Question

A 0.373-g sample of naphthalene (C10H8) is burned in a bomb calorimeter and the temperature increases from 24.90 °C to 27.80 °C. The calorimeter contains 1.05E3 g of water and the bomb has a heat capacity of 836 J/°C. Based on this experiment, calculate ΔE for the combustion reaction per mole of naphthalene burned (kJ/mol).

Answer 1

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

From the data, the number of moles of Naphthalene used is:

Molar mass of C₁₀H₈ = 128.2g/mol

Moles of C₁₀H₈ = 0.373 g / 128.2 g/mol = 2.90 * 10^-3mol

The change in temperature is:

27.80 °C - 24.90 °C = 2.9 °C

If only the calorimeter changed temperature (no outside loss or water jacket), then the change in heat energy is: (1KJ = 1000J)

ΔE = 2.9 °C * 836 J/°C = 2424.4 J/°C = 2.42 KJ

To get the ΔE_rxn (per mole) for the combustion, divide the ΔE above by the moles of C₁₀H₈ used.

As heat of combustion is negative,Therefore

ΔE_rxn (per mole) for the combustion = - (2.42 KJ ) / (2.90 * 10^-3mol) = -8.36*10^-4 KJ/mol