Question

Consider a general reaction $aA+bB\to cC+dD$. The rate expression for the reaction is Rate$=K\left[A{]}^x\right[B{]}^y$
(i) Establish the significance of ${}^{\prime }(a+b{)}^{\prime }$ and ${}^{\prime }(x+y{)}^{\prime }$ in terms of order and molecularity.
(ii) Write any two differences between order and molecularity.

Answer 1

(i) $aA+bB⟶cC+dDRate=K{\left[A\right]}^x[B{]}^y$

If the reaction is an elementary or single step reaction then ${}^{\prime }(a+b{)}^{\prime }$ is equal to ${}^{\prime }(x+y{)}^{\prime }$ and $x$ and $y$ would be called molecularity of the reaction.

If the reaction is not an elementary or ${}^{\prime }(a+b{)}^{\prime }$ not equal to${}^{\prime }(x+y{)}^{\prime }$ and $x$ and $y$ would be called order of the reaction.

(ii)

Molecularity	Order
It is the total number of reacting species (molecules, atoms or ions) which bring the chemical change.	It is the sum of powers of molar concentration of the reacting species in the rate equation of the reaction
It is always a whole number	It may be whole number, fractional or zero.