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# How to Write a Chemical Equation?

Updated: May 4, 2020 A chemical equation gives you the essential information about a chemical reaction, specifically:

• The nature of the reactants (chemicals reacting together) and products (chemicals produced)

• The quantities of reactants and products

• The physical state of the reactants and products (E.g. solid, liquid, gas, aqueous)

### Step by step guide to writing a chemical equation for a chemical reaction

• Write down a word equation for the chemical reaction

• Write down the unbalanced equation using correct chemical symbols for all the reactants and products

• Balance the equation by inspection. (Ensure that the same number of each type of atom appears on both sides of the equation)

• Write the state symbol after each chemical symbol

### Tips for balancing the equation

• Typically, you balance H (hydrogen) atoms first, followed by O (oxygen) atoms, than any other atoms. OR you can start by balancing the most complicated molecule.

• Never change the chemical symbol of a substance. (Do not change the subscripts in the chemical symbols of reactants or products)

• A number in front of a formula multiplies every symbol that follows it

• Remember that atoms cannot be created or destroyed in a chemical reaction (i.e. do not ‘eat’ any chemical symbols)

### Examples:

Formation of iron(III) chloride from the reacting iron with chlorine gas

• Iron + chlorine → iron(III) chloride

• Before balancing: Fe + Cl2 → FeCl3

• Balance number of Cl atoms: There are 2 Cl atoms on the left and 3 Cl atoms on the right. The simplest way to balance Cl atoms is to multiply 3 on the left and 2 on the right to make a total of 6 Cl atoms.

• We obtain: Fe + 3 Cl2 → 2 FeCl3

• Notice that the number of Fe atoms is not the same for both sides: 1 on the left and 2 on the right. Hence, we will just have to multiply 2 on the left.

• We have: 2 Fe + 3 Cl2 → 2 FeCl3

• Not forgetting to include the state symbols: 2 Fe(s) + 3 Cl2(g) → 2 FeCl3(s) Formation of carbon dioxide and water from methane and oxygen. (or known as Combustion of methane in oxygen)

• methane + oxygen → carbon dioxide + water

• Convert the above to chemical symbols: CH4 + O2 → CO2 + H2O

• Balance H first: There are 4 H atoms on the left and 2 on the right. Hence, we will have to multiply the chemical symbol containing H on the left by 2. (i.e. multiply H2O by 2)

• We have: CH4 + O2 → CO2 + 2 H2O

• Balance O: There are 2 O atoms on the left and 4 O atoms on the right. We will have to multiply the O2 on the left by 2.

• We have: CH4 + 2 O2 → CO2 + 2 H2O

• The carbon atoms are balanced so we do not have to multiply them.

• Including the state symbol: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

Balance: Pb3O4 + HNO3 → PbO2 + Pb(NO3)2 + H2O

• Notice that there are 3 Pb atoms on the left and 2 Pb atoms on the right. There are several ways to go about balancing the Pb atoms – we can multiply PbO2 by 2 or Pb(NO3)2 by 2 or multiply both sides of the equation. Any of the ways is correct. But some ‘ideal’ way will be shorter than the others.

• We shall look at multiplying Pb(NO3)2 by 2. (shortest method in this case)

• We have Pb3O4 + HNO3 → PbO2 + 2Pb(NO3)2 + H2O

• We have 1 N atom on the left and 4 N atoms on the right. We will have to multiply the HNO3 by 4

• We have: Pb3O4 + 4HNO3 → PbO2 + 2Pb(NO3)2 + H2O

• Balance H atom: Pb3O4 + 4HNO3 → PbO2 + 2Pb(NO3)2 + 2H2O

• Notice that the number of O atoms is balanced. We’re done!

• Including state symbol: Pb3O4(s) + 4 HNO3(aq) → PbO2(s) + 2 Pb(NO3)2(aq) + 2 H2O(l)

Important: Always express your equation as the simplest form.

• For example:  2 Pb3O4(s) + 8 HNO3(aq) → 2 PbO2(s) + 4 Pb(NO3)2(aq) + 4 H2O(l)

• The above equation is not correct as it is not in its simplest form. (You can divide throughout by 2) 