Limiting Reactants and % Yield

Limiting Factors

Limiting factors are those factors which control the speed or the outcome of a reaction or a situation Have you ever been behind a school bus

in the morning? You can only go as fast as the bus in

front of you. The bus one limiting factor preventing

you from getting to school on time.

Limiting ReactantsIngredients for making “S’mores”:

2 graham crackers

1 marshmallow

1 Hershey barHow many “S’mores” can you make with 14 graham crackers, 5 marshmallows, and 6 Hershey bars?

5 “S’mores”

How many of each ingredient are left over?

0 marshmallows

4 graham crackers

1 Hershey bar

Limiting Reactant

The limiting reactant is the reactant which controls the rate of reaction or the amount of product that can be made

The limiting reactant is determined using stoichiometric relationships

Excess

The reactant that you have more than you need to perform a chemical reaction is called the excess reactant

It is not completely used up in a chemical reaction.

Here’s an example

You are making ham and cheese sandwiches and you have: 5 pieces of ham 5 pieces of cheese 8 pieces of bread

Which of these ingredients do you have more than enough of (excess reactant)?

Which of these do you not have enough of (limiting reactant?

Yield

Yield is how much product that you produce

In industry, the amount of yield needs to be calculated so that production schedules can be made

You also need to know how much product you are going to be making so that you will have the appropriate size collection container

Real or Not?

Actual yield refers to the amount of product that is actually generated

Theoretical yield refers to the amount of product that you expect to generate

Calculating Yield

In order to calculate the theoretical yield you need to Write a balanced chemical equation. Calculate the amount of product produced

from each reactant individually. The reactant that produces the least

amount of product is the limiting reactant. The reactant that you have enough of is

the excess reactant.

Percentage Yield

You recently bought a new car. The EPA sticker says that you should get 28 mpg in “average driving”.

Your actual miles per gallon turns out to be less than 28 mpg.

Why?

Theoretical Yield vs. Actual Yield

You perform a mass-mass calculation to determine how much chemical should be produced in a reaction.

The actual results of the experiment produce less chemical than calculated.

Why?

Calculating Percent Yield

1. Calculate the theoretical yield (mass-mass calculation).

2. Determine the actual yield.3. Calculate the % yield

% Yield = Actual yield x 100Theoretical yield

A Percent Yield Problem

A chemist starts with 1.75 g of salicylic acid (C7H6O3) and excess methanol (CH3OH) and reports the production of 1.42 g oil of wintergreen (C8H8O3) in the following reaction:

C7H6O3 + CH3OH C8H8O3 + H2O

What is the percent yield for this reaction?

Solving the Problem

C7H6O3 + CH3OH C8H8O3 + H2O1.

2.

1.75 g ?g

3. 1.75 g x 1 mol =

138 g

0.0127 mol C7H6O3

4. 0. 0127 mol C7H6O3 x 1 molC8H8O3 = 1 mol C7H6O3

0.0127 mol C8H8O3

5. 0.0127 mol C8H8O3 x 152 g = 1 mol

1.93 g C8H8O3

% Yield cont.

6. (Actual Yield ÷ Theoretical Yield) x 100

(1.42 g ÷ 1.93 g) x 100 =

73.6 % Yield

Practice Problems1. A chemist carried out a reaction that should produce 21.8 g

of a product, according to a mass-mass calculation. However, the chemist was able to recover only 13.9 g of the product. What percentage yield did the chemist get?

2. A calculation indicates that 82.2 g of a product should be obtained from a certain reaction. If a chemist actually gets 30.7 g, what is the percentage yield?

3. Chromium(III) hydroxide will dissolve in sodium hydroxide according to the following equation:

NaOH + Cr(OH)3 NaCr(OH)4

If you begin with 66.0 g of Cr(OH)3 and obtain 38.4 g of product, what is your % yield?

Solutions

#1. (13.9g / 21.8g) x 100 = 63.8%63.8%

#2. (30.7g / 82.2g) x 100 = 37.3%37.3%

#3. NaOH + Cr(OH)3 NaCr(OH)4

66.0g ?g66.0g Cr(OH)3 x 1mol/103g = 0.641mol Cr(OH)3

= 0.641mol NaCr(OH)4 x 143g / 1mol NaCr(OH)4 = 91.66g NaCr(OH)4

(38.4g / 91.7g) x 100 = 41.9%41.9%

Practice ProblemIdentify the limiting reactant and the theoretical yield of H3PO3 if 225 g of PCl3 is mixed with 125 g of H2O

PCl3 + 3H2O H3PO3 + 3HCl

Convert each mass to moles:225 g PCl3 x 1 mol/137 g = 1.64 mol PCl3

125 g H2O x 1 mol/ 18 g = 6.94 mol H2O1.64 mol PCl3 requires 4.92 mol H2O

PCl3 is the limiting reactant

1.64 mol PCl3 = 1.64 mol H3PO3

1.64 mol H3PO3 x 82 g/1 mol = 134 g H3PO3

225g 125g ?g

Classwork

P. 314 #1-3p. 317 #1-3p. 318 #1-3

Homework

P. 319 1-13