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chemistry igcse acapprinted 93 · PDF 93
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printed 93 · PDF 93
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HCl (aq) → H⁺ (aq) + Cl⁻ (aq)
This reaction shows hydrochloric acid breaking down entirely into its ions, leading to a very acidic
solution.
Definition of a Weak Acid:
Weak acids only partially dissociate in water, meaning that only some of the acid molecules release
H⁺ ions, while others remain intact. This partial dissociation leads to an equilibrium between
dissociated ions and undissociated acid molecules. The pH of weak acids is still below 7 but closer
to neutral.
For example, with propanoic acid (CH₃CH₂COOH), equilibrium is established as follows:
CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO⁻
The equilibrium favors the intact acid molecules, resulting in a lower concentration of H⁺ ions in
the solution.
Another example of a weak acid is ethanoic acid, which can react with an alkali like sodium
hydroxide to form ethanoate salts:
CH₃COOH + NaOH → CH₃COONa + H₂O
Concentration vs. Strength of Acids
Understanding Acid Concentration:
The concentration of an acid refers to how many acid molecules are present per cubic decimeter
(dm³) of solution. A concentrated acid solution contains more acid molecules, while a dilute
solution has fewer.
Strength vs. Concentration:
It's important to note that concentration and strength are not the same. Strength refers to how
completely an acid dissociates in water, while concentration refers to the number of acid molecules