Heat of Combustion Question?

Question

A 1.397 g sample of Thymol, C10H14O is burned in a bomb calorimeter assembly. The temp. increase is 11.23C, and the heat capacity of the bomb calorimeter is 4.68 kJ/C. What is the heat of combustion of thymol, expressed in kJ/mol of thymol?

Answer 1

ANSWER: -5645kJ/mol

SHOWING WORK:
qrxn = - qcalorimeter = - (Ccal . t)calorimeter
qrxn = - (4.68 kJ/(oC)) . (11.23 oC) = -52.56 kJ

1.397g thymol (1mol thymol/150.14 g thymol) = 0.009305 mol thymol.

Heat of Comb. Thymol =-52.56 kJ / 0.009305mol = -5645 kJ/mol


____________________________________________________

EXPLAINATION (If you care...) :
When measuring the heat flow in a reaction, it is typically carried out in a device known as a calorimeter. The device contains water or other materials of known heat capacity. The walls of the calorimeter are insulated in order to eliminate any exchange of heat with air outside of the device. This means that there is only heat flow between the reaction system and the calorimeter (surroundings). The magnitude of this heat flow must be equal but opposite in sign meaning

qrxn = - qcalorimeter = - (m . c . t)calorimeter (1)

However, a special device known as a bomb calorimeter is used when gases or high temperatures are involved. Use of a bomb calorimeter involves a different equation because it can be calibrated for the amount of heat absorbed by the water and the metal parts of the calorimeter per oC change. Equation 1 can be rewritten to the following:

qrxn = - qcalorimeter = - (Ccal . t)calorimeter (2)

In your problem we are using a bomb calorimeter; therefore, we will be using equation 2. Plugging in the values for this problem we obtain

qrxn = - (4.68 kJ/(oC)) . (11.23 oC) = -52.56 kJ

This is the amount of heat for 1.397g of Thymol. To determine the heat of combustion expressed in kJ/mol of thymol, you have to convert the this mass into mols, then divide the qrxn found above by the mol of thymol.

Thus, 1.397g thymol (1mol thymol/150.14 g thymol) = 0.009305 mol thymol.

Heat of Comb. Thymol =-52.56 kJ / 0.009305mol = -5645 kJ/mol