AP Chemistry

Rocket Chemistry

Rockets have evolved from “arrows of flying fire” to devices capable of putting humans on the moon.

Rocket engines generate the highest temperatures and the highest exhaust velocities but some of the lowest efficiencies of any engine type. Despite the low efficiencies rockets can burn in space, underwater, and at high altitude where there is not enough air to allow other engine types to work. Rockets can also produce more power than other types of engine.

High powered rockets:

Rockets use redox reactions to generate energy. These reactions require a fuel and an oxidizer. Rockets are different than other engines because they carry their own oxidizer. Jet engines, Otto cycle engines, diesels and steam engines all use air to obtain oxygen for combustion. There are three main types of rocket engines, solid, liquid, and hybrid. Solid rockets have solid fuel and oxidizer while liquid rockets have liquids. Hybrid rockets have one component solid and the other liquid.

Combustion provides the energy for a rocket. Combustion is a redox reaction that requires a fuel (reducing agent) and an oxidizing agent. Some liquid fuel rockets use liquid hydrogen and liquid oxygen as fuel and oxidizer respectively. The product of the reaction is water vapor. This water vapor exits the rocket engine at high velocity and propels it. The ideal rocket propellant is a mix of fuel and oxidizer with these characteristics (the importance of the characteristics varies by application):

  1. Easy to store at a variety of ambient temperatures
  2. Cheap
  3. Safe
  4. Releases lots of energy upon combustion
  5. The products have low molar mass so they exit the rocket at higher velocity for a given temperature

No propellant meets all of the above so a variety of propellants are used. Let’s look at some:

1. Liquid hydrogenand oxygen

2. Liquid hydrocarbon and oxygen

3. Liquid methane and hydrogen

4. Hybrid rubber and nitrous oxide

5. Aluminum and ammonium perchlorate

6. Liquid hydrogen and fluorine

7. Hydrazine and nitrogen tetroxide

8. Aniline and nitric acid

9. Hydrazine monopropellant

10. Hydrogen peroxide monopropellant

11. ALICE

Liquid oxygen and hydrogen produce a great deal of energy and the exhaust product (water) is light and environmentally safe. The fuel and oxidizer must be kept very cold to stay liquid. That means they can’t be stored in a rocket for very long. Oxygen can be replaced by fluorine as an oxidizer. Fluorine has one main advantage over oxygen, it releases more heat when it reacts with hydrogen. There are two disadvantages. One is that the fluorine itself is dangerous, as is the combustion product hydrogen fluoride. The other is that it takes one mole of fluorine to react with one mole of hydrogen while it only takes half a mole of oxygen to react with one mole of hydrogen. Liquid methane is proposed as a fuel because it can be used to cool the rocket engine. Liquid hydrocarbon usually is kerosene. It does not need to be refrigerated to stay liquid. It is dense and produces a great deal of energy. It produces carbon dioxide which is a less efficient exhaust product than water because it has a higher molar mass. Nitric acid is a good oxidizer as it contains three oxygen atoms and a nitrogen atom in the +5 oxidation state. It is a liquid at room temperature but it corrodes many materials that are used to store it. Aniline and nitric acid can be used as rocket propellant since they are hypergolic, that is, they react as soon as they come into contact. This eliminates the need for some kind of starter in the rocket. Hydrazine, N2H4, and Hydrogen peroxide, H2O2 are both capable of releasing energy when they decompose. They can be used alone or hydrazine can be combined with an oxidizer or peroxide with a fuel. ALICE stands for aluminum ice. The reaction between aluminum and ice is exothermic and the products are aluminum hydroxide and hydrogen. The water is frozen into ice for this rocket to ease storage.

Military rockets have constraints that other rockets do not. Military rockets must be much safer than commercial rockets. This seems counterintuitive but the objective of military rockets is to deliver explosives to the enemy, not to explode and harm the user. Military rockets must be able to be fired given very little warning, seconds in most cases. These rockets must be capable of withstanding considerable jostling and even damage and keep their ability to function. Military rockets must be employed by people who aren’t rocket scientists. These factors preclude the use of cryogenic liquids. Storable liquids are liquids like red fuming nitric acid and aniline. Although liquid they can be stored for long periods. Unfortunately the machinery required to burn liquids is fragile. Almost all military rockets use solid propellant. The fuel is usually aluminum powder and the oxidizer is usually ammonium perchlorate. A problem with this system is that the exhaust is visible. In civilian rockets this is not a concern. In military rockets it provides a visible trail to the source of the rocket. Another problem with solid propellant is that it does not produce as much energy as liquids. The Meteor missile actually uses a gel of compounds including boron compounds.

