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COMEDK2025ChemistryThermodynamics (C)Actual

When the combustion of one molecule of liquid heptane is carried out at a constant temperature ' T ', the difference between H^o and U^o is equal to:

Options

  1. AU^o- H^o=3 R T
  2. BU^o- H^o=4 R T
  3. CH^o- U^o=4 R T
  4. DH^o- U^o=-3 R T

Correct answer

B. U^o- H^o=4 R T

Step-by-step solution

The combustion reaction of liquid heptane ( C₇H₁₆ ) is given by: C₇H₁₆(l) + 11O₂(g) 7CO₂(g) + 8H₂O(l) The relationship between enthalpy change ( H^ o ) and internal energy change ( U^ o ) is given by the equation: H^ o = U^ o + n_ g RT where n_ g is the change in the number of moles of gaseous species in the reaction. Calculating n_ g : n_ g = n_ g, products - n_ g, reactants n_ g = 7 - 11 = -4 Substituting this value into the equation: H^ o = U^ o - 4RT Rearranging the terms to find the difference H^ o - U^ o : H^

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