1. Reactions in Chemistry
Writing Balanced Chemical Equations

2. Writing reaction equations
Before you can write a balanced chemical equation for a reaction, you need to write an unbalanced equation. To do this you need to identify several things:
What chemicals are reacting?
What is the proper formula for each chemical?
What type of reaction is it?
What are the products that will be produced?
What are their proper formulas?

3. Types of chemical reactions
Remember, the five basic categories of reactions are:
Synthesis (a.k.a. Direct Combination)
Decomposition

4. Types of chemical reactions
Single Replacement
Double Replacement

5. Types of chemical reactions
Combustion
An element or compound reacts with oxygen, often producing energy in the form of heat and light.
If a hydrocarbon undergoes combustion, the products are normally CO2 and water.

6. Things to remember When writing the chemical formulas:
Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine are diatomic molecules.
When deciding on formulas for binary ionic compounds, use the crisscross method and remember to reduce the ratio of atoms if possible.
Be sure that all chemical formulas are correct and that all reactants and products have been included before trying to balance the equation.

7. Example 1
A strip of magnesium is placed in a hydrochloric acid solution, and hydrogen gas is released. Write a balanced equation for the reaction that takes place.
What are the reactants?
What are their formulas?
Magnesium metal and hydrochloric acid.
Mg and HCl

8. Example 1 (cont)
What type of reaction would this be? What are the products? (Were all of them mentioned?) What are their formulas?
Single replacement
Magnesium chloride and hydrogen gas.
MgCl2 and H2

9. Example 1 (cont)
What would the unbalanced equation be? Now balance it:
Reactants Products
Mg – Mg –
H – H –
Cl – Cl –

10. Example 2 Potassium chlorate decomposes, to evolve oxygen.
What are the reactants?
What are their formulas?
Only potassium chlorate
KClO3

11. Example 2 (cont)
What type of reaction would this be? What are the products? (Were all of them mentioned?) What are their formulas?
Decomposition
Oxygen and potassium chloride
O2 and KCl

12. Example 2 (cont)
What would the unbalanced equation be? Now balance it:
Reactants Products
K – K –
Cl – Cl –
O – O –

13. Example 3 Copper(II) sulfate reacts with a sodium hydroxide solution.
What are the reactants?
What are their formulas?
Copper(II) sulfate and sodium hydroxide.
CuSO4 and NaOH

14. Example 3 (cont)
What type of reaction would this be? What would the products be? What are their formulas?
Double replacement
Copper(II) hydroxide and sodium sulfate
Cu(OH)2 and Na2SO4

15. Example 3 (cont)
What would the unbalanced equation be? Now balance it:
Reactants Products
Cu – Cu –
SO4 – SO4 –
Na – Na –
OH – OH –

16. Example 4
The reaction that takes place in a butane lighter. (butane is C4H10)
What type of a reaction is this?
What would the reactants be?
Burning would indicate combustion
Butane and oxygen

17. Example 4 (cont)
What are the formulas for the reactants? What would the products be? What are their formulas?
C4H10 and O2
Combustion results in carbon dioxide and water
CO2 and H2O

18. Example 4 (cont)
What would the unbalanced equation be? Now balance it:
Reactants Products
C – C –
H – H –
O – O –

19. Example 5
Soda pop gets its acidic taste because as carbon dioxide is forced to dissolve in the pop, it combines with water forming an acid.
What type of a reaction is this?
What would the reactants be?
Synthesis (or direct combination)
Carbon dioxide and water

20. Example 5 (cont)
What are the formulas for the reactants? What would the products be? What are their formulas?
CO2 and H2O
Synthesis results in only one product.
In this case it would be carbonic acid.
Only H2CO3

21. Example 5 (cont)
What would the unbalanced equation be? Now balance it:
Reactants Products
C – C –
O – O –
H – H –

22. Resources:
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