0.7 Representing chemical change  (Page 3/6)

Remember that if you put a number in front of a molecule, that number applies to the whole molecule. For example, if you write $2\mathrm{H}{}_2\mathrm{O}$ , this means that there are 2 molecules of water. In other words, there are 4 hydrogen atoms and 2 oxygen atoms. If we write $3\mathrm{HCl}$ , this means that there are 3 molecules of $\mathrm{HCl}$ . In other words there are 3 hydrogen atoms and 3 chlorine atoms in total. In the first example, 2 is the coefficient and in the second example, 3 is the coefficient.

Activity: balancing chemical equations

You will need: coloured balls (or marbles), prestik, a sheet of paper and coloured pens.

We will try to balance the following equation:

Count the number of balls on the left and the number on the right. Do you have the same number of each colour on both sides? If not the equation is not balanced. How many balls will you have to add to each side to make the number of balls the same? How would you add these balls?

You should find that you need 4 balls of one colour for $\mathrm{Al}$ and 3 pairs of balls of another colour (i.e. 6 balls in total) for ${\mathrm{O}}_2$ on the left side. On the right side you should find that you need 2 clusters of balls for ${\mathrm{Al}}_2{\mathrm{O}}_3$ . We say that the balanced equation is:

Repeat this process for the following reactions:

• $${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$$
• $$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$$
• $$\mathrm{Zn}+2\mathrm{HCl}\to {\mathrm{ZnCl}}_2+{\mathrm{H}}_2$$

Steps to balance a chemical equation

When balancing a chemical equation, there are a number of steps that need to be followed.

• STEP 1: Identify the reactants and the products in the reaction and write their chemical formulae.
• STEP 2: Write the equation by putting the reactants on the left of the arrow and the products on the right.
• STEP 3: Count the number of atoms of each element in the reactants and the number of atoms of each element in the products.
• STEP 4: If the equation is not balanced, change the coefficients of the molecules until the number of atoms of each element on either side of the equation balance.
• STEP 5: Check that the atoms are in fact balanced.
• STEP 6 (we will look at this a little later): Add any extra details to the equation e.g. phase.