Limiting Reagents and Percent Yield Prentice-Hall Chapter 12.3 Dr. Yager

Objectives Identify the limiting reagents in a reaction. Calculate theoretical yield, actual yield, or percent yield given the appropriate information.

If a carpenter had two tabletops and seven table legs, he could only build one four-legged table. The number of table legs is the limiting factor in the construction of four-legged tables. Similarly, in chemistry, the amount of product made in a chemical reaction may be limited by the amount of one or more of the reactants.

In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of product that forms. The limiting reagent (aka limiting reactant) is the reagent that determines the amount of product that can be formed by a reaction.

N2(g) + 3H2(g) 2NH3(g) Preparation of Ammonia

N2(g) + 3H2(g) 2NH3(g) Suppose you have 10 moles of N2 and 6 moles of H2. Which is limiting and which is excess? How much NH3 can be made? Limiting reagent is H2 Excess reagent is N2 Product tracks limiting reagent H2 For every 3 moles of H2 you get 2 moles of NH3 so 6 moles of H2 will produce 4 moles of NH3.

O2 is the limiting reagent.

43.2 g H2O

Percent Yield The percent yield is the ratio of the actual yield to the theoretical yield expressed as a percent.

The percent yield is a measure of the efficiency of a reaction carried out in the laboratory. A batting average is actually a percent yield.

The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants. In contrast, the amount of product that actually forms when the reaction is carried out in the laboratory is called the actual yield.

59.3 g Fe

83.5%

3NO2 + H2O  2HNO3 + NO How many grams of HNO3 can form when 1.00 g of NO2 and 2.25 g of H2O are allowed to react? 0.913 g 0.667 g 15.7 g 1.37 g

3NO2 + H2O  2HNO3 + NO How many grams of HNO3 can form when 1.00 g of NO2 and 2.25 g of H2O are allowed to react? 0.913 g 0.667 g 15.7 g 1.37 g

2. How many grams of H2O can be formed from 24.0 g O2 and 6.00 g H2?

2. How many grams of H2O can be formed from 24.0 g O2 and 6.00 g H2?

3. Octane burns according to the following equation: 2C8H18 + 25O2  16CO2 + 18H2O What is the percent yield if 14.6 g of CO2 are produced when 5.00 g of C8H18 are burned? 106% 94.8% 34.2% 62.5%

3. Octane burns according to the following equation: 2C8H18 + 25O2  16CO2 + 18H2O What is the percent yield if 14.6 g of CO2 are produced when 5.00 g of C8H18 are burned? 106% 94.8% 34.2% 62.5%