Reversible and irreversible reactions

Reversible reactions

Reversible reactions are those that can proceed in both directions. They can go forward and backward depending on the conditions applied.

Examples of reversible reactions

• Example 1: When you heat blue crystals of copper (II) sulphate, they break down into anhydrous copper (II) sulphate (a white powder): $\text{CuSO}_4\cdot5\text{H}_2\text{O(s)} \rightleftharpoons \text{CuSO}_4\text{(s)} + 5\text{H}_2\text{O(g)}$ The reaction can be reversed by adding water to the white powder, which quickly turns the substance back into blue crystals: $\text{CuSO}_4\text{(s)} + 5\text{H}_2\text{O(l)} \rightleftharpoons \text{CuSO}_4\cdot5\text{H}_2\text{O(s)}$ This is often used as a test for the presence of water.
• Example 2: Heating ammonium chloride (a solid) in the bottom of a test tube breaks it down into ammonia and hydrogen chloride gases. The gases will readily combine at the top of the tube where the temperature is cooler: $\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}$ This is another example of a reversible reaction.

Irreversible reactions

Irreversible reactions proceed in only one direction, and the products cannot be easily converted back into the original reactants under normal conditions.

Examples of irreversible reactions

• Example 1: The combustion of fuels, such as the burning of methane in oxygen: $\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(g)}$ This reaction produces carbon dioxide and water and cannot easily be reversed.
• Example 2: The reaction between sodium and water to produce sodium hydroxide and hydrogen gas: $2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)}$ Sodium reacts violently with water, and once the reaction occurs, the products cannot be easily converted back into sodium and water.
• Example 3: The rusting of iron: $3\text{Fe(s)} + 4\text{O}_2\text{(g)} + 6\text{H}_2\text{O(l)} \rightarrow \text{Fe}_2\text{O}_3\cdot3\text{H}_2\text{O(s)}$ Rusting is a slow, irreversible process where iron reacts with oxygen and water to form hydrated iron (III) oxide. Once rust forms, it is difficult to reverse the reaction.
• Example 4: The decomposition of calcium carbonate when heated: $\text{CaCO}_3\text{(s)} \rightarrow \text{CaO(s)} + \text{CO}_2\text{(g)}$ When calcium carbonate (lime) is heated, it decomposes to form calcium oxide and carbon dioxide, and this reaction is irreversible.