notes #14, ch. 9

Notes #14, Ch. 9

Stoichiometry and Theoretical Yield

What is Stoichiometry? It is a way to calculate how much of a

chemical is consumed or produced in a chemical reaction using Dimensional Analysis.

Propanol (rubbing alcohol) C3H7OH,

burns in air. Write and balance the reaction:

2C3H7OH + 9O2 6CO2 + 8H2O

What are the Equivalents?2 mols C3H7OH 9 mols O2 6 mols CO2 8 mols

H2O

This reaction will be used in the next few examples.

Example 1:

How many moles of water are produced from the combustion of 20.0 g propanol?

The arrow: 20.0 g C3H7OH ? mols H2O

Chemistry Tip of the Year: “When in doubt, go to moles!” Why? So you can use the mole ratios in

the reaction as equivalents.

Solution:

So, 1.33 mols of H2O is produced

OH mols 1.33OHHC mol 2

OH mols 8

OHHC 60.0g

OHHC mol 1OHHC 20.0g 2

73

2

73

7373

Example 2:

How many grams of oxygen is needed to react with 12.0 grams of propanol?

12.0 g C3H7OH ? g O2

Solution:

So, 28.8 grams of oxygen is needed for the 12.0 grams of propanol to burn

22

2

73

2

73

7373 O g8.28

O mol 1

O 32.0g

OHHC mol 2

O mols 9

OHHC 60.0g

OHHC mol 1OHHC 12.0g

2O g

Example 3:

If 25.3 g of carbon dioxide is produced, how many grams of propanol was burned?

25.3g CO2 ? g C3H7OH

Solution:

So 11.5 grams of propanol was burned to produce the 25.3 grams of CO2.

OHHC g5.11OHHC mol 1

OHHC 60.0g

CO mol 6

OHHC mol 2

CO 44.0g

CO mol 1CO 25.3g 73

73

73

2

73

2

22

The Theoretical Yield Concept

What if the reacts aren’t present in the exact ratio needed?

What theoretical yield is all about:Ordinarily, reactants are not present in the exact

ratio needed to complete a reaction. One reactant is used up, or “limits the reaction.”

The other reactant has some left over, unused.

Steps for calculating theoretical yield:

1. Calculate the yield as if the first reactant was limiting how much product is made.

2. Calculate the yield as if the second reactant was limiting the amount of product.

3. The Lesser of these two yields is the theoretical yield (T.Y.), or how much product you could ideally make.

4. The reactant in the D.A. which leads to the T.Y. is used up and is your “limiting reactant.”

Hot Dog Example:

8 buns + 10 wieners ? hot dogs?

Step 1. 8 buns ? hot dogs

hotdogs 8bun 1

hotdog 1Buns 8

Step 2. 10 wieners ? hot dogs

hotdogs 01 wiener1hotdog 1

wieners10

Step 3: TY & LR

Possible Yield from step 1 was 8 hot dogsPossible Yield from step 2 was 10 hot dogs

You can make lesser yield, 8 hot dogs. This is the Theoretical YieldThe buns are the limiting reactant.There are extra wienies.

Skateboard Factory Example:

You inherit the remnants of a skateboard store which went under.

You have:167 wheels93 wheel “trucks”71 boards

How many skateboards can you make?

So, 41 skateboards is the theoretical yield.The limiting reactant is the wheels.

Skatebds 46 trucks2

Skatebd 1 trucks93

Skatebds 41 wheels4Skatebd 1

wheels167

Skatebds 71board 1

Skatebd 1 boards 71

Skateboard 1 wheels4 board 1 trucks2

Chemistry Example:

2Ag + I2 2AgIIf you start with 1.00 g of each reactant,

how much silver iodide could you make?

2Ag + I2 2AgIStep 1. 1.00 g Ag ? g AgI

AgIgAgImolAgIg

AgmolAgImol

AggAgmol

gAg 18.2 1 235

2 2

108 1

00.1

Possible Yield: 2.18 g AgI

2Ag + I2 2AgIStep 2. 1.00g I2 ? g AgI

AgIgAgImolAgIg

ImolAgImol

IgImol

gI 85.1 1 235

1 2

542 1

00.122

22

Possible Yield: 1.85 g AgI

Step 3: So, the theoretical Yield is 1.85 g AgI, lesser of the two.Limiting Reactant is iodine, it’s all used

up. There’s left over silver.The actual yield is always a little less, due

to unwanted side reactions and loss.

Percent Yield Example:

If you do the reaction in lab, mixing 1.0 grams of each reactant, and you get 1.50 g AgI in your filter paper, what is the actual percent yield?

%1.8110085.1

50.1%

gAgI

gAgIyield

100% lTheoretica

Actualyield