Rules for Balancing Equations

1
Rules for Balancing Equations

• Determine the correct formula for all reactants
  and products.
• Write the formulas for the reactants on the left
  side of arrow and the products on the right

2

• Count the number of atoms on each side of the
  reaction. If a polyatomic ion appears unchanged
  on both sides count it as one unit.
• Balance the elements one at a time using
  coefficients.
• NEVER CHANGE SUBSCRIPTS TO BALANCE

3
Combination reactions

• Two or more substances come together to form a
  compound
• A B è AB
• Mg O2 è MgO

4

• Combination
• Reactions
• 2H2 O2 gt 2H2O

5
Special combination reactions

• A metal oxide and water give a base
• A base is a compound with hydroxide
• K2O H2O è KOH
• CuO H2O è ?
• Right !! Copper(II) hydroxide

6
Special combination reactions

• A nonmetal oxide and water give an acid
• An acid is a compound with hydrogen
• SO3 H2O è H2 SO3 and/or H2 SO4
• NO2 H2O è ?
• Right !! Nitric or nitrous acid

7
Important fact

• Combination reactions are responsible for acid
  rain.
• Sulfuric acid, nitric acid and carbonic acid are
  involved.
• What nonmetal oxides are involved?
• Where do they come from?

8
Decomposition reaction

• A substance breaks down forming two or more
  substances
• ABè A B
• NI3è N2 I2
• MnO2
• H2O2 è H2O O2

9
Single replacement reactions

• A BC è AC B
• Aluminum Iron(II) oxideè Aluminum oxide Iron
  
• 2Al(s) Fe2O3(s) --gt Al2O3(s) 2Fe(s)

10
Single replacement will it or will it not
happen?

• For metals, check the reactivity series
• A more reactive metal will bump a less reactive
  one
• Zn HCl è ZnCl2 H2

11
Single replacement will it or will it not
happen?

• Magnesium is placed in sulfuric acid
• Mg H2SO4 è MgSO4 H2
• zinc is placed in phosphoric acid
• 3Zn 2H3PO4 è Zn3(PO4)2 3H2
• Potassium is placed in contact with aluminum
  chloride
• 3K AlCl3è3KCl Al

12
Single replacement will it or will it not
happen?

• For non metals look to the periodic table (as
  an activity series)
• F is more reactive than Cl
• F2 HCl è HF Cl2
• But
• Cl2 HF è no reaction

13
Write single replacement equations

• Magnesium is added to lead nitrate
• Iron is added to aluminum sulfide
• Sodium is added to calcium carbonate

14
Write single replacement equations

• Cl2 NaI gt ?
• I2 NaCl gt ?
• K NaCl gt ?

15
Double Replacement

• Two compounds interact
• The cations trade places
• AB CD è AD CB
• Na3PO4 FeCl3è Fe3PO4 NaCl

16
What is the precipitate

• Silver nitrate and sodium hydroxide react
• AgNO3 NaOH è NaNO3 AgOH

17
Check the solubility tables

• AlCl3 NaOH è Al(OH)3 (s) NaCl (aq)
• How do we know that the aluminum hydroxide is the
  precipitate? We look at a solubility table
• If the solubility table indicates I
  (insoluble), expect the substance to be a
  precipitate

18
Combustion reactions

• Fuel oxygen gives carbondioxide, water, and
  energy
• C3H8 (g) 5O2 (g) -----gt 3CO2 (g) 4H2O (g)
• Fuels have carbon, hydrogen, and sometimes oxygen

19
Organic molecules The alkanes CnH2n2

• Methane CH4
• Ethane C2H6
• Propane C3H8
• Butane C4H10
• Pentane (five carbons) C5H12
• Hexane (six carbons) C6H14
• Heptane (seven carbons) C7H16
• Octane (eight carbons) C8H18

20
Can you write balanced equations for the
combustion of these hydrocarbons?

• Methane (one carbon)
• Ethane (two carbons)
• Propane (three carbons)
• Butane (four carbons)
• Pentane (five carbons)
• Hexane (six carbons)
• Heptane (seven carbons)
• Octane (eight carbons)