1. 0.568 g of acetone (molecular weight 58.08 g mol-s burned in an adiabatic bom

Question

1. 0.568 g of acetone (molecular weight 58.08 g mol-s burned in an adiabatic bomb calorimeter (constant volume and constant temperature of 298 K) and heat release of 7.84 kJ was observed. Assume that the combustion reaction proceeds completely according to the following reaction: (CHs)2 CO(I) + 402 (g) 3CO2 (g) + 3H20 (1) Calculate a) the change in internal energy per mole and b) the change in enthalpy per mole (i.e., the reaction enthalpy). In part b) you may assume that the gas products behave according to the ideal gas law and that the vapor pressure of liquid compounds is negligible.

Explanation / Answer

(a) At constant volume, energy changes are equal to chang ein internal energy.

SO,

deltaU = q / n

Moles of urea = mass / molar mass = 0.568 / 58.08 = 0.00978 mol

Therefore,

deltaU = 7.84 / 0.00978 = 801.7 kJ/mol

(b)

Relation between deltaU and deltaH is,

deltaH = deltaU + delta ng R T

deltaH = 801.1 + [(3 - 4) * 0.008314 * 298)] = 801.1 - 2.48 = 798.6 kJ/mol

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