why is it that [H2O] is NOT included in the rate law, even though it is a reactant?
The reason why water ([H2O]) is not usually included in the rate law of a chemical reaction, even though it is a reactant, is because it is typically present in large excess compared to the other reactants.
The rate law is an expression that describes the relationship between the concentrations of the reactants and the rate of the reaction. It reveals how changes in the concentrations of the reactants affect the overall reaction rate.
When water is present in large excess, its concentration remains approximately constant throughout the reaction. Therefore, any changes in the concentration of water would have little effect on the reaction rate. Consequently, it is common practice to disregard the concentration of water and include only the reactants whose concentrations significantly impact the rate of the reaction. This simplifies the rate expression and makes it easier to determine the dependencies of the reaction rate on the relevant reactants.