Writing Balanced Chemical Equations
Writing balanced chemical equations is one of the most challenging aspects of O level chemistry. O level Chemistry fundamentally revolves around calculations, symbols and formula of chemical substances. This tutorial is about balancing chemical equations but before we start balancing chemical equations we are going to start by learning how to write chemical formulae.
A chemical equation is a summary of what happens during a chemical equation. A balanced chemical equation provides the following information:
- nature of reactants (reacting chemicals) and the products.
- ratio of moles of reactants and of the products.
- physical states of the reactants and of the products.
Writing a chemical equation
To easily write a chemical equation:
- write down a word equation using correct names of the reactants and the products.
- change the names of the reactants and products into their corresponding chemical formulae. You will produce an unbalanced chemical equation but with correct formulae for all the reactants and products.
- balance the chemical equation by inspection, starting with the most complicated molecule. You balance a chemical equation by writing the appropriate numbers in front of each chemical formula until the there are the same number of each type of atoms on both sides of the equation. Never adjust the formula of a substance, i.e. by changing the subscript values in the formula to suit your balanced equation.
- finally add state symbols after each chemical formula to show the physical state of the substance.
Example of balancing chemical equations
You are given that when a mixture of hydrogen and oxygen it explode to produce steam. Write down a balanced chemical equation to show the chemical equation that happens.
First you write down the word equation, taking it into consideration that steam is water in its gaseous form. So we write water in place of steam:
Hydrogen + Oxygen → Water
Convert the chemical names into chemical formulae. Hydrogen gas and oxygen gas are diatomic gases so in their natural state, their atoms exist in pairs:
H2 + O2 → H2O
Now we have an unbalanced chemical equation. Its unbalanced because the number of oxygen on the left hand side is more than the number of oxygen atoms on the right hand side of the equation. Its tempting but wrong to balance the equation by changing the subscripts on the formulae:
H2 + O → H2O (looks balanced but its wrong because oxygen gas is diatomic)
H2 + O2 → H2O2 (looks balanced but its wrong because H2O2 is the formula of hydrogen peroxide not water. The equation should continue to produce water, whether balanced or not)
The correct way to balance the chemical equation is adjust the coefficients of the formulae. Lets start by adding 2 to the formulae of water.
H2 + O2 → 2H2O
Now the oxygen atoms are balanced but the hydrogen is still not balanced. We can then balance the hydrogen atoms by adding 2 to formulae of hydrogen gas.
2H2 + O2 → 2H2O
Now our equation is balanced because as you can see we now have 4 atoms of hydrogen and 2 atoms of oxygen on both sides. Now lets add the state symbols:
2H2(g) + O2(g) → 2H2O(g)
Hydrogen and oxygen are in their natural gaseous form and water is produced as a gas (steam), hence the (g) state symbols.
Balance the following chemical equations and add to state symbols:
Na + Cl2 → NaCl
Na + Cl2 → NaCl
We have 2 chlorine atoms on the LHS and 1 chlorine atom on the RHS which means the equation is not balanced. So lets put 2 on the RHS to make it 2 sodium atoms and 2 chlorine atoms
Na + Cl2 → 2NaCl
Now that the chlorine is balanced, let us balance the sodium. We have 1 sodium atom on the LHS and 2 sodium atoms on the RHS, so let us put on the sodium.
2Na + Cl2 → 2NaCl
Finally the state symbols. Sodium is a metal which is a solid at room temperature, chlorine is a gas at room temperature and sodium chloride is a solid at room temperature.
2Na(s) + Cl2(g) → 2NaCl(s)
Fe + Br2 → FeBr3
Fe + Br2 → FeBr3
The Br2 on the LHS is diatomic which restricts the number of bromine atoms to being always an even number. So lets start by making the Br atoms on the RHS an even number. Put 2 on the RHS formulae.
Fe + Br2 → 2FeBr3
This makes it 6 Br atoms on the RHS. Now multiply the Br2 on the LHS by 3 to make it 6 Br atoms there.
Fe + 3Br2 → 2FeBr3
Then balance the Fe.
2Fe + 3Br2 → 2FeBr3
Finally add state symbols:
2Fe(s) + 3Br2(g) → 2FeBr3(s)