Question

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3) The molar enthalpy of combustion (...H) of naphthalene is 5100 kJ/mel. Calculate the molar energy of combustion (AcmU) for naphthalene at 25°C CoHe(s)+ 120 (6) ► 10C02(6) + 4H2O(1) (hint: use AcnH=AcmU + AngaRT) Sciences Osh I SCIENCES CIENCES

Answer 1

Solution:

The given balanced combustion reaction of naphthalene is

C₁₀H₈(s) + 12O₂(g) $\rightarrow$ 10CO₂(g) + 4H₂O(l)

It is given that the enthalpy of combustion at constant pressure,

$\Delta$_cmH = - 5100 kJ/mol, negative sign indicates that the reaction is exothermic in nature.

Temperature, T = 25 ^oC = (25 + 298.15) K = 298.15 K

$\Delta$n_gas = (Sum of stoichiometric coefficients of gas products) - (Sum of stoichiometric coefficients of gas reactants)

$\Delta$n_gas = (10 + 0) - (0 + 12) = - 2

Gas constant, R = 0.008314 kJ/mol-K

We have to find the heat of combustion at constant volume, $\Delta$ _cmU,

These two enthalpies are related as

AcmH = SemU + Angas RT

$\Delta_c_mU = \Delta _c_mH-\Delta n_{gas}RT$

$\Delta_c_mU = (-5157kJ/mol)-(-2)(0.008314kJ/mol.K)(298.15K)$

$\Delta$_cmU = - 5152.0424 kJ/mol