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
^{2 }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**^{-2}s^{-1 }And to 3 sig figs,** k = 1.88M**^{-2}s^{-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=**M**olarity or **
***M*, and [B]=**M**olarity or
*M*

*Rate units are then equal to ***M****s-1***/(***M**^{1}**M**^{2}*) or *

*M***s**^{-1}*/***M**^{3} or
**s**^{-1}**M**^{-2}

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

*Cody*

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