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# Chemistry Help Please!

The rate of reaction in terms of the "rate law expression" includes the rate constant (k), the concentration of the reactants, and the orders of the reaction with respect to the different reactants. Consider the following reaction:

A+B→C+D

The initial concentrations of the reactants A and B are 0.220 M and 0.400 M, respectively. The rate of reaction is 0.060 M⋅s^−1, and the orders of the reaction, with respect to reactants A and B, are 1 and 2, respectively.

*Determine the rate constant (k) for the reaction using the rate law.*
Express your answer in M^−2⋅s^−1 to three significant figures.

### 1 Answer by Expert Tutors

Cody P. | Math, Science, Test Prep, & College CounselingMath, Science, Test Prep, & College Coun...
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The rate of reaction in terms of the "rate law expression" includes the rate constant (k), the [concentration] of the reactants, and the orders of the reaction with respect to the different reactants. Consider the following reaction:

A+B→C+D

The initial concentrations of the reactants A and B are 0.220 M and 0.400 M, respectively. The rate of reaction is 0.060 M⋅s^−1, and the orders of the reaction, with respect to reactants A and B, are 1 and 2, respectively.

*Determine the rate constant (k) for the reaction using the rate law.*
Express your answer in M^−2⋅s^−1 to three significant figures.]

Original Problem Above
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Hi Meg,

I made some recognizable changes to the quantities you will need from your original problem above.
First lets start by examining what a rate law tells us.  Since chemists can measure quantities of reactants and products at different times during a reaction, they used this ability to measure to determine a rate for a reaction experimentally.

You need to know that brackets or [ ] mean concentration of reactant. So use the initial [molarity] of each reactant, and raise it to the order given in the problem:  (for A it was 1 and B it was for respective reaction orders)

For your reaction, A + B --> C + D, your rate law should be:

Rate =  k[A]1st order[B]2nd order

What they want you to solve for is the k. If you divide the equation above by [A]1[B]2 you get k by itself.

k = Rate/([A]1[B]2)

Since you have every piece of information highlighted in the problem, just plug the data in where it belongs:

k = 0.060Ms-1/([.2]1[.4]2) and solve with a calculator.

k = 1.875 M-2s-1  And to 3 sig figs, k = 1.88M-2s-1

**A side note on units too.  Just take the units associated with each part of the rate law above and  see what you get when you plug them in.

Rate has units of M.s-1

[A]1=Molarity or M, and [B]=Molarity or M

Rate units are then equal to  Ms-1/(M1M2) or
Ms-1/M3  or s-1M-2

I hope that helped!!!  :-)  Good luck,

Cody