35. Strong Acids vs. Weak Acids

The strength of an acid is determined by how much it dissociates (splits) in an aqueous solution to release hydrogen ions (H⁺).

Strong Acids: Ionize completely in water. Every acid molecule dissociates to release a hydrogen ion.
- Examples: Hydrochloric acid (HCl), Nitric acid (HNO₃), Sulfuric acid (H₂SO₄).
Weak Acids: Do not fully ionize. Only a small proportion of the acid particles dissociate. This is because the reaction is reversible, creating an equilibrium that usually lies to the left (more undissociated molecules).
- Examples: Ethanoic acid, Citric acid, Carbonic acid.

It is vital to distinguish between these two terms:

Acid Strength: Refers to the proportion of acid molecules that ionize in water (Strong vs. Weak).
Concentration: Refers to the total amount of acid dissolved in a certain volume of water (Concentrated vs. Dilute).
Note: You can have a dilute solution of a strong acid or a concentrated solution of a weak acid.

The pH scale is a measure of the concentration of H⁺ ions in a solution.

The Inverse Rule: As the concentration of H⁺ ions increases, the pH decreases.
The Factor of 10 Rule: Each change of 1 on the pH scale represents a 10-fold change in the concentration of hydrogen ions.
- Example: Moving from pH 5 to pH 3 means the concentration of H⁺ ions is 100 times higher ($10 \times 10 = 100$).
- Example: Moving from pH 8 to pH 11 means the concentration of H⁺ ions is 1,000 times lower ($10 \times 10 \times 10 = 1,000$).

At the same concentration: A strong acid will always have a lower pH than a weak acid because it releases more H⁺ ions.
To lower the pH of a weak acid: You must increase its concentration significantly to compensate for the fact that so few particles ionize.