Science 10

Balancing Chemical Equations

There are two types of numbers found in chemical equations:

  • Subscripts – the small numbers to the lower right of chemical symbols show the number of atoms of each element. There are two atoms of oxygen in a molecule of oxygen gas, O2.
  • Coefficients – the large numbers in front of chemical formulas. Coefficients show the number of molecules of the substance used in the reaction. The coefficient is a “multiplier”.

You can change any coefficient in a chemical equation. This shows more or less of a substance is needed. You can not change the subscripts in a chemical formula. To do so would change the substance.

Chemical equations must be balanced because atoms can be neither created nor destroyed in an ordinary chemical reaction. (What goes in must come out!) This is the Law of Conservation of Mass.

There must be the same number of each type of atom on each side of the chemical equation. For example – we know a reaction occurs between oxygen and hydrogen to make water:

hydrogen gas +oxygen gas  water

A chemical equation shows a chemical change using symbols. The “skeleton” equation uses the formula for each reactant and product.

H2+O2H2O

two hydrogens + two oxygens  two hydrogens and one oxygen

Just by looking, we can see that this equation is NOT balanced. There are two oxygen molecules on the left in the reactants, but only one on the right with the products. To balance it, we must figure out how many of each reactant and product will make it balance.

2H2 + O2 2H2O 

x2 x2

four hydrogen + two oxygen four hydrogen and two oxygen

The coefficient multiplies each element that appears in the formula!

Example

2NaOH  Na2O + H2O

On the left side of the equation: On the right side of the equation:

2 sodium (Na) atoms2 sodium (Na) atoms

2 oxygen (O) atoms2 oxygen (O) atoms

2 hydrogen (H) atoms2 hydrogen (H) atoms

Balancing Equations by Inspection (by looking)

Do this in order:

  1. Write the “skeleton” equation – the correct formulae for the reactants and products.
  2. If any element or polyatomic ionsshows up only once on both sides of the reaction, balance them first.
  3. Balance anyother elements that occur only once on each side.
  4. Balance any remaining elements.
  5. Check your work by recounting all atoms.

If every coefficient can be divided by the same number, rewrite the equation in the simplest whole-number ratio.

Example: each of the coefficients below can be evenly divided by 2

4Fe2O3 4Fe + 6O2

2Fe2O3 2Fe + 3O2 Now, it is properly balanced!

An equation is not properly balanced if the coefficients are not written in their lowest whole-number ratio.

Note:diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2)

These are often a gas (except iodine). If these elements appear by themselves (not in a compound) they must be written with the subscript 2.

Examples:

calcium chloride + silver nitratecalcium nitrate + silver chloride

  1. skeleton

CaCl2 + AgNO3 Ca(NO3)2 + AgCl

  1. calciumappears only once on each side – it seems balanced
  2. nitrateappears once on each side – it is not balanced

try doubling the silver nitrate

CaCl2 + 2AgNO3 Ca(NO3)2 + AgCl

calcium is still okay, and now the nitrate ions balance

  1. silver is not balanced – try doubling the amount of silver chloride

CaCl2 + 2AgNO3 Ca(NO3)2 + 2 AgCl

  1. check your work

Left side:Right side:

Ca – 1atomCa – 1atom

Cl – 2 atoms Cl – 2 atoms

Ag – 1 atomAg – 1 atom

NO3 – 2 ionsNO3 – 2 ions

Practice:

zinc + tin(II)chloride zinc chloride + tin

copper + silver nitrate copper (II) nitrate + silver

potassium iodide + lead (II) nitrate  potassium nitrate + lead (II) iodide

barium chloride + potassium sulfate  barium sulfate + potassium chloride

Answer Questions 1- 6 on page 174