# Balancing Equations

This is part of Year 11 HSC Chemistry course under the topic of Chemical Reactions and Stoichiometry

### HSC Chemistry Syllabus

• Relate stoichiometry to the law of conservation of mass in chemical reactions by investigating:
• Balancing chemical equations (ACSCH039)
• Solving problems regarding mass changes in chemical reactions (ACSCH046)

## What are Chemical equations

A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae. They describe a chemical change which occurs as a result of chemical reaction. An example of this is the formation of water from hydrogen and oxygen gases.

$$H_2(g) + O_2(g) \rightarrow H_2O(l)$$

In an expression such as this, the formulas and symbols on the left-hand side are the reacting substances which are called the reactants. These chemical species are used up in the chemical reaction and become transformed into new species which are shown on the right-hand side. These chemical species on the right-hand side of the equation are called the products and this is because they are produced by the chemical reaction. The arrow is read as ‘moves towards’, ‘gives’ or ‘forms’, and the plus sign is read as ‘and’. When the plus sign (+) appears between the formulas for the two reactants, it can also be read as “reacts with”. Thus the equation above can be read as hydrogen gas and/reacts with oxygen gas gives water.

Sometimes energy may be required to start a reaction or become formed by the reaction and these would be included just as a word equation.

### State Symbols

Next to the chemical formula for any reactant or product, you will find the state of the substance in brackets.

• Solid (s)
• Liquid (l)
• Gas (g)
• Aqueous (aq) – This means that the substance was dissolved in water or is in solution form

### Balancing chemical equations

As it is written, the equation indicates in a qualitative way what substances are consumed in the reaction and what new substances are formed. To have quantitative information about the reaction, the equation must be balanced so that it conforms to the rules of the law of conservation of matter. That is, there must be and equal number of atoms of each element on the right-hand side of the equation as there are on the left-hand side.

If the number of atoms of each element in the equation above are counted, we observe that there are 2 hydrogen and 2 oxygen atoms on the left side, and 2 hydrogen and 1 oxygen atom on the left side

$$H_2(g) + O_2(g) \rightarrow H_2O(l)$$

 Left Side Right Side 2 atoms of H 2 atoms of H 2 atoms of O 1 atom of O

The balancing of the equation is accomplished by introducing the proper number or coefficient betfore each formula. To balance the number of O atoms, there must either be a half added to the number of O on the left, or a 2 must be added in front of the H2 and the H2O

$$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$$

or

$$H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(l)$$

 Left Side Right Side 2 or 4 atoms of H 2 or 4 atoms of H 1 or 2 atoms of O 1 or 2 atoms of O