How to Balance Equations for Acid Reactions

This article covers $\mathrm{4.1.1.1}$, $\mathrm{4.3.1.1}$ and $4.4.2.1-4.4.2.2$ of the AQA 8462 specification.

To write a balanced equation for an acid reaction, predict the products, write their correct formulae, then add coefficients so each element has the same number of atoms on both sides.

This assumes AQA GCSE Chemistry 8462, the separate-science qualification. The content applies to both Foundation and Higher Tier.

Step 1: Identify the reaction type

Learn these general equations:

$$\mathrm{acid}+\mathrm{metal}\to \mathrm{salt}+\mathrm{hydrogen}$$ $$\mathrm{acid}+\mathrm{base}\to \mathrm{salt}+\mathrm{water}$$ $$\mathrm{acid}+\mathrm{carbonate}\to \mathrm{salt}+\mathrm{water}+\mathrm{carbon}\mathrm{dioxide}$$

An alkali is a soluble base, so acid and alkali reactions also produce salt and water.

Step 2: Name and write the salt

The acid determines the ending of the salt:

• hydrochloric acid produces chlorides
• nitric acid produces nitrates
• sulfuric acid produces sulfates

For example, magnesium reacting with hydrochloric acid produces magnesium chloride and hydrogen. The correct formula for magnesium chloride is $\mathrm{MgC}{\mathrm{l}}_2$.

Step 3: Balance the equation

Begin with the unbalanced equation:

$$\mathrm{Mg}+\mathrm{HCl}\to \mathrm{MgC}{\mathrm{l}}_2+{\mathrm{H}}_2$$

There are two chlorine atoms and two hydrogen atoms on the right, so place a coefficient of $2$ before $\mathrm{HCl}$:

$$\mathrm{Mg}+2HCl\to \mathrm{MgC}{\mathrm{l}}_2+{\mathrm{H}}_2$$

The equation is now balanced because both sides contain the same number of atoms of each element.

A second worked example

Sulfuric acid reacting with sodium carbonate follows the acid and carbonate pattern. Sulfuric acid produces a sulfate, and sodium carbonate supplies sodium, so the salt is sodium sulfate.

$${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4+\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3\to \mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$$

Count the atoms: each side has two sodium atoms, one sulfur atom, seven oxygen atoms, two hydrogen atoms and one carbon atom. No extra coefficients are needed, so the equation is already balanced.

This example shows why product formulae must be correct before balancing. Coefficients can change how many particles react, but they cannot repair an incorrect formula.

Common exam mistake

Never change the small subscript numbers inside a chemical formula. Changing $\mathrm{HCl}$ to $\mathrm{HC}{\mathrm{l}}_2$, for example, changes the substance. Balance equations only by placing coefficients in front of complete formulae.

Finally, check every element separately and use the smallest possible whole-number coefficients.