Name: ___________________________ ( ) Class:_______Date: ________

Chemical equationsA chemical equation is an equation that involves the writing of chemical names or chemical formulae.

Eg. Word Equation: Carbon + Oxygen Carbon dioxide

Chemical Equation: ______ + ________ ___________________________

• Carbon and oxygen (at the left hand side) are the reactants while carbon dioxide (at the right hand side is a product.

• The arrow shows that the reaction proceeds from left to right.

Writing Balanced equations

An equation is balanced when there is an equal number of atoms of each element on both sides of the equation.

Steps in writing balanced equations:1. Write down the equation in words.

2. Write down the formulae of the reactants and products.

3. Check the number of atoms of each element on both sides of the equation. If the equation is not balanced, proceed to step 4.

4. Balance the equation by placing numbers in front of the formulae of the substances in the equation. The number ‘1’ is not written.

5. Write down the state symbols in the equation.

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State symbols Representss solidl liquidg gasaq aqueous

(solutions)

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Secondary 3 NA Science (Chemistry)Chapter 8: Writing EquationsNotes 1

Example: (Refer to Textbook Pg. 108, Example 1)Sodium reacts with chlorine gas to form sodium chloride.

Step 1: Write the equation in words.

Sodium + Chlorine Sodium chloride

Step 2: Write down the formulae of the reactants and products.

________ + ________ ____________

Step 3: Check the number of atoms of each element on both sides of the equation.

Left hand side (LHS) , i.e. before the arrow sign: Right hand side (RHS):

_________ sodium atom _________ sodium atoms

__________ chlorine atoms _________ chlorine atoms

Therefore, the equation is not balanced. If the equation is not balanced, proceed to step 4.

Step 4: Balance the equation by placing numbers in front of the formulae of the substances in the equation. The number ‘1’ is not written.

(a) To balance the number of chlorine atoms, we need to put a ‘2’ in front of ‘NaCl’

Na + Cl2 2 NaCl

Left hand side (LHS) : Right hand side (RHS):

_________ sodium atom _________ sodium atoms

__________ chlorine atoms _________ chlorine atoms

The equation is still not balanced because there are 2 sodium atoms on right-hand side and one sodium atom on left hand side.

(b) To balance the equation, we need to put a ‘2’ in front of ‘Na’.

________ + ________ ____________

Step 5: Write down the state symbols in the equation.

____________ ( ) + _______________ ( ) ______________ ( )

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Example: (Refer to Textbook Pg. 109, Example 2)Methane reacts with oxygen to form carbon dioxide and water vapour.

Step 1: Write the equation in words.Methane + Oxygen Carbon dioxide + Water Vapour

Step 2: Write down the formulae of the reactants and products.

____________ + ____________ ____________ + _____________

Step 3: Check the number of atoms of each element on both sides of the equation.

Left hand side (LHS) , i.e. before the arrow sign: Right hand side (RHS):

_________ carbon atom _________ carbon atom

_________ hydrogen atoms _________ hydrogen atoms

_________ oxygen atoms _________ oxygen atoms

Therefore, the equation is not balanced. If the equation is not balanced, proceed to step 4.

Step 4: Balance the equation by placing numbers in front of the formulae of the substances in the equation. The number ‘1’ is not written.

(a) To balance the number of hydrogen atoms, we need to put a ‘2’ in front of ‘H2O’

CH4 + O2 CO2 + 2 H2O

Left hand side (LHS) : Right hand side (RHS):

Left hand side (LHS) , i.e. before the arrow sign: Right hand side (RHS):

_________ carbon atom _________ carbon atom

_________ hydrogen atoms _________ hydrogen atoms

_________ oxygen atoms _________ oxygen atoms

The equation is still not balanced because there are 2 oxygen atoms on left-hand side and 4 oxygen atoms on right hand side.

(b) To balance the equation, we need to put a ‘2’ in front of ‘O2’.

____________ + ____________ ____________ + _____________

Step 5: Write down the state symbols in the equation.

____________ ( ) + _______________ ( ) __________ ( ) + ___________ ( )

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