For the combustion of liquid benzene at 298 K: C6H6(l) + 15/2 O2(g) -> 6 CO2(g) + 3 H2O(l), what is the difference between the enthalpy of reaction and internal energy change (ΔH - ΔU)?
Answer and explanation
Correct answer: A. -1.5 R T
- Gaseous products: ,
- Gaseous reactants: .
- (Liquids and are excluded from ).
- Gaseous reactants: .
- (Liquids and are excluded from ).
- Step 1: The relationship between and is given by: , which rearranges to .
- Step 2: is the change in moles of gaseous substances: .
- Step 3: In this reaction:
- Step 4: .
- Step 5: Therefore, .
Answer
-1.5 R T
Why not the others: '+1.5 R T' inverts the sign of ; '-0.5 R T' incorrectly includes liquid water as a gas (); '+6.0 R T' counts only the gaseous product while ignoring .
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