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

• 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

Combination reactions

• Two or more substances come together to form a compound

• A + B AB

• Mg + O2 MgO

CombinationReactions2H2 + O2 =>

2H2O

Special combination reactions

• A metal oxide and water give a base

• A base is a compound with hydroxide

• K2O + H2O KOH

• CuO + H2O ?

• Right !! Copper(II) hydroxide

Special combination reactions

• A nonmetal oxide and water give an acid

• An acid is a compound with hydrogen

• SO3 + H2O H2 SO3 and/or H2 SO4

• NO2 + H2O ?

• Right !! Nitric or nitrous acid

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?

Decomposition reaction• A substance breaks down

forming two or more substances:

• AB A + B

• NI3= N2 + I2

MnO2

• H2O2 H2O + O2

Single replacement reactions

• A + BC AC + B

• Aluminum + Iron(II) oxide Aluminum oxide + Iron

• 2Al(s) Fe2O3(s) + --> Al2O3(s) + 2Fe(s)

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 ZnCl2 +

H2

Single replacement: will it or will it not happen?

• Magnesium is placed in sulfuric acid

• Mg + H2SO4 MgSO4 + H2

• zinc is placed in phosphoric acid

• 3Zn + 2H3PO4 Zn3(PO4)2 + 3H2

• Potassium is placed in contact with aluminum chloride

• 3K+ AlCl33KCl + Al

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 HF + Cl2

But –

• Cl2 + HF no reaction

Write single replacement equations

•Magnesium is added to lead nitrate

• Iron is added to aluminum sulfide

•Sodium is added to calcium carbonate

Write single replacement equations

•Cl2 + NaI => ?

• I2 + NaCl => ?

•K + NaCl => ? At

I

Br

Cl

F

Group VIIA

Double Replacement

• Two compounds interact

• The cations trade places

• AB + CD AD +CB

• Na3PO4 + FeCl3 Fe3PO4 + NaCl

What is the precipitate

•Silver nitrate and sodium hydroxide react:

•AgNO3 + NaOH NaNO3 + AgOH

Check the solubility tables

AlCl3 + NaOH 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

Combustion reactions

• Fuel + oxygen gives carbondioxide, water, and energy

• C3H8 (g) + 5O2 (g) -----> 3CO2 (g) + 4H2O (g)

• Fuels have carbon, hydrogen, and sometimes oxygen

Organic molecules: The alkanes CnH2n+2

• Methane CH4

• Ethane C2H6

• Propane C3H8

• Butane C4H10

• Pentane (five carbons) C5H12

• Hexane (six carbons) C6H14

• Heptane (seven carbons) C7H16

• Octane (eight carbons) C8H18

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)