Liquid hydrazine, N2H4, is sometimes used as a rocket propellant. — Thermodynamics Chemistry Question
Problem Context
Liquid hydrazine, N2H4, is sometimes used as a rocket propellant.
Write an equation for the formation of hydrazine from its elements and use the combustion equations below to derive an equation in which ∆Hf˚ for liquid hydrazine is expressed in terms of ∆H1, ∆H2 and ∆H3.
1/2N2(g) + O2(g) → NO2(g) ∆H1
H2(g) + 1/2O2(g) → H2O(g) ∆H2
N2H4(l) + 3O2(g) → 2NO2(g) + 2H2O(g) ∆H3
Model Answer
looking at N2 + 2H2 → N2H4
we must use the equations,
N2 + O2 → NO2 2ΔH1
2H2 + O2 → 2H2O 2ΔH2
2NO2 + 2H2O → 3O2 + N2H4 – ΔH3
N2 + 2H2 → N2H4
ΔHf = 2ΔH1 + 2ΔH2 – ΔH3
In a rocket, liquid hydrazine is reacted with liquid hydrogen peroxide to produce nitrogen and water vapor. Write a balanced equation for this reaction.
Model Answer
N2H4(l) + 2H2O2(l) → N2(g) + 4H2O(g)
Calculate ∆Hrxn˚ for the reaction represented in 2b.
Model Answer
The structures of hydrazine and peroxide are,
Thus,
= – 1143.2 + 325.0
= – 818.2 kJ
Calculate ∆Hrxn˚ for the reaction in 2b using bond dissociation energies.
Model Answer
(note : 4O – H cancel from each side)
= 167 + 4(386) + 2(142) – [942 + 4(459)]
= – 783 kJ
Which value of ∆Hrxn˚ (that calculated in part c or part d) is likely to be more accurate? Justify your answer.
Model Answer
ΔH from part c should be more accurate. ΔHf values are determined for each compound individually, whereas bond energies are average values. We should expect the actual values for the compounds in this problem to vary from these averages.
Calculate the maximum temperature of the combustion gases if all the energy generated in the reaction goes into raising the temperature of those gases. [The heat capacities of N2(g) and H2O(g) are 29.1 J/(mol.˚C) and 33.6 J/(mol.˚C), respectively.]
Model Answer
C = 1 mol × (29.1 J⋅mol-1⋅oC-1) + 4 mol × (33.6 J⋅mol-1⋅oC-1) so C=163.5 J⋅oC-1
q = CΔT 818200 J = 163.5 J⋅oC-1 × ΔT so ΔT = 5004 oC.