AP Chemistry
Reading #1
Read the paragraph and then answer the questions.
The Type 93 was a torpedo used by the Imperial Japanese Navy in World War 2. It
differed from other torpedoes in that it was powered by oxygen and “oil” rather than air
and alcohol (C2H6O). All torpedoes need to carry both fuel and oxidizer because they are
underwater and cannot use air to support combustion. Speed and range are two
important factors in torpedoes. The faster and farther they can go, the better. Originally,
torpedoes were powered by tanks of compressed air that ran through an engine. The
force to power the engine came from the fact the air was compressed and was allowed to
escape into the water after going through the engine. Later, a heater was added to the
engine so that the engine would not get too cold from the expanding gas. This allowed
more work to be done by the same amount of gas. In a further improvement, the engines
were modified to run on alcohol (fuel), air (provides the oxidizer oxygen), and water
which helps cool the engine and also does work in the engine as it expands from a liquid
to a gas (steam). The Type 93 doubled the range of other torpedoes by using oxygen
instead of compressed air and “oil” instead of alcohol. An added advantage of the Type
93 was that its wake of exhaust gas that appeared at the surface of the ocean was smaller
than the wakes of torpedoes that used air. This was an early example of “stealth”. You
can learn more about this topic with a google search.
1. Why does pure oxygen deliver greater range and speed for the torpedo compared to
air?
2. Why is the wake of the oxygen torpedo less visible than the wake of a similar torpedo
driven by air?
3. If the Type 93 had a 980L tank that stored 98% oxygen at 225 atm how many moles
of oxygen were carried in this tank? Assume 298K.
4. The engine is said to be fueled by “oil”. We will assume that the oil in question had
the formula C12H26. This substance has a density of .75gcm-3 at 25C. How big would the
fuel tank need to be for the torpedo?
5. Why does “oil” give a greater range than alcohol?
6. One of the engineering challenges in building this device was the need to avoid 90
degree turns in the piping that carried the oxygen from the tank to the engine. The piping
was ok when the angle was more obtuse. Explain why this was an issue.
7. What is the drawback of using pure oxygen in devices? Today torpedoes use batteries
or Otto Fuel (look it up).
8. Hydrogen peroxide is a substitute for oxygen in some torpedoes. Hydrogen peroxide
is a liquid at room temperature. What is an advantage of storing oxygen as peroxide
rather than as pure oxygen?
9. The hydrogen peroxide used in torpedoes is 90% peroxide and 10% water. We’re
going to use 1.11 for the density of this solution. Compare the moles of oxygen stored in
a peroxide tank with a 980L volume with your answer to #3.
Answers:
1. A tank of pure oxygen contains 5 times as much oxygen as the same tank of air. Air is
21% oxygen and 78% nitrogen with 1% argon and traces of other gases.
2. The wake is the trail left in the water by a body moving through it. A torpedo with a
combustion engine exhausts the products of combustion into the water behind the
torpedo. If the torpedo uses air the exhaust contains nitrogen in addition to carbon
dioxide and water vapor. If it uses pure oxygen the exhaust contains water vapor and
carbon dioxide only. The water vapor condenses and mixes with seawater while carbon
dioxide is more soluble in water than nitrogen. Interestingly the visible wake at night is
phosphorescence from sea creatures disturbed by the passage of the object through the
water and the visibility of this wake depends upon the size, speed, and depth of the
torpedo.
3. Use the ideal gas law and multiply the answer by .98. 8666 moles of oxygen.
4. To do this problem you need to assume a stoichiometric ratio of oxygen and oil in the
engine and write a balanced equation for the reaction between “oil” and oxygen. You can
use the number of moles of oxygen from problem 3. Take the mass and convert it to a
volume using density.
2C12H26 + 37O2 = 24CO2 + 26H2O
106L tank of “oil”.
5. Alcohol is already partially oxidized so it releases less energy when it burns in the
engine.
6. With a 90 degree bend the oxygen has to change direction (accelerate). In order to
accelerate the oxygen must have a force applied to it by the piping. The force used by the
piping to do this is friction. Friction generates heat so the piping gets hot enough to burn
in the oxygen. Obtuse angles spread the area of the acceleration out so the pipes don’t
get hot enough to burn.
7. The pure oxygen is highly reactive and likely to explode.
8. Hydrogen peroxide is a liquid that can be stored at ambient temperature and pressure.
9. First write a balanced equation for the decomposition of hydrogen peroxide:
2H2O2 = 2H2O + O2
Then calculate the mass of peroxide in the 980L tank using density. Follow this by
turning mass of peroxide into moles of peroxide then use the mole ratio from the equation
to get moles of oxygen.
15997 moles of oxygen. We could multiply this by 90% if we want to account for the
solution being 10% water: 14397 moles of oxygen. Storing the oxygen as peroxide is
much more efficient and has the bonus that the storage tank can be lighter because it does
not have to withstand the astounding figure of 225 atmospheres of pressure. The Type 93
pressure tanks were machined from solid pieces of nickel alloy armor steel, a costly and
time consuming process.
Analyze your performance:
Everyone needs to get #3.
1 requires knowledge of the composition of air.
2 is difficult unless you infer that water vapor turns to water underwater and you know
that carbon dioxide is especially soluble in water (soda pop)
No part of 4 is difficult but you have to map out where you are starting and where you
will end
5 requires some knowledge of thermochemistry
6 is tough
7 requires some background knowledge of the problems with stored oxygen
8 is doable
9 is only a bit tougher than 3 and easier than 4