Reaction Rates
Definition of Rate of Reaction
Rate or speed refers to the amount of change in something per unit of time. The speed of a car might be 30 km/h, in this case the thing which changes was the position of the car and the unit of time was one hour. In a chemical reaction the quantity changing is the concentration of reactants and products or some physical property which is related to one of the concentrations.
Consider the reaction: 2N2O5(g) ==> 4NO2(g) + O2(g)
The rate of reaction can be expressed in terms of the rate of change in concentration (M) of the reactant or either of the products. Two conditions we impose on reaction rate is that it be a positive quantity and of course it must be the same quantity regardless of which substance we use to express it.
First lets write expressions for the change in concentrations of each of the substances in the reaction per unit time
rate(N2O5) = DConc N2O5 / Dt
rate(O2) = DConc O2 / Dt
We can see from the stoichiometry of the reaction above that rate(O2) = 1/2 rate(N2O5), that is every time a molecule of O2 is formed two molecules of N2O5 are consumed. By the same reasoning rate(O2) = 1/4 rate( NO2). Furthermore we should note that rate(N2O5) is a negative quantity since N2O5 is a reactant and DConc N2O5 is negative. We can now write an expression for the rate of the reaction in terms of any of the substances involved in the reaction.
rate = DConc O2 / Dt = 1/4 DConc NO2 / Dt = -1/2 DConc N2O5 / Dt