Question

For the reaction$\underset{\mathrm{Hydrogen}\mathrm{gas}}{\underbrace{{\mathrm{H}}_2\left(\mathrm{g}\right)}}+\underset{\mathrm{chlorine}\mathrm{gas}}{\underbrace{{\mathrm{Cl}}_2\left(\mathrm{g}\right)}}\stackrel{\mathrm{Sunlight}}{\to }\underset{\mathrm{Hydrochloric}\mathrm{acid}}{\underbrace{2\mathrm{HCl}\left(\mathrm{l}\right)}}$. The order of the reaction is

• A. $0$
• B. $1$
• C. $2$
• D. $3$

Answer 1

The correct option is A

$0$

The explanation for the correct answer:

Option (A) $0$:

1. A photochemical reaction is a light-induced process in which every molecule exposed to the light conducts the reaction by absorbing light of a certain wavelength.
2. Because photochemical reactions involve either overcoming the activation barrier or not, and nothing in between, these processes are unaffected by the concentration of your molecules.
3. When the rate law is unaffected by the concentration, the rate law is given as $\mathrm{r}=\mathrm{k}{\left[\mathrm{A}\right]}^0=\mathrm{k}$ i.e. zero-order reaction.
4. Hence, this is the correct option.

The explanation for the incorrect answer:

Option (B) $1$:

1. A first-order reaction's rate is proportional to the reactant's concentration. In other words, double the concentration twice the rate of reaction.
2. The reaction is unaffected by the concentration change in this reaction.
3. Hence, it is an incorrect option.

Option (C) $2$:

1. When the overall order is two, the reaction is called a second-order reaction. The square of one reactant's concentration determines the rate of these reactions.
2. Some combustion and decomposition reactions are examples of second-order reactions. The reaction we have been given is a photochemical reaction that is unaffected by reactant concentration.
3. Hence, it is an incorrect option.

Option (D) $3$:

1. If the rate of a reaction is determined by the three terms of concentration variation, it is said to be of the third order.
2. Because the rate of the given reaction is independent of the concentration, it is not a third-order reaction.
3. Hence, it is an incorrect option.

Therefore, the reaction in question is of zero order.