Question

a 5.00 g sample of TNT (C7H5N2O6) is burned in a bomb calorimeter with a heat capacity of 420 J/degreeC. The calorimeter contained 610 grams od water (4.18J/gdegreeC) and the temperature of the water was measured to go from 20.0 degree C to 22.5 degree C. What is the heat of combustion of TNT?.

I know the answer, but can you give me a step by step explanation of how to get it?

Answer 1

Since the reaction is combustion, heat flow from system to surrounding as it is exothermic. The heat re;eased will be absorbed by i) water in the calorimeter and the calorimeter itself.

A. Calculate the heat absorbed by the water (q_water)

m = 610 g

c_water = 4.18J/g^oC

B) Calculate the heat absorbed by the calorimeter (q_cal)

the temperature change of the calorimeter is the same as the temperature change for water. In this step, however, the heat capacity of the calorimeter should be used which is already knowm\n. When using heat capacity the mass of the calorimeter is not required for the calculation

C_cal = 420J/^oC

the total heat absorbed by the water and the calorimeter

q_comb= q_water + q_cal

q_comb= 6.374 + 1.050 = + 7.424 kJ. (q is is positive as heat is absorbed)

the amount of heat released by the reaction is equal to the amount of heat absorbed by the water and the calorimeter/ We jsut need to change the sign. So q_reaction = - 7.424kJ

Since 5 g of TNT was burned the heat of combustion for TNT is equal to -7.424 kJ/g. In other words when one mole of TNT is burneed 7.424kJ of heat is released from the reaction

1 mol of TNT weigh 227.13 g mol^-1

so (7.424)(227.13) = 1686.21 kJ/mol

OR

moles (C7H5N2O6) = 5 g(1 mole (C7H5N2O6)/(227.13 g) = 0.022 moles (C7H5N2O6)

molar heat of combustion = -(7.424 kJ)/(0.022 moles) = -337.45 kJ/mole