1.2 The rate expression and order of reaction

The rate expression tells us by how much a reactant participates in the reaction, and the contribution they have. In a simple reaction, one species may have more of an effect then another. Also, species that are not involved in the chemical reaction could also affect the rate of reaction, e.g. catalysts.

This is expressed via reaction ORDERS. An order is a reaction expresses to which power the concentration of that species affects the rate. How the rate dpeends on that chemical and it’s concentration.

Order Zero: A change in concentration of the species does NOT affect the rate.

Order First: A change is concentration of the species has exactly the same effect on the rate. (If the concentration doubles, the rate doubles.)

Order Second: A change in concentration of the species has a squared effect on the rate. If the concentration doubles, the rate would quadruple. Or, if it tripled, the rate would multiply by nine, and so on.

A rate equation is set out as: Rate = k[A][B].

The letters in square brackets represent the concentrations of particular species. The orders apply to these. So, if A was order two, and B was order one, the rate equation would look like this:

Rate=k[A]2[B]

(There need not be a ‘1’ to show first order, as like ‘x’ meaning ‘1x’ in algebra, the single, un-numbered bracket shows order one.)

When a sepecies is order zero, it is not shown in the rate equation. So, if A was order 1, B was order 0, and C was order 2, it would look like this:

Rate=k[A][C]2

K is the equilibrium constant. K is different for every different reaction, dependant upon what is reacting, and the conditions.

To work out K’s units, you use cancelling. In Rate=k[A][B],

moldm-3s-1 = k x moldm-3 x moldm-3

moldm-3s-1= Units of rate.

moldm-3 = Units of species concetration.

Cancelling… moldm-3s-1 = k x moldm-3 x moldm-3

Therefore, K= mol-1dm3s-1, as the s-1 has multiplied with the moldm-3.

The overall order of a reaction, is all of the orders added together. So, if Rate=k[A][B]2, the overall order would be 1+2=3.