Reaction mechanism is the path by which a reaction takes place.
Rate of reaction
The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.
So this can be expressed as Δ concentration/Δ time. That is change in concenation divided by time taken for the change.
Molar concentration i.e., moles per liter (M), is used in these equations.
The brackets, [ ] are always used to to indicate molar concentrations.
The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.
For the reaction aA + bB → products
The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.
Rate = k[A]^x[B]^y
Law of Mass Action
In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.
For the reaction aA + bB → cC (when it is a rate determining step)
Rate of reaction is proportional to [A]^a[B]^b
The above proportionality can be written as an equation, by putting in a proportionality constant k.
Rate = k *[A]^a[B]^b
K is called the specific rate constant
Note rate law and law of mass action are different expressions.