Question

A 1.00g sample of the rocket fuel hydrazine N2H4 is burned in a bomb calorimeter containing 12.00g of water. The temperature of the water and bomb calorimeter rises from 24.62 degrees Celsius to 28.16 degrees Celsius. Assuming the heat capacity of the empty bomb calorimeter is 837J/degrees Celsius, calculate the heat of combustion of 1 mol of hydrazine in the bomb calorimeter. (The specific heat capacity of water is 4.184 J/g*degree Celsius .

Answer 1

Amount of energy or heat : q = (mass)(specific heat capacity)(change in temperature) = m x c x change in temp.

1) Total energy produced by combusting hydrazine = (change in heat of water + change in heat of bomb calorimeter)
= [(12.00 g)(4.184 J/degree C)(28.16 - 24.62 degree C)] + [(837 J/degree)(28.16 - 24.62 degree C)]

= 177.74 + 2962.98
= 3140.72 J = 3.14 kJ

2) q system = q surroundings
So, heat released by hydrazine = - q surroungs = -3.14 kJ (indicating exothermic reaction)

3) Now just use ratio to find out molar heat of combustion for hydrazine.
moles N2H4 = (1 g N2H2 / molar mass N2H2) = 1 g / 32 g

= 0.03125 moles

If 0.03125 mol N2H2 produces -3.14 kJ of energy, then energy produced by 1 mol N2H2

= [(1 mol x -3.14 kJ) / 0.03125 mol] =
-100.48 kJ.. (exothermic reaction!)