Discussion:
This is a spectacular demonstration which can challenge students in diverse ways: to describe the visible features of the reaction, to interpret observations in terms of reacting volumes of gases, and of equilibrium between gases. Effects of gas diffusion and solubility in water, of changing pressure within a gas, of reaction rates can all be involved in discussion of the reaction. Discussion is limited only by the ability of students to grasp the various subtleties of what they observe.
It can be fruitful, when the demonstration is set up and ready to run, to allow students to discuss their predictions of what may happen, and why, before the clamps are loosened.
It is not an easy demonstration to set up without practice, although it becomes quite easy with practice. I have actually allowed a small group of students to set it up, successfully, under close direction and supervision. It can take well over half an hour to set it up properly.
The essential features of the demonstration are shown in the diagram below: a one liter flask full of water is connected to a 250 mL flask of oxygen which is connected in turn to a 500 mL flask full of nitric oxide, by single tubes which can be closed by a clamp. The one liter flask of water has also a clamped tube through which air can enter freely.
Flat bottom flasks can of course, be used; whichever kind is used, they must be held securely on stands. Glass tubing should be inserted through the stoppers, but plastic connectors allow a Hoffman or Mohr clip to be used to clamp the tubes between the flasks.
When it is time for the demonstration to be started all the clamps must be loosened. The reaction may start very slowly, but at a critical moment it speeds up and is complete within about twenty seconds. If it takes too long to start (sometimes 30 minutes or longer), it can be hurried up by blowing gently but firmly with the mouth (or suitable air-pump) into the open tube leading into the top of the water flask until the reaction in the flasks is clearly starting to occur. At this stage, blowing should stop: the reaction will accelerate now without assistance.
What to Expect and Why
Initially, O2 and NO will meet in the tube connecting the two
smaller flasks, due to gas diffusion, and wisps of brown NO2 may become visible. Air in the connecting tube may slow this process.
As NO and O2 react, the volume starts to decrease, and brown NO2 is seen to increase in amount.
2NO (g) + O2 (g) --------------> 2NO2 (g) <==========> N2O4 (g)
As can be seen 3 volumes become 2 volumes and finally approach 1 volume. The decreasing volume causes a decrease in pressure, so water starts to flow towards the middle flask. Oxygen flows towards the NO flask as the pressure falls and water enters the middle flask.
As bulk flow of oxygen enters the NO flask, the reaction becomes fast, and the gas in the end flask suddenly becomes quite brown.
When the middle flask is full of water, the process would stop if only NO2 were formed, since the total gas volume would have decreased to that of the end flask. (Actually, a very slow movement of water would continue as the NO2 dissolves and reacts in the water.) However, some of the NO2 dimerizes to N2O4,causing a further decrease in volume and pressure, so water continues to flow towards the500 mL flask. The water dissolves the gas (and reacts with it),so the pressure stays low and water fills the 500 mL flask.
N2O4 (aq) + H2O (l) -----------> HNO3 (aq) + HNO2 (aq)
If any gas is still present in the 500 mL flask when the water stops flowing, it may be either air trapped from the setting-up process, or excess O2 or NO if the flask volumes were not exact.
Air in the connecting tubes is a likely cause of a slow start to the reaction: care is needed in setting up the flasks to minimize the amount of air left in the connecting tubes. Slight pressurization of the flasks, as mentioned above, usually overcomes this problem.
Setting up
Both oxygen and nitric oxide have low solubility in water, so both flasks can be filled with gas by displacement of water. It is suggested that they be filled in a large basin of water, with the stoppers and tubes in place, (loosely for the NO flask) so that the flasks and tubes are full of gas, and the only air that might get in eventually is around the connections of the tubes. One successful approach is to use a piece of plastic tube at the end of each tube coming from a flask, with a clamp on each piece near its end. The tubes can be connected with a small piece of glass tubing. See the diagram below.
The 500 mL flask may be held inverted and full of water in the basin and be filled through its tube while water escapes past the loosened stopper. When it is full with NO, both stopper and clamp can be tightened before the flask is taken out of the water.
Oxygen is generated easily by dropping hydrogen peroxide solution onto solid manganese dioxide.
Nitric oxide can be generated by gently warming dilute nitric acid (about 2M or 3M) with pieces of copper metal. If the reaction is too slow, it can be either heated more strongly,or more concentrated nitric acid used. If the gas produced from the mixture is brown, it contains NO2 signifying that the nitric acid is too concentrated. Any nitrogen dioxide formed will dissolve in the water as the flask is filled, but it is wasteful and inefficient if this is allowed to happen.
Questions? Comments??
Mike Clark