3.4 Limiting Reagents

Chemical Concepts Demonstrated: Limiting and excess reagents

Demonstration:

A round-bottom flask is filled with O2 and sealed with a rubber stopper holding a deflagrating spoon.. The deflagrating spoon is filled with red phosphorus, ignited, and plunged into the oxygen filled flask. A magnesium strip is ignited by the burner.

Observations:

    After a few moments in the flask, the red phosphorus will stop burning but a small amount will still be present on the deflagrating spoon.
    The magnesium strip will burn brightly until there is none remaining in the tongs.

Explanations (including important equations):

    P₄ (s) + 5 O₂ (g) --> 4 P₄O₁₀ (s)

    According to the balanced equation, if the spoon holds 2.0 g of phosporus, it would take 2.6 g of O₂ to consume all of the phosphorus in the reaction.  At 25°C and 1 atm, 2.6 g of O₂ is slightly less than 2 L in volume. The flask can only hold 1 L of O₂.  There is not enough oxygen within the flask to burn all of the phosphorus present.  Therefore oxygen is the limiting reagent.  However, the magnesium strip is burned in an open room with a large excess of oxgyen.  In this case, the magnesium ribbon is the limiting reagent.