Multiple Choice
Calculate the ∆Hrxn for the following thermochemical equation:
When given the following:
7. Energy, Rate and Equilibrium
Hess's Law
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Calculate the ∆Hrxn for
Given the following set of reactions:
A
-494.4 kJ
B
-692.8 kJ
C
494.4 kJ
D
-346.4 kJ
Verified step by step guidance1
Identify the target reaction: S (s) + \frac{3}{2} O_2 (g) \rightarrow SO_3 (g). We need to find the \Delta H_{rxn} for this reaction.
Examine the given reactions and their enthalpy changes: \frac{1}{2} S (s) + \frac{1}{2} O_2 (g) \rightarrow \frac{1}{2} SO_2 (g) with \Delta H^\circ = -296.8 \text{ kJ}, and 2 SO_3 (g) \rightarrow 2 SO_2 (g) + O_2 (g) with \Delta H^\circ = 198.4 \text{ kJ}.
Use Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step. We need to manipulate the given reactions to match the target reaction.
Reverse the second reaction to get: 2 SO_2 (g) + O_2 (g) \rightarrow 2 SO_3 (g). This changes the sign of \Delta H^\circ to -198.4 \text{ kJ}.
Add the modified reactions: \frac{1}{2} S (s) + \frac{1}{2} O_2 (g) \rightarrow \frac{1}{2} SO_2 (g) and 2 SO_2 (g) + O_2 (g) \rightarrow 2 SO_3 (g). Adjust stoichiometry and sum the enthalpy changes to find \Delta H_{rxn} for the target reaction.
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