How to write balanced chemical equations
core
• Construct word equations and simple balanced chemical equations
Simple Balanced Chemical equations
A balanced equation shows the number of atoms for each substance in the reaction and the total number of atoms for the elements in both the reactants and the products is the same. A balanced chemical equation can include state symbols in brackets after each formula as requested by the question. You may want to revise other topics such as chemical bonding or refer to solubility table for the correct state symbols of the substances.
| State symbols |
Meaning |
| (g) |
gas |
| (I) |
Liquid |
| (s) |
Solid |
| (aq) |
Aqueous (dissolved in water) |
How to write balanced chemical equations.
You can only change the coefficients in front of the formula of the substance to make sure the number of atoms of each element at the reactant side is same as product side.
2H2 + O2 → 2H2O
↑ ↑ ↑
Coefficients
2 is the coefficient in front of H2 and H2O. If there is no number in front of the formula, coefficient is 1. Number 1 is not written for balancing equations. The coefficient for oxygen is 1.
Suggested steps to follow.
Step 1.Always write the correct formula for the reactants and products first.
Step 2. Count the number of each element in the reactant and product side and change the coefficient to make sure the atoms of the elements are the same on the products as reactants
E.g. 1: Methane reacts with oxygen to form carbon dioxide and water.
Step 1 : CH4 + O2 → CO2 + H2O
Step 2: atoms on reactant side atoms on product side
1C 4H 2O 1C 2H 3O
Since oxygen gas is alone, oxygen will be balanced the last. Balance H first and put 2 in front of H2O to become 2H2O, the number of atoms of each element will be multiplied by the coefficient now. There are 4H and 2O in 2H2O.
CH4 + O2 → CO2 + 2H2O
atoms on reactant side atoms on product side
1C 4H 2O 1C 4H 4O
Next, balance oxygen by placing 2 in front of O2 to become 2O2. So, the equation will look like this now: CH4 + 2O2 → CO2 + 2H2O
Now 2O2 will results in 4O.
atoms on reactant side atoms on product side
1C 4H 4O 1C 4H 4O
Equation is now balanced. You could put in the state symbols if the question requested for them.
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
E.g. 2: dilute hydrochloric acid reacts with magnesium carbonate to form magnesium chloride, carbon dioxide and water.
Step 1: HCl + MgCO3 →MgCl2 + H2O + CO2
Step 2: atoms on reactant side atoms on product side
1H 1Cl 1Mg 1C 3O 2H 2Cl 1Mg 1C 3O
Try to balance hydrogen or chlorine first. Place a 2 in front of HCl to become 2HCl, so now there is 2H and 2Cl
2HCl + MgCO3 →MgCl2 + H2O + CO2
atoms on reactant side atoms on product side
2H 2Cl 1Mg 1C 3O 2H 2Cl 1Mg 1C 3O
Equation is now balanced. You could put in the state symbols if the question requested for them.
2HCl (aq) + MgCO3 (s) →MgCl2 (aq) + H2O (l) + CO2 (g)
E.g. 3: Propene burns in oxygen to form carbon dioxide and water.
step 1: C3H6 + O2 → CO2 + H2O
Balance oxygen the last as it is alone.
Balance the carbon by placing 3 in front of CO2 to become 3CO2 and balance the hydrogen atoms by placing 3 in front of H2O to become 3H2O
The equation looks like this: C3H6 + O2 → 3CO2 + 3H2O
Hydrogen and carbon atoms are balanced but not oxygen.
atoms on reactant side atoms on product side
3C 6H 2O 3C 6H 9O
Place a 9/2 in front of O2 to get 9O.
C3H6 + 
9/2O2 → 3CO2 + 3H2O
For O level and IGCSE level, usually we present whole numbers for balanced equation. To get rid of the denominator of 2, multiply 2 throughout the equation.
2C3H6 + 9O2 → 6CO2 + 6H2O
Equation is now balanced. You could put in the state symbols if the question requested for them.
2C3H6 (g) + 9O2 (g) → 6CO2 (g) + 6H2O (l)
How to balance simple equation with polyatomic ion.
See polyatomic ion as a unit.
E.g. 4: dilute nitric acid reacts with calcium hydroxide to form calcium nitrate and water.
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Ca(NO3)2 shows multiplication of 2 X NO3, I will balance by placing 2 in front of HNO3 so that there is 2 X NO3 on the reactant side. you can also try placing 2 in front of H2O as OH looks similar to H2O.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
atoms on reactant side atoms on product side
2H 2N 8O 1Ca 2H 2N 8O 1Ca
Equation is now balanced. You could put in the state symbols if the question requested for them.
2HNO3 (aq) + Ca(OH)2 (aq) → Ca(NO3)2 (aq) + 2H2O (l)
E.g. 5: Iron (III) sulfate with sodium hydroxide gives iron (III) hydroxide and sodium sulfate
FeSO4 + NaOH → Na2SO4 + Fe(OH)2
Fe(OH)2 shows multiplication of 2 X OH, balance oxygen and hydrogen by placing 2 in front of NaOH so that there is 2 X NaOH on the reactant side.
FeSO4 + 2NaOH → Na2SO4 + Fe(OH)2
atoms on reactant side atoms on product side
1Fe 1S 6O 2Na 1Fe 1S 6O 2Na
The equation is now balanced. You could put in the state symbols if the question requested for them.
FeSO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + Fe(OH)2 (s)