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.

N 2 (g) + 3H 2 (g) 2NH 3 (g) Preparation of Ammonia

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

O 2 is the limiting reagent.

43.2 g H 2 O

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

 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%

1.3NO 2 + H 2 O  2HNO 3 + NO How many grams of HNO 3 can form when 1.00 g of NO 2 and 2.25 g of H 2 O are allowed to react? How many grams of HNO 3 can form when 1.00 g of NO 2 and 2.25 g of H 2 O are allowed to react? a)0.913 g b)0.667 g c)15.7 g d)1.37 g

1.3NO 2 + H 2 O  2HNO 3 + NO How many grams of HNO 3 can form when 1.00 g of NO 2 and 2.25 g of H 2 O are allowed to react? How many grams of HNO 3 can form when 1.00 g of NO 2 and 2.25 g of H 2 O are allowed to react? a)0.913 g b)0.667 g c)15.7 g d)1.37 g

2. How many grams of H 2 O can be formed from 24.0 g O 2 and 6.00 g H 2 ? a)30.0 g b)27.0 g c)54.0 g d)13.5 g

2. How many grams of H 2 O can be formed from 24.0 g O 2 and 6.00 g H 2 ? a)30.0 g b)27.0 g c)54.0 g d)13.5 g

3. Octane burns according to the following equation: 2C 8 H O 2  16CO H 2 O 2C 8 H O 2  16CO H 2 O What is the percent yield if 14.6 g of CO 2 are produced when 5.00 g of C 8 H 18 are burned? What is the percent yield if 14.6 g of CO 2 are produced when 5.00 g of C 8 H 18 are burned? a)106% b)94.8% c)34.2% d)62.5%

3. Octane burns according to the following equation: 2C 8 H O 2  16CO H 2 O 2C 8 H O 2  16CO H 2 O What is the percent yield if 14.6 g of CO 2 are produced when 5.00 g of C 8 H 18 are burned? What is the percent yield if 14.6 g of CO 2 are produced when 5.00 g of C 8 H 18 are burned? a)106% b)94.8% c)34.2% d)62.5%