Chemistry 11 Stoichiometry Reality II: Percent Yield

A reaction is never 100% efficient. In other words: Not all of the reactants in a chemical reaction are converted into the predicted products. The products either don’t form or other products form in unpredictable “side reactions”.

Why Calculate Percent Yield? When a chemist wants to analyze the efficiency of a reaction, they calculate the percent yield. Under different conditions of temperature, pressure, etc. a reaction efficiency may change. This calculation allows a chemist to compare a reaction under different conditions to determine what conditions give maximum efficiency for a reaction.

Percent Yield The calculation requires two bits of information: 1. Actual Yield - The Sweat The mass of the product measured in an experiment. 2. Theoretical Yield - The Stoichiometry The mass of the product calculated by stoichiometry – the ideal amount.

Percent Yield To calculate the Percent Yield, use this equation:

Expected Percent Yield The percent yield is expected to be less than 100%, and there are two reasons for this. a.Most of the reactants in a reaction don’t have enough energy to react. b.Some of the reactants may react to form other products in “side reactions” and these products may not be measured in the Actual Yield.

Example Percent Yield Question Two industrious Chem students mixed g of Aluminum with g of chlorine gas. When the reaction was complete, they found the mass of the product, aluminum chloride, to be g. What was the percent yield?

Example Percent Yield Question Two industrious Chem students mixed g of Aluminum with g of chlorine gas. When the reaction was complete, they found the mass of the product, aluminum chloride, to be g. In a stoichiometry question, the mass of the product that is given is the Actual Yield.

Example Percent Yield Question How would you find the actual yield of a reaction? You measure the mass of a product in an experiment. OR The Actual Yield is given in the question.

Example Percent Yield Question Determine the theoretical yield of a reaction. 1) Balanced reaction equation: 2 Al (s) + 3 Cl 2(g) → 2 AlCl 3(s) 2)Moles of each reactant:

Example Percent Yield Question Determine the theoretical yield of a reaction. 3) Limiting reactant: Cl 2 is the limiting reactant, we predict a theoretical yield of mol of AlCl 3 which translates into:

Example Percent Yield Question Determine the Percent Yield of this reaction.

Percent Yield How do you interpret a Percent Yield greater than 100%? A percent yield greater than 100% means that the Actual Yield is falsely high. This is most likely due to impurities in the product. In this example, the impurities could be excess Al or unknown products of side reactions that were not separated out before the final mass was measured.