Question

4(a) Derive an expression for the difference in the heat capacities at constant volume and constant pressure for an ideal gas. (3) 4(b) The fuel used in the engine of the Apollo rockets was a mixture of dimethylhydrazine (CH3)2NNH2 and dinitrogen tetroxide (N204) which ignites on contact to give gaseous N2, H2O, and CO2 as products. (i) Balance the equation for the reaction. (2) (ii) Enthalpies of formation of ((CH3)2NNH2) and of liquid N204 are, respectively, +49.4 and -19.5 kJ mol-!. Calculate the enthalpies of reaction per mole of dimethylhydrazine consumed. (5)

Answer 1

4a. Heat neat a cp. Capacity at Constant volume capacity at constant pressure ce (da ( v cpa (dane . cp-cra (4). - ( v We howe H 2 U + PV upp + pap - 0 Therefore, we know that u=f(VT) du ald), do + ( u dT deviding both side by dut and using restriction constant P. from eauh 0 km 4) pa lover Hp+)r +P (3%) e Cp-Co - Color (ar) p + 12 + P. (24) P - (0) (2) p [P+ ( 21 ) ) for ideal gas (2) 28. Co-cur P (2) P. Now PV =RT va Rp 2p 2 R. t. cp-wra R.. (CH3)2NH NH2 + 2N209 - 312 +4420 + 2002 Enthalpy of reaction is = {49.4 + 2X (-19.5) KI moth (49.4 + - 39) 2 10.4 kJ mert