Specific impulse is a measure of rocket propellant efficiency. Depending upon the units input into calculating it, the units can vary. Let’s use seconds. When we measure it in seconds we could think of it as the amount of time that a pound of propellant would produce a pound of force. Longer time means more efficiency. Note that if everything else is equal the exhaust products with lower molar mass will have higher exhaust velocity and higher specific impulse. Go here for more:

A chemist proposes fluorine perchlorate as an oxidizer. Let’s investigate this molecule in depth:

  1. Draw a Lewis structure for fluorine perchlorate.
  2. Find the formal charge of each atom in the molecule.
  3. Find the oxidation state of each atom in the molecule.
  4. State why this molecule would make a good oxidizing agent in theory.
  5. Fluorine perchlorate turns out to be very unstable. Write an equation for it decomposing. Predict whether its decomposition is endo or exothermic using a table of bond energies.
  6. Write a balanced equation for the combustion of hydrogen with fluorine perchlorate.
  7. Calculate how much more hydrogen can be burned with a tank of fluorine perchlorate vs a tank of oxygen given the same volume per mole of oxidant.
  8. Calculate which will burn more hydrogen per gram of oxidant, oxygen or fluorine perchlorate.
  9. Calculate which is a more efficient oxidizer by mass oxygen or fluorine.
  10. Calculate which is a more efficient oxidizer by volume, oxygen or fluorine.
  11. In many of our calculations we assume that the rocket is burning a stoichiometric mixture of fuel and oxidizer. However, some rockets that use hydrogen as a fuel are run fuel rich. This means that there is an excess of hydrogen. Why is this done?
  12. Benzene is a hydrocarbon that has a high octane rating. It is suitable for gasoline engines but it has never been used for rockets. Why?
  13. Syntin is a very expensive fuel that was used in the USSR. Its structure is shown below:

Syntin is a mixture of cis and trans isomers. The average heat of formation of the isomers is 133KJ/mol and the density is .851g/ml. We will compare the efficiency of syntin to RP-1 (Rocket Propellant 1). Since it is difficult to find the composition of RP-1 we will assume the density of RP-1 is .81g/ml. We will assume that the heat of formation of RP-1 is the same as dodecane since we can find a value for it.
2 C12H26(l) + 37 O2(g)→ 24 CO2(g)+ 26 H2O(g)

∆H˚= -7513 kJ

Calculate how much more energy 1L of syntin produces compared to 1L of RP-1.

14. It has been proposed to use ozone as an oxidizer. Give two reasons why it would make a good oxidizer for rockets.

15. Ozone has not been used as an oxidizer because it is unstable, decomposing into oxygen.Calculate how much more energy would be derived from a tank of liquid ozone than a tank of liquid oxygen. Assume they have the same density per mole. You can assume whatever fuel you want.

16. Dinitrogen tetroxide is an effective oxidizer. It is in equilibrium with nitrogen dioxide. List the conditions that favor the formation of N2O4 over NO2.

17. It has been proposed to use atomic hydrogen, H, as a rocket propellant. It can only be stored at low temperature. We could store the hydrogen atoms in a matrix of solidified low temperature gas such as argon or neon. How much energy would be released by a mole of H atoms forming H2?

18. Calculate which of the two monopropellants will produce more energy per mole, hydrazine or hydrogen peroxide.

19. Which monopropellant has a higher boiling point? Explain.

20. Which monopropellant likely has a higher exhaust velocity?

21. Make an inference about the activation energy of the reaction between hypergolic fuel and oxidizer. Explain.

22. Kerosene and gasoline both work in rockets but kerosene is safer to handle. Explain why.

23. Trinitramide is a possible oxidizer with the formula N4O6. It has an N in the middle surrounded by 3 NO2 groups. Draw a Lewis structure for trinitramide.

24. Assign oxidation states to each atom in trinitramide.

25. Write a balanced equation for the reaction between aluminum powder and trinitramide. Assume that the N goes to N2.

26. ClF3 and ClF5 have been proposed as oxidizers. Write separate balanced equations for the reactions between the two and H2.

27. Explain why liquid oxygen/kerosene has a higher specific impulse than hydrogen peroxide/kerosene.

28. The Soviet SS-18 ICBM used unsymmetrical dimethyl hydrazine (UDMH) as a fuel and nitrogen tetroxide as an oxidizer. Draw Lewis structures for both reactants. The UDMH has formula C2H8N2. Both C are on the same N.

29. Assume that the reactants in #28 produce water, carbon dioxide, and nitrogen. Write a balanced equation for the reaction.

30. Why might the VASIMR engine (see below) use argon or xenon instead of hydrogen as the propellant?

Answers:

