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Order of reaction
The order of reaction shows the effect a particular reactant has on the rate of reaction. It is determined by experiment and can be worked out if you have the rate equation for the reaction. The order of reaction is the power of the concentration term in the rate equation for a particular reactant. The overall order of a reaction is all the individual orders added together. In more advanced degree level work you will find some very strange fractional orders, but at "A" level the reaction will only be zero, first or second order.
In a zero order reaction the reactant has no effect on the rate of reaction and so does not appear in the rate equation. For example, in the following general equation:
|
A + B + C ⇒ D + E |
Rate = k[B][C]2
The reactant, A, does not appear in the rate equation so this reaction is said to be zero order with respect to A. Technically, we can put A into the equation raised to the power zero. Any number to the power zero is 1, and so it has no effect on the equation:
Rate = k[A]0[B][C]2
In a first order reaction the reactant appears to the power 1. However, it is unusual for the power 1 to be written in an equation, the term usually just appears on its own. This is a linear relationship. If we double the concentration, the rate doubles. If we treble the concentration, the rate trebles, and so on. Using the same example above, the reaction is first order with respect to B. Writing the rate equation out in full:
Rate = k[A]0[B]1[C]2
In a second order reaction the reactant appears to the power 2. This is a square relationship. If we double the concentration, the rate quadruples (2 × 2 or 22). If we treble the concentration, the rate goes up by 9 (3 ×3 or 32), and so on. Using the same example above, the reaction is second order with respect to C.
The reaction shown is third order overall (0 + 1 + 2).