A complex reaction occurs through a number of steps i.e., elementary reactions. Number of molecules involved in each elementary reaction may be different, i.e., the molecularity of each step may be different. Therefore, it is meaningless to talk of molecularity of the overall complex reaction.

On the other hand, order of complex reaction depends upon the molecularity of the slowest step. Hence, it is not meaningless to talk of the order of a complex reaction.

Balanced chemical equation often leads to incorrect order or rate law. e.g., the following reaction seems to be a tenth order reaction

$$\mathrm{KClO}_3+6 \mathrm{FeSO}_4+3 \mathrm{H}_2 \mathrm{SO}_4 \longrightarrow \mathrm{KCl}+3 \mathrm{H}_2 \mathrm{O}+3 \mathrm{Fe}_2\left(\mathrm{SO}_4\right)_3$$

This is actually a second order reaction. Actually the reaction is complex and occurs in several steps. The order of such reaction is determined by the slowest step in the reaction mechanism.

Order is determined experimentally and is confined to the dependence of observed rate of reaction on the concentration of reactants.

A. $\rightarrow$ (1) B. $\rightarrow$ (2) C. $\rightarrow$ (2) D. $\rightarrow$ (1)

For zero order reaction rate equation may be written as

$$[R]=-k t+\left[R_0\right]\quad\text{.... (i)}$$

Which denotes a straight line equation similar to $y=m x+c$

On transforming (i)

$$\begin{aligned} \frac{[R]-\left[R_0\right]}{t} & =-k \\ k & =\frac{\left[R_0\right]-[R]}{t} \end{aligned}$$

$$\begin{aligned} k & =\text { Rate } \\ \text { Rate } & =k \cdot[t]^0 \\ \text { Rate } & \propto[t]^0 \end{aligned}$$

For a first order reaction $\frac{d x}{d t} \propto[\text{concentration}]^{1}$

$\therefore$ Graph between rate and concentration may be drawn as

$$\begin{aligned} k & =\frac{2.303}{t} \log \frac{[R]_0}{[R]} \\ \frac{k t}{2.303} & =\log \left[\frac{R^{\circ}}{R}\right] \\ \frac{k t}{2.303} & =\log [R]_0-\log [R]\\ \log [R]&=\underset{\text { Slope }}{\left(\frac{-k}{2.303}\right)} t+\underset{\text { Intercept }}{\log [R]_0} \end{aligned}$$

A. $\rightarrow(3)$ B. $\rightarrow$ (1) C. $\rightarrow$ (4) D. $\rightarrow$ (6) E. $\rightarrow$ (2) F. $\rightarrow$ (5)

1. Catalyst alters the rate of reaction by lowering activation energy.

2. Molecularity can't be fraction or zero. If molecularity is zero, then reaction is not possible.

3. Second half-life of first order reaction is same as first because half-life time is temperature independent.

4. $e^{-E_Q / R T}$ refers to the fraction of molecules with kinetic energy equal to or greater than activation energy.

5. Energetically favourable reactions are sometimes slow due to improper orientation of molecule cause some ineffective collision of molecules.

6. Area under the Maxwell, Boltzmann curve is constant because total probability of molecule taking part in a chemical reaction is equal to one.

A. $\rightarrow(2)$ B. $\rightarrow$ (1) C. $\rightarrow$ (3)

1. Diamond can't be converted into graphite under ordinary condition.

2. Instantaneous rate of reaction completes at very short span of time.

3. Average rate of reaction occurs to a long duration of time.