Question

1. 0.1964 g sample of the solid quinone (C6H4O2) is combusted in a bomb calorimeter in the presence of excess oxygen. The total heat capacity of the calorimeter including water is 1.560 kJ/°C. The temperature of the calorimeter increases initially from 22.000˚C to 25.200˚C. Write the balanced combustion reaction: (diff=3) a. Calculate the enthalpy of combustion of quinone in kJ/mol. b. Determine the enthalpy of formation of quinone (in kJ/mol). Use Appendix C from your textbook as needed. Hint: Write a balanced combustion reaction first, where water is formed as a liquid. Look up standard enthalpy of formation values of CO2 (g) and H2O (l) in your textbook Appendix C (in the end pages of the eText).

Answer 1

1)

a)

balanced equation :

C₆H₄O₂ (s) + 6O₂ (g) -------------> 6CO₂ (g) + 2H₂O (l)

b)

temperature difference = 25.2 - 22 = 3.2 oC

heat capacity of the calorimeter = 1.560 kJ / oC

Q = Cp dT = 1.560 x 3.2

   = 4.992 kJ

moles of quinone = 0.1964 / 108.096 = 1.817 x 10^-3 mol

delta H = - Q / n = - 4.992 / 1.817 x 10^-3

delta H = 2747.5 kJ /mol

enthalpy of combustion of quinone = - 2747.5 kJ /mol

b)

C₆H₄O₂ (s) + 6O₂ (g) -------------> 6CO₂ (g) + 2H₂O (l)

enthalpy of formation of quinone = - 185.1 kJ/mol