Balancing Chemical Equations

What is a chemical equation?

When a chemical reaction occurs, it can be described by an equation. This shows the chemicals that react (called the reactants) on the left-hand side, and the chemicals that they produce (called the products) on the right-hand side. The chemicals can be represented by their names or by their chemical symbols.

Unlike mathematical equations, the two sides are separated by an arrow, that indicates that the reactants form the products and not the other way round.

The concept of balancing equations

Take a look at this chemical word equation:

Aluminium + Oxygen → Aluminium Oxide

This is the equation for the burning of aluminium in oxygen. If we convert each of the chemical names into the appropriate symbols, we get the following:

Al + O₂ → Al₂O₃

Note that oxygen gas is diatomic, which means that the oxygen atoms, like policemen, go around in pairs. A molecule of aluminium oxide consists of two aluminium atoms combined with three oxygen atoms. Actually, technically the word "molecule" is inappropriate in that previous sentence. The formula simply tells us the ratio of aluminium atoms to oxygen atoms in the compound.

You can see by looking at it that there is something wrong with this equation. If you count the number of atoms of each type on each side, you will see that there is only one aluminium atom on the left side whereas there are two on the right. There are two oxygen atoms on the left side, as compared to three on the right side. This clearly doesn't match.

Left side:

Right side:

We can balance the equation by mutiplying the different atoms and molecules on each side by different amounts. Firstly, multiply the aluminium atoms on the left side by 2:

Al + O₂ → Al₂O₃

Left side:

Right side:

Now there are the same number of aluminium atoms on each side of the equation. We could also multiply the number of oxygen molecules on each side by one and a half (1.5), which would give three oxygen atoms on the left side (1.5 x 2 = 3) to match the three oxygen atoms on the right side:

2 Al + 1.5 O₂ → Al₂O₃

Left side:

Right side:

This is now balanced, but that 1.5 is a horrible thing to have in an equation - how can you have one and a half molecules? We can solve this problem by multiplying everything throughout by 2:

Al +

O₂ →

Al₂O₃

Left side:			Right side:

If you count the number of atoms on each side, you will find that there are four aluminium atoms on each side and six oxygen atoms.

Write your answers in the boxes. Every box will require a coefficient.

1. H₂ + O₂ → H₂O

2. H₂ + N₂ → NH₃

3. Al₂O₃ → Al + O₂

4. KClO₃ → KCl + O₂

5. S₈ + O₂ → SO₂

6. C₂H₆ + O₂ → CO₂ + H₂O

7. Al₂(SO₄)₃ + Ca(OH)₂ → Al(OH)₃ + CaSO₄
8. P₄ + O₂ → P₂O₅

9. Ag + S₈ → Ag₂S

10. Al + Br₂ → AlBr₃

11. Cr + O₂ → Cr₂O₃

12. NaClO₃ → NaCl + O₂

13. AlBr₃ + Cl₂ → AlCl₃ + Br₂

14. Na + H₂O → NaOH + H₂

15. AlI₃ + HgCl₂ → AlCl₃ + HgI₂

16. Ca(OH)₂ + H₃PO₄ → Ca₃(PO₄)₂ + H₂O

17. AgNO₃ + K₃PO₄ → Ag₃PO₄ + KNO₃

18. C₃H₈ + O₂ → CO₂ + H₂O

19. C₂H₂ + O₂ → CO₂ + H₂O

20. C₆H₆ + O₂ → CO₂ + H₂O

WRITE THE COMPLETE PRODUCT AND BALANCE THE EQUATION

When heated, aluminium reacts with solid copper oxide to produce copper metal and aluminium oxide:
Al + CuO → Al₂O₃ + ___
Potassium oxide is not a stable compound. In the presence of water (or even water vapour in the air), it readily converts into potassium hydroxide:
K₂O + H₂O → _____

When calcium chloride solution is mixed with silver nitrate solution, a thick white precipitate (solid) of silver chloride appears. This is because silver chloride is insoluble: when silver ions and chloride ions find themselves together in solution, they immediately react together to form the solid. This leaves the calcium ions and nitrate ions in solution, effectively forming calcium nitrate solution.
CaCl₂ + AgNO₃ → AgCl + ____

Magnesium will react with phosphorus to form magnesium phosphide, a chemical widely used in industry.
Mg + P₄ → ____

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Monday, March 21, 2011

CHEMICAL FORMULA WRITING AND NAMING