Love, Life and Stoichiometry

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Love, Life and Stoichiometry
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Stoichiometry

• Greek for measuring elements
• The calculations of quantities in chemical
  reactions based on a balanced equation.
• We can interpret balanced chemical equations
  several ways.

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In terms of Particles

• Element- atoms
• Molecular compound (non- metals)- molecule
• Ionic Compounds (Metal and non-metal) - formula
  unit

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2H2 O2 2H2O

• Two molecules of hydrogen and one molecule of
  oxygen form two molecules of water.
• 2 Al2O3 4Al 3O2

2
formula units
Al2O3
form
4
atoms
Al
and
3
molecules
O2
2Na 2H2O 2NaOH H2
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Look at it differently

• 2H2 O2 2H2O
• 2 dozen molecules of hydrogen and 1 dozen
  molecules of oxygen form 2 dozen molecules of
  water.
• 2 x (6.02 x 1023) molecules of hydrogen and 1 x
  (6.02 x 1023) molecules of oxygen form 2 x (6.02
  x 1023) molecules of water.
• 2 moles of hydrogen and 1 mole of oxygen form 2
  moles of water.

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In terms of Moles

• 2 Al2O3 4Al 3O2
• 2Na 2H2O 2NaOH H2
• The coefficients tell us how many moles of each
  kind

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In terms of mass

• The law of conservation of mass applies
• We can check using moles
• 2H2 O2 2H2O

2.02 g H2
2 moles H2
4.04 g H2

1 moles H2
32.00 g O2

1 moles O2
32.00 g O2
1 moles O2
36.04 g H2
36.04 g Reactants
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Therefore

• 2H2 O2 2H2O

18.02 g H2O
36.04 g H2O
2 moles H2O

1 mole H2O
2H2 O2 2H2O
36.04 g H2 O2

36.04 g H2O
No mass has been created or destroyed
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Your turn

• Show that the following equation follows the Law
  of conservation of mass.
• 2 Al2O3 4Al 3O2

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Mole to mole conversions

• 2 Al2O3 4Al 3O2
• every time we use 2 moles of Al2O3 we make 3
  moles of O2

2 moles Al2O3
3 mole O2
or
3 mole O2
2 moles Al2O3
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Mole to Mole conversions

• How many moles of O2 are produced when 3.34 moles
  of Al2O3 decompose?
• 2 Al2O3 4Al 3O2

3.34 moles Al2O3
3 mole O2

5.01 moles O2
2 moles Al2O3
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Your Turn

• 2C2H2 5 O2 4CO2 2 H2O
• If 3.84 moles of C2H2 are burned, how many moles
  of O2 are needed?
• How many moles of C2H2 are needed to produce
  8.95 mole of H2O?
• If 2.47 moles of C2H2 are burned, how many moles
  of CO2 are formed?

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How do you get good at this?
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Stoichiometry Practice
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• Given the following equation
• 2 KClO3 ? 2KCl 3 O2
• How many moles of O2 can be produced by letting
  12.00 moles of KClO3 react?

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• 2 KClO3 ? 2KCl 3 O2
• The strategy will be to convert
  to . Remember, conversion
  factors are what you over what you
  .

moles of KClO3
moles of O2
want
have
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KClO3 ? 2KCl O2
2
3

• 12.00 moles of KClO3

------------------
moles of O2
moles of KClO3
------------------
12 x (3/2)
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• If ammonia, NH3, is burned in air, the following
  reaction takes place
• 4NH3 3O2 ? 2 N2 6H2O
• Given that you started with 51.0 g of NH3, how
  many grams of water will be produced?

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4NH3 3O2 ? 2 N2 6H2O

• The overall strategy will be to convert the
  of ammonia of ammonia, then convert
  the of ammonia of water, and then
  finally convert the of water of water.

grams
to moles
moles
to moles
moles
to grams
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4NH3 3O2 ? 2 N2 6H2O

• grams NH3 moles NH3 moles H2O
  grams of H2O
• 51.0 g NH3

?
?
?
1 mole NH3
18 g H2O
6 mole H2O
4 mole NH3
1 mole H2O
17 g NH3
81 g of H2O
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• 20.0 g of silver(I)nitrate is reacted with an
  excess of sodium choride to produce silver(I)
  chloride
• AgNO3 NaCl gt AgCl NaNO3

What mass of silver(I) chloride is produced?
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AgNO3 NaCl gt AgCl NaNO3

• The overall strategy will be to convert the
  of silver nitrate of silver nitrate ,
  then convert the of silver nitrate of
  silver chloride, and then finally convert the
  of silver chloride of silver chloride.

grams
to moles
moles
to moles
moles
to grams
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AgNO3 NaCl gt AgCl NaNO3

• 20.0 g AgNO3

?
?
?
grams AgNO3
moles AgNO3
moles AgNO3
moles AgCl
moles AgCl
grams AgCl
------------------
1 mole AgNO3
1 mole AgCl
------------------
143.4 g AgCl
------------------
169.9 g AgNO3
1 mole AgNO3
1 mole AgCl
----------------
------------------
------------------
16.9
g AgCl
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• Now go to mass to mass problems

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Mass in Chemical Reactions

• How much do you make?
• How much do you need?

