Question

Zero order reaction is:

Hint:

Zero-order reactions are rare and typically occur in photochemical processes or when a catalyst surface is saturated.

Answer 1

Step 1: Understanding zero-order reactions.
In zero-order reactions, the rate of reaction is independent of the concentration of reactants. Such reactions often occur under special conditions like the presence of a catalyst or on metal surfaces.

Step 2: Analyze the given reactions.
- (A) $2FeCl_{3} + SnCl_{2} \to 2FeCl_{2} + SnCl_{4}$: This is a redox reaction, not zero-order.
- (B) $H_{2} + Cl_{2} \to 2HCl$: This reaction proceeds in the presence of light (photochemical reaction). Under high light intensity, the rate becomes independent of reactant concentration, hence it is zero-order.
- (C) $CH_{3}COOC_{2}H_{5} + NaOH \to CH_{3}COONa + C_{2}H_{5}OH$: This is saponification, usually a second-order reaction.
- (D) $CH_{3}COOCH_{3} + H_{2}O \to CH_{3}COOH + CH_{3}OH$: This is acid-catalyzed hydrolysis of ester, which is generally a pseudo-first-order reaction.

Step 3: Conclusion.
Thus, the only zero-order reaction among the options is the photochemical reaction of $H_{2}$ and $Cl_{2}$ to form $HCl$.

Final Answer: \[ \boxed{(B) \, H_{2} + Cl_{2} \longrightarrow 2HCl} \]