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Kinetics 189
4. Physical state of reactants—Gases and liquids tend to react faster than
solids because of the increase in surface area of the gases and liquids versus
the solid.
5. Catalysts—Using a catalyst increases the reaction rate.
The rate of reaction is the change in concentration per change in time. It is possible
to find the rate of reaction from a graph of concentration of a reactant versus
time. The procedure involves drawing a tangent to the curve at the point in
the reaction where we wish to know the rate.
M
∆M
∆t
Time
The slope of the tangent (∆M/∆t) is the instantaneous rate of the reaction at this
time. To determine the rate at a different time, we would need to draw another tangent
line. In most kinetic studies, we wish to know the initial rate. The initial rate
comes from a tangent drawn to the curve at the very beginning of the reaction.
13-2 Rate Laws
Let’s consider those cases in which the reactant concentration may affect the
speed of reaction. For the general reaction:
a A b B . . . . l c C d D . . .
Where the lower case letters are the coefficients in the balanced chemical equation,
the upper case letters stand for the reactant, and product chemical species
and initial rates are used, then the rate equation (rate law) is:
Rate k[A] m [B] n . . . .
In this expression, k is the rate constant (for the chemical reaction at a given
temperature). The exponents, m and n, are the orders of reaction. The orders
indicate what effect a change in concentration of that particular reactant species
will have on the reaction rate. If, for example, m 1 and n 2, then if the