# Rate of a Chemical Reaction

2 Fe^{3+}(aq) + Sn^{2+}(aq) ->2 Fe^{2+}(aq) + Sn^{4+}(aq)

Lets say that 40 seconds after the reaction occurs, [Fe2+] is found to be 0.001 M. So, the change in time, is 38.5 seconds, and the change in concentration is (0.001 - 0) M. The average rate, is the change in molarity divided by the change in time. So, the rate of formation is 2.6 x 10^-5 M/s.

Because one Sn4+ ion is produced for every two Fe2+ ions, so the molarity is 0.0005 M. Therefore, the rate of formation is 1.3 x 10^-5 M/s.

We can generalize this:

Given the following equation:

*a *A + *b *B -> *c *C + *d *D

The rate of reaction can be expressed as so:

## -1/a * [A]/t = -1/b * [B]/t = 1/c * [C]/t = 1/d * [D]/t

# Effect of Concentration on Reaction Rates: The Rate Law

An equation that relates the rate of reaction and the concentrations of reactants is called the **rate law**.

*a *A + *b *B -> *c *C + *d *D

rate of reaction = k[A]^{m}[B]^{n} ...

[A] and [B] are molarities of the reactants, and m and n are usually small, positive, whole numbers, except they can be 0, fractional, or negative.

### Order:

Exponent in the rate law.

For example, if m = 1, this reaction is first order in A, and if n = 2, the reaction is second order in B.

### Overall Order:

Sum of all the exponents: m + n + ...

### Rate Constant:

This constant relates the rate of reaction to concentrations. The larger the value of k, the faster a reaction goes.

# Zero-Order Reactions

An overall zero-order reaction has a rate law where the sum of the exponents equals 0.

### Integrated Rate Law:

Expresses the concentration of a