1. BALANCING CHEMICAL EQUATIONS
Chemical reactions and chemical equations
Writing & balancing chemical equations

2. Chemical formula: shorthand method for showing the composition of a compound using element symbols and subscripts.
H2O, CaCl2, C6H12O6
Subscript: number that appears partially below the line and to the right of a symbol in a chemical formula.
C6H12O6 = 6 carbon, 12 hydrogen, 6 oxygen
Ca(NO3)2 = 1 calcium, 1 X 2 = 2 nitrogen, 3 X 2 = 6 oxygen
Chemical reaction: a chemical change; atoms, molecules, or ions rearrange to form new substances.
Substances entering the reaction are called reactants.
Substances formed in the reaction are called products.

3. Reaction of hydrogen (H2) and oxygen (O2) to form water (H2O)
Law of Conservation of Matter: atoms (matter) cannot be created or destroyed.
Therefore, all atoms present in the reactants must also be present in the products, although they may rearrange.
During a reaction:
Chemical bonds that hold atoms together in the reactants are broken.
Some (or all) of the atoms rearrange.
New bonds are formed as atoms come together to form products.
Reaction of hydrogen (H2) and oxygen (O2) to form water (H2O)

4. Coefficient: number that appears before a chemical formula
Chemical equation: shorthand expression showing the reactants and products of a chemical change.
H2 + O2 → H2O
Coefficient: number that appears before a chemical formula
Shows how many molecules are present
Used to balance equations
When none is shown, is understood to be one.
2 H2O = 2 water molecules
2 X 2 = 4 H 2 X 1 =2 O

5. aqueous solution (aq): dissolved in water
Balanced chemical equation: contains same number of each kind of atom on each side of the equation.
2 H2 (g) + 1 O2 (g) → 2 H2O (g)
2 X 2 = 4 H 2 X 2 = 4 H
1 X 2 = 2 O 2 X 1 = 2 O
Includes:
A summary of the reaction.
Formulas and numbers of reactants and products present.
States of reactants and products:
solids (s)
liquids (l)
gases (g)
aqueous solution (aq): dissolved in water

6. How to balance chemical equations:
Write the unbalanced equation.
Inventory the atoms in the reactants and products.
Balance the equation.
Hint: balance elements left-to-right on the Periodic Table (metals first, then nonmetals)
Leave H and O until last

7. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
1. Write the unbalanced equation.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

8. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
2. Inventory reactants and products.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) Zn Cl H 1 Zn Cl H 1 2

9. 3. Balance the equation (by changing coefficients.)
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 2 Zn Cl H 1 Zn Cl H 1 2
BALANCED ! 2
BALANCED !
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 1 2 1 1

10. Try to balance this reaction:
__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) 1 2 1 2 C H O 1 4 2 C H O 1 2 3 4 4 4

11. K Cl O 1 3 2 K Cl O 1 2 2 2 2 6 6 __KClO3 (s)  __KCl (s) + __O2 (g) 2
Try to balance this reaction:
__KClO3 (s)  __KCl (s) + __O2 (g) 2 2 3 K Cl O 1 3 2 K Cl O 1 2 2 2 2 6 6

12. Try to balance this reaction:
__Al(OH)3 (s) + __H2SO4 (aq) 
__ Al2(SO4)3 (aq) + __H2O (l) 2 3 1 6 Al S H O 1 5 7 2 Al S H O 2 3 13 3 8 12 12 10 18 18

13. A balanced reaction means: 2 H2 (g) + O2 (g) → 2 H2O (g)
“1 H2O” means:
1 molecule of water = 2 H atoms and 1 O atom
1 mole of water = 6.02 X 1023 molecules = 18g H2O
A balanced reaction means:
2 H2 (g) + O2 (g) → 2 H2O (g)
2 molecules H2 1 molecule O2 2 molecules H2O
2 moles H2 1 mole O2 2 moles H2O