Chapter 3 Stoichiometry

1
Chapter 3Stoichiometry

• Sections 7-10
• Chemical Equations
• Stoichiometry
• Limiting Reactant

2
Section 7 and 8

• Chemical Equations
• Balancing Chemical Equations

3
Characteristics of a Chemical Reaction

• A reorganization of the atoms in one or more
  substances.
• Bonds have been broken
• New bonds have been formed
• The law of conservation of mass must be satisfied
  there must be the same number of each type of
  atom on the product and reactant side.
• Coefficients a whole number that appears in
  front of a formula in a chemical equation.

4
Characteristics of a Chemical Reaction

• The equation must represent known facts.
• The equation must contain the correct formulas
  for the reactants and products.
• Diatomic elements I2 Br2 Cl2 F2 O2 N2 H2
• Molecular elements S8 P4

5
Diatomic Elements
6
Chemical Reactions

• methane oxygen ? carbon dioxide water

PRODUCTS
REACTANTS
YIELDS
7
The Meaning of a Chemical Equation

• methane oxygen ? carbon dioxide water

CH4 (g) O2 (g) ? CO2 (g)
H2O(g)
STATE OF MATTER SYMBOLS
Solid (s)Liquid (l) Gas
(g)Aqueous (aq)
? Dissolved in water
8
The Meaning of a Chemical Equation
CH4 (g) 2 O2 (g) ? CO2 (g)
2 H2O(g)

• 1 molecule 2 molecules ? 1 molecule 2
  molecules
• 1 mole 2 moles ? 1
  mole 2 moles
• 6.022 x 1023 2(6.022 x 1023 ) ? 6.022 x 1023
  2(6.022 x 1023)
• 16 g 2(32 g) ? 44g
  2(18 g)
• 80 g reactants 80 g products

?
9
Balancing Chemical Equations

1. If the reaction is described by a paragraph,
   write the word equation.
2. Write the formula for each reactant and product
   to get the formula equation.
3. Balance the equation.
4. Insert state of matter symbols and other
   additional symbols.

10
Balancing Chemical Equations

• TIPS FOR PLAY
• Balance the different types of atoms one at a
  time.
• First balance elements that appear only once on
  each side.
• Balance polyatomic ions that appear on both sides
  as a single unit.
• Balance H and O last.

11
Balancing Chemical Equations

• TIPS FOR PLAY
• Try keeping a tally for each element on each side
  below the equation.
• If it could be balanced by a coefficient of 1½ -
  use it- then multiply all coefficients in the
  equation by 2.

12
Ammonium Dichromate Volcano

• See Ammonium Dichromate Volcano on Houghton
  Mifflin Chemistry DVD

13
Sample Problem p.100

• Chromium compounds exhibit a variety of bright
  colors. When solid ammonium dichromate, a vivid
  orange compound, is ignited , a spectacular
  reaction occurs. Lets assume the products are
  solid chromium (III) oxide, nitrogen gas, and
  water vapor.

14
Sample Problem p.100

• Ammonium dichromate ? chromium (III) oxide
  nitrogen water

(NH4)2Cr2O7 ? Cr2O3 N2 H2O
4
N H Cr O
N H Cr O
2 8 2 7
2 2 2 4
2 8 2 7
(NH4)2CrrO7 (s) ? Cr2O3(s) N2 (g) 4 H2O(g)
15
Sample Problem

• Ammonia gas reacts with oxygen gas to form
  gaseous nitrogen monoxide and water vapor. This
  reaction is the first step in the commercial
  production of nitric acid by the Ostwald process.
  Balance the equation for this reaction.

16
Sample Problem

• ammonia oxygen ? nitrogen monoxide and
  water

NH3 O2 ? NO H2 O
3
2
N H O
N H O
1 3 2
1 2 2
1 6 4
2 6 2
17
Sample Problem

• ammonia oxygen ? nitrogen monoxide water

NH3 O2 ? NO H2 O
3
2
2½
2
N H O
N H O
1 6 4
2 6 2
2 6 5
2 6 5
Multiply all coefficients by 2!
4NH3 (g) 5O2 (g) ? 4NO (g) 6H2 O (g)
18
Section 9

• Stoichiometric Calculations

19
Calculating Masses of Reactants and Products in
Chemical Reactions

1. Balance the equation for the reaction.
2. Convert the known mass of the reactant or product
   to moles of that substance
3. Use the balanced equation to set up the
   appropriate mole ratio
4. Use the appropriate mole ratio to calculate the
   number of moles of the desired reactant or
   product
5. Convert from moles back to grams if required by
   the problem.

20
Sample Problem p.121 90

• The reaction between potassium chlorate and red
  phosphorous takes place when you strike a match
  on a matchbox. If you were to react 52.9 g of
  potassium chlorate with excess red phosphorous,
  what mass of tetraphosphorous decaoxide would be
  produced?

KClO3 (s) P4(s) ? P4O10 (s) KCl
(s) (unbalanced)
21
Sample Problem p.121 89

• Over the years, the thermite reaction has been
  used for welding railroad rails, in incendiary
  bombs, and to ignite solid-fuel rocket motors.
  The reaction is
• What masses of iron (III) oxide and aluminum must
  be used to produce 15.0 g of iron? What is the
  maximum mass of aluminum oxide that could be
  produced?

Fe2O3 (s) 2Al(s) ? 2Fe (l) Al2O3 (s)
22
Section 10

• Stoichiometry Limiting Reactants

23
Sample Problem p.121 100

• Consider the following unbalanced equation
• What masses of calcium sulfate and phosphoric
  acid can be produced from the reaction of 1.0 Kg
  calcium phosphate with 1.0 kg concentrated
  sulfuric acid (98 sulfuric acid by mass)?

Ca3(PO4)2 (s) H2SO4(aq) ? CaSO4 (s)
H3PO4 (aq)
24
Sample Problem p.121 99

• What mass of hydrogen peroxide should result when
  1.50 g of barium peroxide is treated with 25.0 mL
  of hydrochloric acid solution containing 0.0272 g
  of HCl per mL? What mass of reagent is left
  unreacted?

BaO2(s) 2HCl(aq) ? H2O2 (aq) BaCl2 (aq)
25
Stoichiometry Limiting Reactant

• Theoretical Yield the amount of product formed
  when the limiting reactant is completely
  consumed.
• Percent Yield __Actual Yield__ x 100
• Theoretical Yield

26
Sample Problem p.122 104

• DDT, an insecticide harmful to fish, birds, and
  humans, is produced by the following reaction
• In a government lab, 1142 g of chlorobenzene is
  reacted with 485 of chloral.
• a. What mass of DDT is formed?
• b. Which reactant is limiting? Which is in
  excess?
• c. What mass of the excess reactant is left
  over?
• d. If the actual yield of DDT is 200.0 g, what
  is the percent yield?

2C6H5Cl C2HOCl3 ? C14H9Cl5
H2O chlorobenzene chloral DDT