Determine the rate constant for the reaction. Include units.                

c) What is the initial rate of reaction when [A] is 0.075 M and [B] is 0.025 M?   

Answer 1

In problems such as these, you want to look for runs that have only 1 variable, i.e. where only the concentration of 1 reactant changes at a time.

(a)

So, for A, we'd want to compare run 2 to run 3, where [A] doubles and [B] remains the same. Under these conditions, the rate stays the same and doesn't change. This tells us that the reaction is ZERO ORDER in A.

Next, compare run 2 to run 1, where [B] doubles and [A] remains the same. Under these conditions, the rate also doubles. This tells us that the reaction is FIRST ORDER in B

Once we know the order for each reactant, we can write the rate law...

Rate = k[B] ... This is the rate law

note that [A] is NOT included since the reaction is zero order in A

(b)

To determine the rate constant, choose the data from any run (I'll use run 1) and plug the values into the rate law equation and solve for k (the rate constant)

1.24x10^-2 M/s = k(0.05 M)

k = 1.24x10^-2 M/s ÷ 0.05 M

k = 0.248 s^-1

(c) Rate = 0.248 s-1 [0.025 M]

rate = 0.0062 M/s = 6.2x10^-3 M/s

(note that M/s is the same as mol/Ls)