Rate of a Chemical Reaction
2 Fe3+(aq) + Sn2+(aq) ->2 Fe2+(aq) + Sn4+(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