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Question

For the decomposition of 1 mole of sodium chlorate, determine ΔHreaction.
(ΔH0f values: NaClO3(s)=85.7 kcal/mol, NaCl(s)=98.2 kcal/mol, O2(g)=0 kcal/mol).

A
183.9 kcal
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B
91.9 kcal
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C
+45.3 kcal
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D
+22.5 kcal
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E
12.5 kcal
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Solution

The correct option is E 12.5 kcal
The decomposition reaction of sodium chlorate is as shown:
2NaClO32NaCl+3O2
ΔHreaction=2ΔHf,NaCl+3ΔHf,O22ΔHf,NaCl
ΔHreaction=2×(98.2 kcal/mol)+3×(0 kcal/mol)2×(85.7 kcal/mol)
ΔHreaction=25 kcal/mol
This is the enthalpy change for decomposition of 2 moles of sodium chlorate.
The enthalpy change for the decomposition of 1 mole of sodium chlorate is 25 kcal/mol 2=12.5kcal/mol

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