Question

HCl(g) can be synthesized from H2(g) and Cl2(g) as represented above. A student studying the kinetics of the reaction proposes the following mechanism Step 1: Cl2(g) → 2 Cl(g) (slow) ∆H° = 242 kJ/molrxn Step 2: H2(g) + Cl(g) → HCl(g) + H(g) (fast) ∆H° = 4 kJ/molrxn Step 3: H(g) + Cl(g) → HCl(g) (fast) ∆H° = -432 kJ/molrxn Which of the following statements identifies the greatest single reason that the value of Kp for the overall reaction at 298 K has such a large magnitude?

Answer 1

The proposed mechanism for the synthesis of HCl(g) involves three steps. The large magnitude of Kp for the overall reaction is due to the high concentration of the products compared to the reactants. The reaction is exothermic and releases excess energy as heat.

Step-by-step explanation:

The proposed mechanism for the synthesis of HCl(g) from H2(g) and Cl2(g) consists of three steps. In the first step, Cl2(g) is converted into 2 Cl(g) slowly, with an enthalpy change (∆H°) of 242 kJ/molrxn. This step is followed by two fast steps: the second step involves the reaction of H2(g) with Cl(g) to form HCl(g) and H(g) with an ∆H° of 4 kJ/molrxn, while the third step involves the reaction of H(g) with Cl(g) to form HCl(g) with an ∆H° of -432 kJ/molrxn.

The value of Kp for the overall reaction at 298 K has a large magnitude due to the high concentration of the products compared to the reactants. Since the second and third steps of the mechanism are fast, H(g) formed in the second step reacts quickly with Cl(g) in the third step to form HCl(g). This leads to the rapid production of HCl and increases the concentration of HCl, resulting in a large Kp value.

This excess energy is released as heat, making the overall reaction exothermic. The value for the standard molar enthalpy of formation of HCl(g) obtained from Appendix G is -92.307 kJ/mol, which confirms that the reaction is exothermic.