Limiting Reagent

Brief Description: 

A Pringles can with a pin hole in the aluminum bottom is filled with hydrogen gas, set onto a lab bench, and the escaping hydrogen is ignited.  After a period of time, the can jumps with a small explosion.

Purpose/Goal: 

To introduce the student to the reactivity of hydrogen with oxygen and to illustrate the concepts of limiting reagents and stoichiometric ratios.

Materials Preparation: 

A Pringles can is prepared by opening and disposing the contents in an appropriate manner.  A pin hole is made in the center of the closed end.  A piece of transparent tape is placed over the hole.  The can is held with the open end facing downward, and is filled with hydrogen gas via rubber tubing attached to a compressed tank.  Alternatively, a balloon may be filled with the gas, clamped, and transported to a remote lecture hall location, and the can may be filled with the gas when needed from the balloon.  The can is then placed on a table top or lab bench, open end downward.  The piece of tape is removed from the pinhole, and the gas is ignited with a lighter.

Presentation: 

As the hydrogen is ignited, introduce the chemical reaction of hydrogen with oxygen which produces water.  Hydrogen is a lighter than air gas, and as such it rises and escapes through the pinhole, thus sustaining the flame just generated.  As the hydrogen is used, it is replaced by the air that is being drawn in at the bottom of can in contact with the bench.  Discuss the concept of stoichiometry in this particular reaction, and that it takes two moles of hydrogen and one mole of oxygen to produce the corresponding molar amount of water.  Discuss the idea of a limiting reactant, and that one must have close to a stoichiometric ratio of reactants in order for a reaction to proceed effectively to completion.  At this time the audience is sufficiently distracted from the experiment taking place.  As the hydrogen is used, the air coming in from the bottom comes in at a faster and faster rate, as evidenced by a high pitched sound that eventually emanates from the setup.  This signals the presenter that there is now close to a stoichiometric ratio of hydrogen and oxygen present.  The flame then is drawn into the can which ignites the explosive amounts of hydrogen and oxygen remaining, resulting in the can jumping into the air with a flame coming from the bottom, accompanied by a bang.  Students are startled by the unexpected explosion because the speaker pretends not to notice what is about to happen.  After the explosion, reinforce the concepts you have just presented, explain what was happening during all stages, and repeat the experiment so that students can now listen for the sound emanating from the can and see what happens when they are paying closer attention to  the experiment being conducted.

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