Chapter 12 Stoichiometry

Quantity

12.1 StoichiometryChemical reactions represent the heart of chemistry: they describe the endless ways that substances

can combine with each other to form new substances. In essence, they describe what chemistry really

is -- the study of matter and its transformations through Chemical

Reactions.

Ch 12 Stoichiometry is the area of chemistry which deals with quantitative relationships in

chemical reactions. It is the area which allows chemists to know how much starting material is

needed to produce a million pounds of sulfuric acid or how many

molecules are in a microgram of a particular hormone.

When I worked in a semi trailer factory, guess who was highest paid?

Not the factory worker (Minimum wage)Not the welders (4 times minimum wage)Not the foreman (5 times minimum wage)It was the warehouse manager (6 times

minimum wage)Why??? He had to know how to do

Stoichiometry to produce the correct amount of chemicals needed for

production.

StoichiometryPredicting the quantities of

products when given the quantities of reagents.

StoichiometryPredicting the quantities of

products when given the quantities of reagents.

Stoichiometry:Quantities are usually given in grams, kilograms, or pounds. Are chemical equations in grams?

StoichiometryPredicting the quantities of

products when given the quantities of reagents.

Quantities are usually given in grams, kilograms, or pounds. Are chemical equations in grams? No, in representative particles or moles.Usually we have to: 1. Convert from grams to moles. 2. Work equations with moles. 3. Convert moles back to grams.

What is the mass of 2 moles of F2?• How much is in 2 moles of F2?

(written as 2 F2 )

• Moles = 2 moles of F2

• Molecules = 2 molecules of F2 (or 2 x 6.02 x 1023 molecules)

• Atoms = 4 atoms F (or 4 x 6.02 x 1023 atoms)

• Grams = 76g (F has atomic mass of 19)(total grams in 2 moles of F2)

Liters = 44.8L (C.F. = 22.4 L per Mole at STP)

Stoichiometry(“stoy-key-AHM-uh-tree”)

The relationship between the quantities of reactants and products in a chemical reaction (Eqn next page.)

P4(s) + 6 Cl2(g) 4 PCl3(s)

From the balanced equation the following information can be obtained.

U try it!

• 2Na + Cl2 2NaCl

• Moles?

• Grams?

• Liters?

U try it!

• 2Na + Cl2 2NaCl

• Moles? 2 1 2

• Grams? 46 70 116

• Liters? 22.4

Mole ratios• The ratio of moles given by the balanced equation.

N2 + 3F2 2NF3

For every 1 mole of N2 there are 2 moles of NF3 formed (assuming that there is at least 3 F2)

Mole Ratios: N2:NF3 is 1 to 2

F2:NF3 is 3 to 2

Mole to Mole calculations

How many moles of ozone O3 will be formed from 6 moles of

oxygen O2?

O2 O3

• Step 1 balance the equation.

How many moles of ozone O3 will be formed from 6 moles of oxygen O2?

• Step 2 Identify what the question is asking… Goal?– List Known’s and Unknown’s

How many moles of ozone O3 will be formed from 6 moles of oxygen O2?

• Step 3 Use dimensional analysis to answer solve the problem

How many moles of ozone O3 will be formed from 6 moles of oxygen O2?• Check your answer to see if it is logical.

Can you make the six mole ratios?4Al + 3O2 2Al2O3

How many moles of aluminum and oxygen are needed to form 3.21 moles of

aluminum oxide?4Al + 3O2 2Al2O3

How many moles of chlorine are needed to make 15 moles of NaCl?

C + O2 CO2

1 C atom reacts with 1 O2 molecule to form 1 CO2 molecule

12 C atoms reacts with 12 O2 molecules to form 12 CO2 molecules

1 mol C atoms reacts with 1 mol O2 molecules to form 1 mol CO2 molecules

What do you need to make 12 mol CO2?

12.2 Chemical calculations

•Start here!

12.2 Chemical calculations

12.2 Chemical calculations

• 1 mole = • 6.02 x 10 23 representative particles = • molar mass (grams) = • 22.4L (gases only)

Mole ratios

• 2 moles of O3 form 3 moles of O2

• 2 O3 3 O2

(Gram, molecules, particles, atoms)

–Remember ALWAYS convert to moles!

–Then use mole ratio.

How many grams of water can be formed from 15g of hydrogen?

(assume there is unlimited oxygen)2H2 + O2 2H2O

– g mol hydrogenThen, mole ratio to find water

then convert moles of water grams

2H2 + O2 2H2O

How many H2 molecules are needed to make 50g of water?

