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)



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