Textbook Question
Show how Equations 18.7 and 18.9, and the combination reaction that leads to the formation of molecular oxygen, 2O(𝑔)⟶O2(𝑔), can be added to give Equation 18.10.
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8. Thermochemistry
Hess's Law
Multiple Choice
Using Hess's Law, calculate ΔH for the reaction 2 S (s) + 3 O2 (g) → 2 SO3 (g) at 298 K given the following reactions and their enthalpy changes:1. S (s) + O2 (g) → SO2 (g) ΔH = -296.1 kJ2. 2 SO3 (g) → 2 SO2 (g) + O2 (g) ΔH = 198.2 kJ
A
-198.0 kJ
B
-792.4 kJ
C
-494.3 kJ
D
-197.8 kJ
Verified step by step guidance1
Identify the target reaction: 2 S (s) + 3 O2 (g) → 2 SO3 (g). We need to manipulate the given reactions to match this target reaction.
Examine the given reactions: Reaction 1 is S (s) + O2 (g) → SO2 (g) with ΔH = -296.1 kJ. Reaction 2 is 2 SO3 (g) → 2 SO2 (g) + O2 (g) with ΔH = 198.2 kJ.
Reverse Reaction 2 to match the formation of SO3: 2 SO2 (g) + O2 (g) → 2 SO3 (g). When reversing a reaction, change the sign of ΔH, so ΔH becomes -198.2 kJ.
Multiply Reaction 1 by 2 to match the stoichiometry of the target reaction: 2 S (s) + 2 O2 (g) → 2 SO2 (g). Multiply ΔH by 2, resulting in ΔH = 2(-296.1 kJ) = -592.2 kJ.
Add the modified reactions: (2 S (s) + 2 O2 (g) → 2 SO2 (g)) + (2 SO2 (g) + O2 (g) → 2 SO3 (g)). The intermediate 2 SO2 (g) cancels out, resulting in the target reaction 2 S (s) + 3 O2 (g) → 2 SO3 (g). Sum the ΔH values: -592.2 kJ + (-198.2 kJ) to find the overall ΔH for the target reaction.
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