Cl2 2Cl 2Cl2

Purpose: Students will be able to balance chemical equations.

Warm-up: What’s the difference between the three formulas below?

one molecule two atoms two molecules

• Molecules may also have brackets to indicate numbers of atoms. E.g. Ca(OH)2• Notice that the OH is a group• The 2 refers to both H and O

O H

O H

Ca

• How many of each atom are in the following?a) NaOHb) Ca(OH)2c) 3Ca(OH)2

Na = 1, O = 1, H = 1Ca = 1, O = 2, H = 2

Ca = 3, O = 6, H = 6

Counting Atoms

Balancing Equations: MgO• Law of Conservation of Mass: matter can neither

be created or destroyed

• Since atoms cannot be created or destroyed, they are only rearranged in a chemical reaction

• The number of atoms must be the same on both sides!

Example: Magnesium + OxygenMg + O2 MgO

• However, this is not balanced• Left: Mg = 1, O = 2• Right: Mg = 1, O = 1

O Mg O+ Mg O

Balancing Equations: MgO

• However, this is not balanced• Left: Mg = 1, O = 2• Right: Mg = 1, O = 1

O Mg O+ Mg O

Balancing Equations: MgO

From: Mg + O2 MgO 2Mg + O2 2MgO is correctMg + ½O2 MgO is incorrectMg2 + O2 2MgO is incorrect4Mg + 2 O2 4MgO is incorrect

Balance Equations

5

2 33.5

2 7 4 6

2 22

2 63

e) C2H6 + O2 CO2 + H2O

a) P4 + O2 P4O10

b) Li + H2O H2 + LiOH

c) Bi(NO3)3 + K2S Bi2S3 + KNO3

d) C2H6 + O2 CO2 + H2O

a) Mg + 2HCl MgCl2 + H2

b) 3Ca + N2 Ca3N2

c) NH4NO3 N2O + 2H2O

d) 2BiCl3 + 3H2S Bi2S3 + 6HCl

e) 2C4H10 + 13O2 8CO2 + 10H2O

f) 6O2 + C6H12O6 6CO2 + 6H2O

g) 3NO2 + H2O 2HNO3 + NO

h) Cr2(SO4)3+ 6NaOH 2Cr(OH)3+ 3Na2SO4

i) Al4C3 + 12H2O 3CH4 + 4Al(OH)3

Balance these skeleton equations:

a) 2KNO3 2KNO2 + O2

b) 2Pb(NO3)2 2PbO + 4NO2 + O2

c) P4 + 6I2 4PI3

d) 3MgO + 2H3PO4 Mg3(PO4)2 + 3H2O

e) Br2 + 2KI I2 + 2KBr

f) Ca(OH)2 + 2HNO3 Ca(NO3)2 + 2H2O

g) Bi2O3 + 3H2 2Bi + 3H2O

h) 3Fe + 2O2 Fe3O4

i) 2CaO + 5C 2CaC2 + CO2

Here are some more to balance: