To know if an acid is strong or weak, the key factor is whether the acid fully dissociates (ionizes) in water:
- A strong acid completely dissociates in water, meaning it breaks up into hydrogen ions (H⁺) and its conjugate base ions 100%. Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and a few others known as the "strong acids." There are about seven common strong acids in general chemistry.
- A weak acid partially dissociates in water, meaning only a fraction of its molecules ionize and the rest remain intact as molecules. Weak acids do not fully ionize and stay mostly bonded together.
Other ways to identify acid strength include:
- Checking if the acid is on the list of known strong acids; if not, it's usually weak.
- Looking at the acid dissociation constant (pKa): a low (negative or very small) pKa means the acid is strong, while a higher pKa indicates a weak acid.
- Bond strength also plays a role: strong acids have weak bonds that break easily to release H⁺, whereas weak acids have stronger bonds holding them together.
Summary table:
Property| Strong Acid| Weak Acid
---|---|---
Degree of Dissociation| Complete (100%)| Partial (less than 100%)
Presence in Solution| Mostly ions| Mostly molecules
Bond Strength (H-A bond)| Weak| Strong
pKa Value| Low (often < 0)| Higher
Examples| HCl, H₂SO₄, HNO₃, HBr| HF, acetic acid (CH₃COOH)
Thus, to know if an acid is strong or weak:
- Check if it is among the well-known strong acids.
- Compare its pKa value if available.
- Understand that strong acids ionize fully in water, whereas weak acids do not.
