A balanced chemical equation has the same number of atoms of each element on both sides. Change coefficients in front of formulae, never the subscripts within a correct formula. For example 2H2+O2→2H2O balances four hydrogen atoms and two oxygen atoms. A coefficient multiplies every atom in its formula, including atoms inside brackets.
Coefficients give ratios of particles and of amounts in moles, not direct mass ratios. The equation N2+3H2→2NH3 means one mole of nitrogen reacts with three moles of hydrogen to form two moles of ammonia. Check net charge for ionic equations. In a closed system total mass is conserved, although an open vessel can lose mass when a gaseous product escapes.
Example 1
Balance Al+O2→Al2O3 with smallest whole-number coefficients.
4Al+3O2→2Al2O3; both sides contain four Al and six O atoms.
Example 2
Interpret 2Mg+O2→2MgO for 0.300 mol Mg with excess O2.
The Mg:MgO ratio is 2:2, so 0.300 mol Mg forms 0.300 mol MgO if the reaction is complete.
Watch out:Changing a subscript changes the substance; balance using coefficients only.
Balanced chemical equations practice | Year 11 Chemistry | Ascent Arcade