In a reaction if the concentration of reactant $A$ is tripled, the rate of reaction becomes twenty seven times. What is the order of the reaction?
Rate of any elementary reaction can be represented as
$$r=k[A]^n$$
After changing concentration to its triple value $A=3 A, r$ becomes $27 r$
$$\begin{aligned} & 27 r=k[3 A]^n \\ & \frac{r}{27 r}=\frac{k[A]^n}{k[3 A]^n} \\ & \frac{1}{27}=\left[\frac{1}{3}\right]^n \Rightarrow\left[\frac{1}{3}\right]^3=\left[\frac{1}{3}\right]^n \end{aligned}$$
Hence, $n=3$
Order of reaction is three.
Derive an expression to calculate time required for completion of zero order reaction.
For zero order reaction $[R]=[R]_0-k t$
For completion of the reaction $[R]=0$
$$\therefore \quad t=\frac{[R]_0}{k}$$
For a reaction $A+B \longrightarrow$ Products, the rate law is - Rate $=k[A][B]^{3 / 2}$. Can the reaction be an elementary reaction? Explain.
During an elementary reaction, the number or atoms or ions colliding to react is referred to as molecularity. Had this been an elementary reaction, the order of reaction with respect to $B$ would have been 1, but in the given rate law it is $\frac{3}{2}$. This indicates that the reaction is not an elementary reaction. Hence, this reaction must be a complex reaction.
For a certain reaction large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why?
According to collision theory apart from the energy considerations, the colliding molecules should also have proper orientation for effective collision.
This condition might not be getting fulfilled in the reaction as it shows the number of reactants taking part in a reaction, which can never be zero.
For a zero order reaction will the molecularity be equal to zero? Explain.
No, the molecularity can never be zero or a fractional number as it shows the number of reactants taking part in a reaction which can never be zero.