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We cant measure moles!!

• What can we do?
• We can convert grams to moles.
• Periodic Table
• Then do the math with the moles.
• Balanced equation
• Then turn the moles back to grams.
• Periodic table

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For example...

• If 10.1 g of Fe are added to a solution of Copper
  (II) Sulfate, how much solid copper would form?
• Fe CuSO4 Fe2(SO4)3 Cu
• 2Fe 3CuSO4 Fe2(SO4)3 3Cu

1 mol Fe
10.1 g Fe
0.181 mol Fe

55.85 g Fe
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2Fe 3CuSO4 Fe2(SO4)3 3Cu
3 mol Cu
0.272 mol Cu
0.181 mol Fe

2 mol Fe
63.55 g Cu
0.272 mol Cu

17.3 g Cu
1 mol Cu
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Could have done it
1 mol Fe
63.55 g Cu
10.1 g Fe
3 mol Cu
55.85 g Fe
2 mol Fe
1 mol Cu

17.3 g Cu
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More Examples

• To make silicon for computer chips they use this
  reaction
• SiCl4 2Mg 2MgCl2 Si
• How many grams of Mg are needed to make 9.3 g of
  Si?
• How many grams of SiCl4 are needed to make 9.3 g
  of Si?
• How many grams of MgCl2 are produced along with
  9.3 g of silicon?

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For Example

• The U. S. Space Shuttle boosters use this
  reaction
• 3 Al(s) 3 NH4ClO4 Al2O3 AlCl3 3 NO
  6H2O
• How much Al must be used to react with 652 g of
  NH4ClO4 ?
• How much water is produced?
• How much AlCl3?

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We can also change

• Liters of a gas to moles
• At STP
• 25ºC and 1 atmosphere pressure
• At STP 22.4 L of a gas 1 mole
• If 6.45 moles of water are decomposed, how many
  liters of oxygen will be produced at STP?

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For Example

• If 6.45 grams of water are decomposed, how many
  liters of oxygen will be produced at STP?
• H2O H2 O2

1 mol H2O
1 mol O2
22.4 L O2
6.45 g H2O
1 mol O2
18.02 g H2O
2 mol H2O
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Your Turn

• How many liters of CO2 at STP will be produced
  from the complete combustion of 23.2 g C4H10 ?
• What volume of oxygen will be required?

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Gases and Reactions

• A few more details

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Example

• How many liters of CH4 at STP are required to
  completely react with 17.5 L of O2 ?
• CH4 2O2 CO2 2H2O

1 mol O2
22.4 L CH4
1 mol CH4
17.5 L O2
2 mol O2
22.4 L O2
1 mol CH4
8.75 L CH4
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Avagadro told us

• Equal volumes of gas, at the same temperature and
  pressure contain the same number of particles.
• Moles are numbers of particles
• You can treat reactions as if they happen liters
  at a time, as long as you keep the temperature
  and pressure the same.

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Example

• How many liters of CO2 at STP are produced by
  completely burning 17.5 L of CH4 ?
• CH4 2O2 CO2 2H2O

2 L CO2
17.5 L CH4
35.0 L CH4
1 L CH4
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Limiting Reagent

• If you are given one dozen loaves of bread, a
  gallon of mustard and three pieces of salami, how
  many salami sandwiches can you make.
• The limiting reagent is the reactant you run out
  of first.
• The excess reagent is the one you have left over.
• The limiting reagent determines how much product
  you can make

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How do you find out?

• Do two stoichiometry problems.
• The one that makes the least product is the
  limiting reagent.
• For example
• Copper reacts with sulfur to form copper ( I )
  sulfide. If 10.6 g of copper reacts with 3.83 g S
  how much product will be formed?

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• If 10.6 g of copper reacts with 3.83 g S. How
  many grams of product will be formed?
• 2Cu S Cu2S

Cu is Limiting Reagent
1 mol Cu
1 mol Cu2S
159.16 g Cu2S
10.6 g Cu
63.55g Cu
2 mol Cu
1 mol Cu2S
13.3 g Cu2S
13.3 g Cu2S
1 mol S
1 mol Cu2S
159.16 g Cu2S
3.83 g S
32.06g S
1 mol S
1 mol Cu2S
19.0 g Cu2S
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Your turn

• If 10.1 g of magnesium and 2.87 L of HCl gas are
  reacted, how many liters of gas will be produced?
• How many grams of solid?
• How much excess reagent remains?

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Your Turn II

• If 10.3 g of aluminum are reacted with 51.7 g of
  CuSO4 how much copper will be produced?
• How much excess reagent will remain?

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Yield

• The amount of product made in a chemical
  reaction.
• There are three types
• Actual yield- what you get in the lab when the
  chemicals are mixed
• Theoretical yield- what the balanced equation
  tells you you should make.
• Percent yield Actual x 100
  Theoretical

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Example

• 6.78 g of copper is produced when 3.92 g of Al
  are reacted with excess copper (II) sulfate.
• 2Al 3 CuSO4 Al2(SO4)3 3Cu
• What is the actual yield?
• What is the theoretical yield?
• What is the percent yield?

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Details

• Percent yield tells us how efficient a reaction
  is.
• Percent yield can not be bigger than 100 .

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Homework!!