# How can a catalyst not be included in a rate equation if, by definition, it speeds up a chemical reaction?

I thought that anything not in a rate equation was automatically zeroth order and therefore did not affect the reaction. However, I have heard that catalysts can be involved in a reaction while not being in a rate equation. How can this be possible?

• Rate equation is generally created on empirical basis to establish a relationship between the rate and reactant concentration. The dependence on catalysis concentration is generally complex and most cases not needed (only later optimised or used in excess quantity).
– Greg
Oct 12 '14 at 19:26
• So if I understand correctly, as with temperature, the rate constant also depends on the particular concentration of any active catalyst which is involved - this being found experimentally. Correct? Oct 12 '14 at 19:29
• >catalyst not be included in a rate equation || WTF? catalyst's concentraion (in case of homogenous catalyst) should be in rate equation (usually, if the mechanism is complicated enough, some wanders may happen). Heterogenous catalyst is usually not included in rate equation as it is, well, heterogenous and not in reaction mixture, so it's concentraion in the mixtire is zero by definition (but concentration of active particles on surface of the catalyst shoud be included into the rate equation) Oct 12 '14 at 19:35
• Granted that the more catalyst there is, the faster the reaction will proceed. Since a catalyst provides a different route to the products, a different rate equation is obtained. This is now a 3 (or more) particle interaction, which is difficult to model. I have not seen any studies with the rate changes with the amount of catalyst.
– LDC3
Oct 12 '14 at 20:18
• @LDC3 From PChem point of view a bound state of one of the reactant and the catalyst is a single particle, and the bound state + other reactant reaction is a separate, two-body elementary reaction. By three body reactions we mean collisions of three independent entity,which has markedly different kinetic and dynamic behavior, and also a pretty low chance to occur at all.
– Greg
Oct 14 '14 at 2:14

• There is a slight difference in the concentrations for catalysts and enzymes. For most non-enzymatic reactions, the catalyst concentration is less than any of the reactants. For most enzymatic reactions, such as the enzyme catalase converting $\ce {H2O2}$, the enzyme has a greater concentration than $\ce {H2O2}$ under normal conditions.