Question

From the diagram

Δ_rH for the reaction C → A is

• A. +35J
• B. -15J
• C. -35J
• D. +15J

Answer 1

The Correct Option is C

The given reactions and their respective enthalpy changes are:

• \( A \to 2B \), \( \Delta H_1 = +10 \, \text{J} \)
• \( 2B \to C \), \( \Delta H_2 = +25 \, \text{J} \)

To find \( \Delta_r H \) for the reaction \( C \to A \):

According to Hess's Law, we reverse the given reactions:

• Reversing \( A \to 2B \) gives \( 2B \to A \), with \( \Delta H = -10 \, \text{J} \).
• Reversing \( 2B \to C \) gives \( C \to 2B \), with \( \Delta H = -25 \, \text{J} \).

Adding the reactions:

\[ \Delta H_{\text{total}} = -25 + (-10) = -35 \, \text{J} \]

The enthalpy change for the reaction \( C \to A \) is \( \Delta_r H = -35 \, \text{J} \), so the correct answer is (C) -35 J.

Answer 2

From the diagram, the total enthalpy change for the reaction \( C \to A \) is the sum of the enthalpy changes of the intermediate steps. \[ \Delta H = \Delta H_1 + \Delta H_2 = 10 \, \text{J} + (-25 \, \text{J}) = -35 \, \text{J} \]