Question

0.5122 g of Napthalene (C10H8) was combusted in a constant-volume bomb calorimeter (Ccal= 5267.8 J/K), where the water temperature increased from 20.17 C to 24.08 C. Calculate the molar delta H and delta U for the combustion of Napthalene in KJ/mol at 20.17 C.

Answer 1

molar mass of C10H8= 128.1705 g/mol

no: moles of C10H8=0.5122 g/128.1705 g/mol=3.99 * 10^-3 mole

change in temperature=24.08 C-20.17 C=3.91C

Heat gained by calorimeter =5267.8 * 3.91=20597.09 J

Heat gained by calorimeter =The heat released by 3.99 * 10^-3 mole C10H8

The heat released by one mole=20597.09 J/3.99 * 10^-3 mole=5172 KJ/mol

heat released per mole, molar ΔU= - 5172 KJ/mol

as all combustion reactions, negative sign indicates that the reaction is exothermic,the reaction is given by

C10H8 (s) + 12 O2 (g) -> 10CO2 (g) + 4 H2O (l)

ΔH= q + ΔnRT Δ n= change in no: of moles R= universal gas constant T= temperature=20.17+273=293.17K

Δ n= 10-12=-2 ( only gaseuos state required)

molar ΔH= - 5172 KJ/mol +(-2) *8.314* (293.17)=-5177KJ/mol   

negative sign indicates that the reaction is exothermic