TOPIC: Stoichiometry

Demo 007

Limiting Reactant

This is an illustration of the concept of limiting reactant using a simple prop. The idea is to make clear the idea of one substance being in short supply and therefore determining the amount of product formed.

MATERIALS

One charcoal briquette (for bbq purposes)
One 1 L wide mouth screw cap jar

PRESENTATION

This should be brought in after the basic stoichiometry calculation has been introduced. Point out that we have tacitly assumed that there was enough of each reactant to allow complete consumption of all substances. Ask the class to consider a single charcoal briquette held in your hand. Point out that in combustion it will react with atmospheric oxygen to form carbon dioxide. The briquette weighs around 50 g. Ask whether it will consume all the oxygen in Earth’s atmosphere or whether some will be left over for everybody to breathe. The 50 g charcoal briquette is the limiting reactant; it determines how many grams of CO2(g) will be produced.

Now put the briquette in the one liter jar and seal it with the lid. Ask the class whether there is enough oxygen in the jar to completely convert the carbon to CO2(g). Most will agree that there is insufficient oxygen. The oxygen is now the limiting reactant.

DISCUSSION

You could then pursue the idea by computing the amount of CO2(g) formed in both scenarios and discussing the fact that it is the limiting reactant, the substance in short supply, that determines the amount of product formed. Also compute the amount of O2(g) consumed in each case. When the O2(g) is the limiting reactant, there is none left. This is also always true of limiting reactants.

 

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