Why is it generally impossible to predict the rate law for a reaction on the basis of the chemical equation only?
Because the rate law (and everything related to kinetics, actually) depends on the mechanism of the reaction and of the kinetics of each of the steps of that mechanism. When we get a chemical reaction, we can see what the reactants and the products are, but we get no information on what happens in between, we don't know how the reaction proceeds step by step and how fast or slow are those steps.
Why is it generally impossible to predict the rate law for a reaction on the basis...