A professor places a 3.252 g sample of ethyl acetate (C4H8O2) in a bomb calorime

Question

A professor places a 3.252 g sample of ethyl acetate (C4H8O2) in a bomb calorimeter, adds 2.50 kg of water to the calorimeter and assembles the apparatus. After charging the device with excess oxygen, the initial temperature is 20.45°C. The professor ignites the ethyl acetate, the combustion reaction takes place, and the final temperature of the system is 27.16°C.

a) What is the heat capacity of the bomb in kJ/°C?

(The constant-volume heat of combustion for ethyl acetate is -2238.1 kJ/mol)-(5 pts)

b) Indicate the Hf (change in heat of formation) reaction for C4H8O2. (2pts)

Explanation / Answer

C4H8O2(g) +   3O2(g) ----------> 4CO2(g) + 4H2O(l)

molar mass of ethyl acetate = 88 g/mole

thus, moles of ethyl acetate in 3.252 g of it = mass/molar mass = 3.252/88 = 0.037

Now, specific heat capacity of water = 4.184 J/g/0C

Total heat gained by water = mass of water*specific heat capacity of water*rise in temperature

                                                = 2500*4.184*(27.16 - 20.45) = 70186.6 J = 70.186 kJ

Now, total heat liberated in the combustion = moles of ethyl acetate*heat of combustion of ethyl acetate

                                                                               = 0.037*2238.1 = 82.8097 kJ

Thus, heat absorbed by bomb calorimeter = Total heat liberated - heat absorbed by water = 12.6237 kJ

Thus, haet capacity of bomb calorimeter = heat absorbed/change in temperature

                                                                          = (12.6237)(/(27.16-20.4) = 1.867 kJ/0C

Heat of formation reaction for ethyl acetete :-

4C(s) + 4H2(g) + O2(g) ----------------> C4H8O2(g)

Now, In the reaction:-

C4H8O2(g) +   3O2(g) ----------> 4CO2(g) + 4H2O(l)

heat of combustion of ethyl acetate = 4*(heat of formation of CO2) + 4*(heat of formation of H2O) - heat of formation of C4H8O2

or, heat of formation of C4H8O2 = 4*(heat of formation of CO2) + 4*(heat of formation of H2O) - heat of combustion of ethyl acetate

or, heat of formation of C4H8O2 = 4*(-393.5 kJ/mole) + 4*(-285.8) - (-2238.1) = -479.1 kJ/mole

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