I know that a species that does not appear in the chemical equation may also affect the rate of a reaction - e.g. a catalyst. But does that mean the catalyst can be present in the rate equation, and if so are catalysts always present in the rate equation?
Any reaction with a finite amount of reactants has a half-life, whether it's first order, second order, zero order or complex order. The half-life (t?) is defined as the time taken for the reaction to go half-way to completion. If the reaction is: A + B ---products and A is in excess, then t? will be the time taken for half of B to be used up. For all reactions, then, you get a decay curve. For zero-order reactions, this 'curve' is a straight line, but for all other orders, the curve is an actual curve and it is quite difficult to distinguish, by visual inspection alone, whether it is exponential (indicating a first-order reaction) or hyperbolic (indicating a second or higher order reaction).
All the factors that affect a reaction rate, except for concentration and reaction order, are taken into account in the rate equation of the reaction. A catalyst: The presence of a catalyst increases the reaction rate (in both the forward and reverse reactions) by providing an alternative pathway with a lower activation energy. en.wikipedia.org/wiki/Reaction_ra...
There are many reactions that would not happen to any extent without a catalyst, yet I have never seen a rate equation that included a catalyst. For example, the Haber reaction requires a catalyst, but the rate equation for it does not include a catalyst. If a reaction will work without a catalyst, you can find its rate under this condition. Then if you run the same reaction with a catalyst you will get a different rate of reaction but the catalyst concentration does not show up in the Rate law for that reaction.
Yes, a catalyst can be included in a rate law. That's because, most reactions occur in a series of step and the rate is based on the rate determining step, which is the slowest step. A catalyst may be a reactant in the rate determining step, and a product in a subsequent step. Therefore, the catalyst is not included in the overall reaction. But a catalyst need not always be in the rate law. The rate law is usually based on the rate determining step. ========== Follow up =========== In the free response questions on the AP chemistry exam there has been at least one case where a rate law included a catalyst( 2002D). Brown and LeMay always include at least one in their examples, and I always cover this situation when I teach rate laws and mechanisms in AP chemistry. Here is one comment: Other examples of species not in the balanced reaction occurring in the rate law would include catalysis, where a catalyst does not normally appear in the balanced reaction but does appear in the rate law. www.chem.arizona.edu/~salzmanr/48... Consider this generalized reaction which is catalyzed by M A + B --C A + M --Q ... slow Q + B --C ... fast M is the catalyst, and Q is the intermediate. The slow, or rate determining step, depends only on the concentrations of A and M, the catalyst. So even though the overall reaction does not include, M, the rate law does. Rate = k[A][M]
