Strong & Weak Acids (Edexcel GCSE Chemistry Combined Science): Revision Notes
4.2.7 Strong & Weak Acids
Strong & Weak Acids
Acids are substances that dissociate in water to release hydrogen ions (H⁺) and anions. The extent of this dissociation determines whether an acid is classified as strong or weak.
Dissociation of Acids in Aqueous Solutions
When acids dissolve in water, they undergo a process called dissociation, where the acid molecules split into hydrogen ions (H⁺) and negative ions (anions). The general dissociation of an acid can be represented as:
Here, HX represents the acid, H⁺ is the hydrogen ion, and X⁻ is the anion. The degree to which an acid dissociates into ions determines its strength.
Strong Acids
Strong acids are those that completely dissociate in water. This means that every molecule of the acid separates fully into its ions. As a result, the concentration of H⁺ ions in the solution is high, leading to a low pH.
Examples of strong acids include:
- Hydrochloric acid (HCl)
- Nitric acid (HNO₃)
- Sulphuric acid (H₂SO₄)
In strong acids, the dissociation is complete, which is why the concentration of hydrogen ions is directly proportional to the concentration of the acid itself.
Weak Acids
Weak acids only partially dissociate in water, meaning that only a small fraction of the acid molecules release hydrogen ions. The dissociation of weak acids is reversible, and an equilibrium is established between the undissociated acid and the ions:
In this equilibrium, HA represents the undissociated acid, H⁺ is the hydrogen ion, and A⁻ is the anion. For weak acids, the equilibrium lies far to the left, indicating that most of the acid remains undissociated in solution. This results in a much lower concentration of H⁺ ions compared to strong acids, and thus a higher pH.
Examples of weak acids include:
- Ethanoic acid (CH₃COOH)
pH and Concentration
The pH of an acid solution is determined by the concentration of H⁺ ions. As the concentration of H⁺ ions increases, the pH decreases. For every tenfold increase in the concentration of H⁺ ions, the pH decreases by 1 unit.
This relationship can be summarised by the formula: Factor change in \ce{[H⁺]=10^−^X}
Where X is the change in pH. For example, if the pH of an acid increases from 1 to 3, the concentration of H⁺ ions decreases by a factor of 100.
Distinguishing Between Strong and Concentrated Acids
It's important to distinguish between an acid's strength and its concentration:
- A strong acid is one that fully dissociates in solution, leading to a high concentration of H⁺ ions and a low pH.
- A weak acid only partially dissociates, so even if it is highly concentrated, it may still have a relatively higher pH compared to a dilute solution of a strong acid. In summary, the strength of an acid is determined by its ability to dissociate in water, while the concentration refers to the amount of acid dissolved in the solution.