The correct option for the question regarding the reaction between hydrogen and iodine, represented by the equation: \[ \text{H}_2(g) + \text{I}_2(g) \rightarrow 2\text{HI}(g) \] is **D. a decrease in temperature will favour the forward reaction**. Let's break down why this is the correct answer and analyze the other options. ### Explanation of the Correct Answer (Option D) 1. **Understanding Exothermic Reactions**: The reaction given is exothermic, meaning it releases heat. In an exothermic reaction, the products (in this case, HI) have lower energy than the reactants (H₂ and I₂). When heat is released, it can be thought of as a product of the reaction. 2. **Le Chatelier's Principle**: This principle states that if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust to counteract that change and restore a new equilibrium. 3. **Effect of Temperature**: For exothermic reactions, if the temperature is decreased, the system will respond by favoring the forward reaction (the formation of products) to produce more heat. Therefore, a decrease in temperature shifts the equilibrium to the right, favoring the production of HI. ### Why the Other Options are Incorrect **Option A: An increase in temperature favours the forward reaction.** - This statement is incorrect because increasing the temperature for an exothermic reaction shifts the equilibrium to the left (toward the reactants). The system tries to absorb the added heat by favoring the endothermic direction (the reverse reaction), which is not what we want if we are trying to produce more HI. **Option B: An increase in pressure favours the backward reaction.** - This option is misleading. While it is true that increasing pressure can favor the side of the reaction with fewer moles of gas, in this case, both sides of the reaction have the same number of moles (1 mole of H₂ + 1 mole of I₂ = 2 moles of HI). Therefore, an increase in pressure does not favor either the forward or backward reaction significantly. **Option C: Both pressure and temperature must be increased to favour the forward reaction.** - This option is incorrect for the same reasons as options A and B. Increasing temperature does not favor the forward reaction in an exothermic process, and pressure does not have a significant effect due to the equal number of moles of gas on both sides of the equation. ### Summary of Key Points - **Exothermic Reaction**: The reaction releases heat, and the products have lower energy than the reactants. - **Le Chatelier's Principle**: A decrease in temperature favors the forward reaction in exothermic processes. - **Pressure Effects**: Increasing pressure does not favor either side of the reaction due to equal moles of gas. - **Temperature Effects**: Increasing temperature shifts the equilibrium toward the reactants in exothermic reactions. ### Revision Summary - The reaction \( \text{H}_2 + \text{I}_2 \rightarrow 2\text{HI} \) is exothermic. - Decreasing temperature favors the formation of products (HI) in exothermic reactions. - Increasing pressure does not favor either side due to equal moles of gas. - Always apply Le Chatelier's Principle to predict the effect of changes in conditions on equilibrium.