The following example is given in my textbook:
If a chemical equation is:
NO2 (g) + CO (g) -> NO (g) + CO2 (g)
The rate law is:
Rate = k[NO2]^2
Usually you put in the reactants in their own sets of brackets, so why isn't the reactant CO involved in the rate law?
I get it now - the order of CO is 0 and anything to the 0 power is 1.
You're correct in observing that typically in a rate law, we include the reactants in separate sets of brackets. The purpose of including reactants in the rate law is to determine how the concentration of each reactant affects the rate of the reaction. However, in this specific example, the reactant CO (carbon monoxide) is not included in the rate law.
The reason for omitting CO in the rate law is that its concentration does not have any effect on the rate of the reaction. In other words, the order of CO in this reaction is zero. When the order of a reactant is zero, it means that its concentration has no impact on the overall rate of the reaction.
Mathematically, any number raised to the power of zero equals 1. Therefore, when we raise the concentration of CO to the power of zero in the rate law equation, it remains as 1. As a result, the rate equation becomes:
Rate = k [NO2]^2
Here, only the concentration of NO2 determines the rate of the reaction, while the concentration of CO does not play a role.
It's important to note that the order of a reactant in the rate law can only be determined experimentally. The rate law is derived from experimental data, and the exponent for each reactant is determined by analyzing the effect of changing the concentration of that particular reactant while keeping the others constant.
In summary, the reactant CO is not included in the rate law because its concentration does not affect the rate of the reaction. Its order is zero, and raising it to the power of zero, as in any number raised to the power of zero, yields 1.