  1. F = 0, the O on F is 0 the Cl is 0, the double bonded O’s are all 0.
  2. F = -1, the O on the F is 0, the Cl is +7, the double bonded O’s are all -2. The sum of them is 0 the same as the charge of the molecule.
  3. There is a lot of oxygen in the molecule for one. Two, the chlorine is in the +7 state so it will take electrons from other atoms in a reaction. There’s also fluorine to react with fuel.
  4. FClO4 = 1/2F2 + 1/2Cl2 + 2O2 The reaction is endo because one F-O is broken (+190KJ/mol), one Cl-O single is broken (+218 KJ/mol) three Cl=O double bonds are broken (249.5KJ/mol) and one half F-F single is formed (-155KJ/mol), one half Cl-Cl single is formed (-240KJ/mol) and two O=O double bonds are formed (-494KJ/mol). It actually turns out that the enthalpy of formation is 9KJ/mol so we may have a bad value for the Cl=O double bond.
  5. 5H2 + FClO4 = HF + HCl + 4H2O
  6. With oxygen 2 moles of hydrogen can be burnt per mole of oxygen, while FClO4 can burn 5 so 2.5 times more with the tank of FClO4.
  7. To burn 200 grams of hydrogen, 1600 grams of oxygen would be required. To burn 200 grams of hydrogen 20 moles, which is 2369.06g is required. The oxygen is more efficient per gram if we just look at the amount of hydrogen consumed.
  8. To burn one mole of hydrogen we need half a mole of oxygen 16 grams. To burn the mole of hydrogen in fluorine we need one mole of fluorine which is 38 grams. That’s if we measure the efficiency of the oxidizer by how much fuel it can react with. If we measure it by the amount of energy released we get about -480KJ/mol for O2 if water is the product, and about -270KJ/mol if we use fluorine and HF is the product. The oxygen would require twice the volume of hydrogen to react as the fluorine does.
  9. Oxygen is twice as efficient if we look at it by the mass of fuel that can be burned with it. If we look at the amount of energy then let’s look at
  10. This is done because the hydrogen has a higher exhaust velocity than water.
  11. Benzene has a low heat of formation so it doesn’t release as much heat as other hydrocarbons. It’s a good fuel in spark ignition (gasoline) engines because it has a high octane rating.
  12. Syntin has the formula C10H16. The equation for its combustion is:

C10H16 + 14O2 = 10CO2 + 8H2O

ΔH = -6002.4KJ/mol

The 1L tank of RP-1 contains 810g. The 810g is 4.76 mol. For the syntin, a 1L tank contains 851g. This is 6.25mol. So, the tank of RP-1 gives 35762KJ while the tank of syntin gives 37515KJ. The syntin gives 4.9% more energy per volume. It is hard to see how the cost of synthesizing syntin is justified. Note that its production has ended when faced with market discipline.

  1. It has 3 O per mole and it has an endothermic heat of formation.
  2. We need to multiply the amount of energy from the tank of O2 by 1.5 because there are 50% more O atoms. Then, we add on a factor accounting for the fact that the O3 is an endothermic compound. If we use a mole of oxygen for two moles of H2 then a mole of oxygen would release 480KJ/mol of energy. The ozone would burn three moles of hydrogen and release about 860KJ/mol. It’s difficult to calculate the efficiency difference unless we’re given the size of the rocket and how big the fuel and oxidizer tanks can be.
  3. High pressure, low temperature. The reaction is exo because a bond is formed.
  4. Find the heat of formation of H atom and give the inverse or find the bond energy of a mole of H-H bonds (-432KJ/mol) negative because we are making a bond here.
  5. The 2 chemical equations are:
    N2H4 = N2 + 2H2
    H2O2= H2O + 1/2O2
    The decomposition of hydrazine will yield it’s the additive inverse of its heat of formation: -50.63KJ/mol
    For peroxide we have to do thermochem and find that the ΔH will be -98KJ/mol.
  6. Peroxide because O is stronger in hydrogen bonding than N.
  7. This is tough but if we assume that the temperature of the molecules in the exhaust is the same we can try the problem. Looking at the peroxide, the 100 or so units of energy are split between 1 molecule with mm = 32 and 2molecules with mm = 18; the hydrazine splits about 50 units of energy between 1 molecule with mm = 28 and 2 with mm= 2. For the peroxide each molecule has 33 units of energy or so. That is their kinetic energy. KE = 1/2mv2. So, 33 = (.5)(32)(v2) and 33 = .5(18)(v2). For hydrazine each molecule has 17 or so units of energy. 17 = (.5)(28)(v2) and 17 = (.5)(2)(v2) Do a weighted average of the velocities to show that the average velocity for the peroxide is1.7 from water at 1.9 and oxygen at 1.4 and the average velocity for the hydrazine is 3.1 from hydrogen 4.1 at and nitrogen at 1.1.
  8. It’s low because they react at room temperature as soon as they come into contact.
  9. Kerosene has a lower vapor pressure than gasoline. Vapors are much easier to ignite than liquids.

  11. The middle N is 0, the other N’s are +4, the O’s are -2.
  12. 4Al + N4O6 = 2N2 + 2Al2O3
  13. 2H2 + ClF3 = HCl + 3HF
    3H2 + ClF5 = HCl + 5HF
  14. Hydrogen peroxide has an exothermic heat of formation. It also produces 2 moles of water for every mole of O2 it makes available for oxidizing. Water has a high heat capacity so it takes a lot of heat to increase its kinetic energy for when it shoots out the back of the rocket.

  1. C2H8N2 + 2N2O4 = 2CO2 + 4H2O + 3N2
  2. Hydrogen would work better because it has a lower mass so a higher exhaust velocity. However, the VASIMR engine ionizes the gas so that it can be accelerated by electromagnetic forces. They need a gas with a lower ionization energy so they choose argon or xenon. If the gas is not ionized they can’t accelerate it.