Q.Define rate law and rate constant.

Step 1. For a general reaction $xA+yB\rightarrow\text{products}$, the rate law is the experimentally-established equation $\text{Rate}=k[A]^m[B]^n$, where the exponents m and n (the individual orders) must be measured, never assumed from the balanced equation's coefficients.

Step 2. The rate constant k is the fixed proportionality factor appearing in that rate law. It is characteristic of a given reaction at a given temperature.

Step 3. Setting $[A]=[B]=1$ mol L$^{-1}$ in the rate law makes $\text{Rate}=k(1)^m(1)^n=k$ -- so k can also be defined as the rate the reaction WOULD have if every reactant were present at unit concentration.

State the general rate law form and identify k as its proportionality constant, then give the unit-concentration interpretation of k as a concrete physical meaning.

• Describing the rate law using the stoichiometric coefficients x, y instead of the experimentally-determined orders m, n -- these coincide only by chance, not by rule.