Calculate ΔH∘r×n for the reaction below given the other two reactions.
8A(g)+2C(g)→A8C2(g)
Given:
A2B2(g)→2A(g)+2B(g)ΔH∘rxn=−231kJA8C2(g)+8B(g)→4A2B2(g)+2C(g)ΔH∘rxn=+702kJ1,626kJ−222kJ471kJ222kJ−1,626kJ
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Answer from Sia
Posted 6 months ago
Solution
1
Identify the target reaction: The target reaction is given as:
8A(g)+2C(g)→A8C2(g)
2
List the given reactions and their enthalpies:
A2B2(g)→2A(g)+2B(g)ΔHrxn∘=−231kJA8C2(g)+8B(g)→4A2B2(g)+2C(g)ΔHrxn∘=+702kJ
3
Manipulate the given reactions to match the target reaction:
- Reverse the first reaction and multiply by 4:
4(2A(g)+2B(g)→A2B2(g))ΔH∘=4×231kJ=924kJ
- Reverse the second reaction:
4A2B2(g)+2C(g)→A8C2(g)+8B(g)ΔH∘=−702kJ
4
Combine the manipulated reactions:
8A(g)+8B(g)→4A2B2(g)ΔH∘=924kJ4A2B2(g)+2C(g)→A8C2(g)+8B(g)ΔH∘=−702kJ
- Add the two reactions:
8A(g)+2C(g)→A8C2(g)ΔH∘=924kJ−702kJ=222kJ
Answer
222kJ
Key Concept
Hess's Law
Explanation
Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction. By manipulating and combining the given reactions, we can determine the enthalpy change for the target reaction.
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