The Limiting Reactant and Percent Yield

Greetings,

In most chemical reactions involving two or more reactants, one of those limits the extent of the reaction and the other reactant(s) is/are in excess.  The limiting reactant is consumed first at which point the reaction stops.  Among the more challenging General Chemistry problems are those involving a determination of the limiting reactant using the balanced equation of the reaction and "actual" masses of reactants used.

Methods of Determining the Limiting Reactant

There are two general ways of getting the limiting reactant; (1) Calculate the stoichiometric yield using all reactants ans select the reactant yielding the lowest product amount and (2) Determine the actual mole ratio of reactants and compare it to the reactant mole ratio obtained from the balanced equation.

The diagram below demonstrates the two methods of limiting reactant determination for the formation of ethylene glycol diacetate.

I've included the molecular structures of the compounds for your review.  Method No. 1 has the advantage in that it provides the theoretical yield of EGDA (121.7 g).  The theoretical yield is the maximum amount of product which can be made from the mass of limiting reactant used to obtain it.  Note also that the Method No. 2 calculations are essentially the "beginning parts" of Method No. 1.

Percent Yield

The actual yield of a reaction compared to the theoretical yield, as a fraction of 100, is the percent yield.  For the reaction above, let's suppose that we actually made 101.2 grams of EGDA.  Then the percent yield of EGDA is (101.2 g/121.7 g) x 100 = 83.16%.

That's all for this post.

Have a good one!

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