A Reaction Between Gases

This is a spectacular demonstration which can challenge students in diverseways:  to describe the visible features of the reaction, to interpretobservations in terms of reacting volumes of gases, and of equilibrium betweengases.  Effects of gas diffusion and solubility in water, of changingpressure within a gas, of reaction rates can all be involved in discussionof the reaction.  Discussion is limited only by the ability of studentsto grasp the various subtleties of what they observe.

It can be fruitful, when the demonstration is set up and ready to run, toallow 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 becomesquite easy with practice.  I have actually allowed a small group ofstudents 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 oxygenwhich is connected in turn to a 500 mL flask full of nitric oxide, by singletubes which can be closed by a clamp.  The one liter flask of waterhas also a clamped tube through which air can enter freely.

Flat bottom flasks can of course, be used; whichever kind is used, they mustbe held securely on stands.  Glass tubing should be inserted throughthe stoppers, but plastic connectors allow a Hoffman or Mohr clip to be usedto clamp the tubes between the flasks.

When it is time for the demonstration to be started all the clamps must beloosened.  The reaction may start very slowly, but at a critical momentit speeds up and is complete within about twenty seconds.  If it takestoo long to start (sometimes 30 minutes or longer), it can be hurried upby blowing gently but firmly with the mouth (or suitable air-pump) into theopen tube leading into the top of the water flask until the reaction in theflasks is clearly starting to occur.  At this stage, blowing shouldstop:  the reaction will accelerate now without assistance.

What to Expect and Why
Initially, O2 and NO will meet in the tube connecting the twosmaller flasks, due to gas diffusion, and wisps of brown NO2 maybecome visible.  Air in the connecting tube may slow this process.  

As NO and O2 react, the volume starts to decrease, and brownNO2 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 startsto flow towards the middle flask.  Oxygen flows towards the NO flaskas the pressure falls and water enters the middle flask.  

As bulk flow of oxygen enters the NO flask, the reaction becomes fast, andthe gas in the end flask suddenly becomes quite brown.  

When the middle flask is full of water, the process would stop if onlyNO2 were formed, since the total gas volume would have decreasedto that of the end flask.  (Actually, a very slow movement of waterwould continue as the NO2 dissolves and reacts in the water.) However, some of the NO2 dimerizes toN2O4,causing a further decrease in volume and pressure,so water continues to flow towards the500 mL flask.  The water dissolvesthe gas (and reacts with it),so the pressure stays low and water fills the500 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 excessO2 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 airleft 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 flaskscan be filled with gas by displacement of water.  It is suggested thatthey be filled in a large basin of water, with the stoppers and tubes inplace, (loosely for the NO flask) so that the flasks and tubes are full ofgas, and the only air that might get in eventually is around the connectionsof the tubes.  One successful approach is to use a piece of plastictube at the end of each tube coming from a flask, with a clamp on each piecenear its end.  The tubes can be connected with a small piece of glasstubing.  See the diagram below.

The 500 mL flask may be held inverted and full of water in the basin andbe filled through its tube while water escapes past the loosened stopper. When it is full with NO, both stopper and clamp can be tightened beforethe flask is taken out of the water.

Oxygen is generated easily by dropping hydrogen peroxide solution onto solidmanganese dioxide.

Nitric oxide can be generated by gently warming dilute nitric acid (about2M 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 containsNO2 signifying that the nitric acid is too concentrated.  Anynitrogen 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