Question

The compound methylhydrazine (CH6N2 and Mm= 46.1 g/mol) was used as a fuel to power the orbital maneuvering thrusters aboard the Space Shuttle. Suppose the complete combustion of a 15.00-g sample of methylhydrazine raises the temperature of a 25.0-kg stardard calorimter composed of DURATHERM-S by +10.6C. Given that the specific heat of DURATHERM-S is 1.61 J/g*C, calculate the molar enthalpy of combustion for methylhydrazine inkJ/mol.

Answer 1

Heating effects

q = m·C·∆T

q = heat in Joules (J)

m = mass in grams (g)

∆T = temperature change = Tfinal - Tinitial

Cs = specific heat = heat required to raise the temp. of 1 g of substance by 1 o C. Units are J/g·o C.

Given Data:

mass of methylhydrazine = 15.00 g

Mass of calorimeter = 25.0 kg = 25000 g , specific heat of DURATHERM-S = 1.61 J/g ^oC

∆t = 10.6^oC , molar mass of methylhydrazine = 46.1 g/mol

Since we can assume, heat loss by methylhydrazine = heat gain by DURATHERM-S the Heat gain by DURATHERM-S is as follows:

q = m* C* ∆t

q = 25000 g * 1.61 J/g ^oC * 10.6^oC

q = 4.266*10⁵ J = 426.65 kJ

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Heat released by methylhydrazine

q = ∆H = - 426.65 kJ

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Moles of methylhydrazine

n = 15.0 g / 46.1 g/mol

n = 0.325 moles CH6N2

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Molar enthalpy of combustion for methylhydrazine

∆H (comb) = qCH6N2 / moles CH6N2

∆H (comb) = - 426.65 kJ/0.325 moles

∆H (comb) = - 1311.2 kJ/mol