(assume there is unlimited oxygen)2H2 + O2 2H2O

50g of water2H2 + O2 2H2O

– g H2O moles H2O

• Moles H2O moles H2

–Moles H2 molecules of H2

Greenhouse gases

• North America, consists of the United States and Canada. North America is the highest fossil-fuel, CO2 emitting region of the world with 1.73 billion tons of carbon in 2002. This 2002 total is an all-time high for North America and represents a 1.4% increase from 2001. Because ~92% of current fossil-fuel CO2 emissions from the region are from the United States, the North America data closely resembles that for the United States.

• How many L of CO2(g) will be formed from 16 gallons of gasoline (64,000g)? (assume gas is 100% octane)

2C8H18 + 25O2 16CO2 + 18H2O

• 197,614g = 2.0 x 105

• 100,000L of CO2 made for each 16 gal of gas burned.

12.5 g Li g product

• O2

• Li + O2 Li2O

• S

• Li + S Li2S

12.5 g Li Predict g product

• O2

• Li + O2 Li2O

• S

• Li + S Li2S

12.5 g Li Predict g productStep 1, Balance

• O2

• Li + O2 Li2O

• 4Li + 2O2 2Li2O

• S

• Li + S Li2S

• 2Li + S Li2S

Use last problem to intro 12.3

12.5 g Li g product• What is your a.)Theoretical Yield

b.) Actual Yield c.) Percent Yield

• Li + O2 Li2O

• Previous Ans: You calculate 26.9 g Li2O

• You measure the product and there is 25 grams of Li2O

• Li + S Li2S

• Previous Ans: You calculate 41.4 g Li2S

• You measure the product and there is 37.0 grams of Li2S

12.5 g Li g product• Li + O2 Li2O

• a.)Theoretical Yield 27.0 g Li2O

b.) Actual Yield 25 g Li2O

c.) Percent Yield 25/26.9 x100=92.9%

• Li + S Li2S

• a.)Theoretical Yield 41.4 g Li2S

b.) Actual Yield 37.0 g Li2S

c.) Percent Yield 37.0/41.4 x100=89.4%

Actual Yield = MEASURED amount

(recorded by experiment)

Theoretical Yield = CALCULATED amount

(What balance equation says we should have produced)

Percent Yield is ratio of GRAMS!

12.3 limiting reagents

• limiting reagent – reactant that will be used up first and cause the reaction to stop producing products

Identify the limiting reagent• Example Problem (solved on next slide)

• 2Na + Cl2 2NaCl

– Givens are starting amounts of each reagent

– Which is the limiting reactant?

STEPS to find limiting reagent (Balance equation.)

1.) Calculate amount of product produced by each reagent.

2.) Limiting reagent is the one that produces the least amount of product.

3.) Excess reagent is the leftover amounts of the other reagent

Identify the limiting reagent

• 2Na + Cl2 2NaCl

– 1 mol Na reacts with 1 mol Cl2?

– Which is the limiting reactant?

Identify the excess reagent

• 2Na + Cl2 2NaCl

– 1 mol Na reacts with 1 mol Cl2?

– How much of excess reagent is there?

What is the actual equation?

1 Na + 1 Cl2 1 NaCl + ½ Cl2 Limiting Excess

Reagent Reagent

Identify the limiting reagent

N2 + O2 NO3

– 1.7 mol N2 or 1.3 mol O2

Identify the excess reagent.

N2 + O2 NO3

– 1.7 mol N2 or 1.3 mol O2

58g of Na are reacted with 110g of S how much product will be formed?

2Na + S Na2S

Discuss steps

• What is the percent yield if 170g of sodium sulfide was produced? (see previous problem)

• Because of human error, measurement error, contaminated reactants, experimental error, ect… there will usually be less than 100% yield, meaning that less products will be produced then you calculated for.

15.5g of H2 is reacted with 155g of O2.

What is the percent yield if 136g of H2O has been collected?

2H2 + O2 2H2O

ANSWER:15.5g of H2 is reacted with 155g of O2.

What is the percent yield if 136g of H2O has been collected?

2H2 + O2 2H2O97.1%

• SUMMARY

• Write the balanced equation.• Convert variables to moles. • Find the moles of each reactant needed based on

the amount of the other reactant given (use Na to find S).

• Identify the limiting reagent. • Solve for the amounts of reactants needed and

products formed.