Chemistry-Hess's Law
NEET Chemistry Hess's Law MCQ Question
Type: MCQ-numerical-Hard-Class 11
Using Hess's Law, calculate the standard enthalpy change for the reaction: C(graphite, s) + 2H2(g) → CH4(g), given the following reactions: 1) C(graphite, s) + O2(g) → CO2(g); ∆rH⁰ = -393.5 kJ mol⁻¹, 2) H2(g) + 1/2 O2(g) → H2O(l); ∆rH⁰ = -285.8 kJ mol⁻¹, 3) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l); ∆rH⁰ = -890.3 kJ mol⁻¹.
A
-74.8 kJ mol⁻¹
B
74.8 kJ mol⁻¹
C
-285.8 kJ mol⁻¹
D
285.8 kJ mol⁻¹
Correct Answer
Option A
Detailed Explanation
Hess's Law states that the total enthalpy change is the sum of the enthalpy changes of individual steps. Using the given reactions and their enthalpies, the enthalpy change for C(graphite, s) + 2H2(g) → CH4(g) is calculated as: -393.5 - 2*(-285.8) + 890.3 = -74.8 kJ mol⁻¹.
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