Problem of the Week

Chapter 14: Gases

Problem of the Week

Spontaneous Ignition
Spacecraft move through space by rocket propulsion. A variety of engines are used to propel spacecraft, from the huge Saturn V rockets used to put the Apollo missions into space to the attitude control thrusters that keep the spacecraft level while traveling. The lunar module descent engine burns a liquid fuel made of Aerozine 50, a 50/50 mixture of hydrazine and unsymmetric dimethyl hydrazine, and the oxidizer nitrogen tetroxide. These propellants are storable and ignite on contact. The engine is restartable and can be controlled with a throttle. This type of propulsion allows astronauts the ability to control their descent and to "hover" over the surface and change landing sites as needed. The amount of hypergolic fuel required for the vehicle that lifted the Apollo astronauts off the Moon was so little that it fit in a corner of the vehicle and the fuel tank could have served as a chair.

Hypergolic propellants are fuels and oxidizers which ignite spontaneously on contact with each other and require no ignition source. The easy start and restart capability of hypergolics make them ideal for spacecraft maneuvering systems. Also, since hypergolics remain liquid at normal temperatures, they do not pose the storage problems of cryogenic propellants, such as liquid hydrogen and liquid oxygen. Hypergolics are highly toxic and must be handled with extreme care. The lunar ascent vehicle requires 785 kg of propellants to be transported to the lunar surface, including 214 kg Aerozine and 571 kg of dinitrogen tetroxide (N₂O₄). A liquid hypergolic fuel is the principle behind the rocket's attitude correction motors where the mixing of two liquids produce spontaneous combustion and a jet of gases.

Liquid hydrazine (N₂H₄) and dinitrogen tetroxide (N₂O₄). The two chemicals ignite on contact to release very large amounts of energy as heat:

2 N₂H₄(liq) + N₂O₄ (g) → 3 N₂ (g) + 4 H₂O(g) + heat

If 21.4 g of hydrazine reacts completely with dinitrogen tetroxide and the gaseous products are collected at 27°C in a 250.0 L tank, what is the pressure in the tank?

Rocket propellants are the fuels and the oxidizers carried by the rocket for propulsion. The fuel unsymmetric dimethylhydrazine (UDMH) reacts with the oxidizer dinitrogen tetroxide in the descent engine. When the two lightweight liquids react they produce the gaseous products nitrogen, water vapor and carbon dioxide according to the equation:

(CH₃)₂NNH₂ (l) + N₂O₄ (g) → 3 N₂ (g) + 4 H₂O(g) + 2 CO₂(g).

Determine the volume of product gases if 42.8 g of unsymmetric dimethylhydrazine reacts with an excess of dinitrogen tetroxide at a temperature of -10.0°C and a pressure of 0.00600 atm.