Strong & Weak Acids

• Acid Dissociation: When an acid dissolves in water, it undergoes a process called ionisation or dissociation, where the molecule splits to release hydrogen ions ($H^+$).

• Strong Acids: These substances undergo complete dissociation in water, meaning every molecule of the acid breaks apart into ions. Common examples include hydrochloric acid ($HCl$) and sulfuric acid ($H_2SO_4$).

• Weak Acids: These substances undergo partial dissociation, where only a small fraction of the molecules release $H^+$ ions. Common examples include organic acids like ethanoic acid ($CH_3COOH$) and hydrofluoric acid ($HF$).

• Equilibrium State: In weak acids, the dissociation is a reversible process, establishing a dynamic equilibrium between the intact molecules and the released ions.

• Reversibility: The dissociation of a weak acid is represented by a reversible arrow ($\rightleftharpoons$), indicating that the ions can recombine to form the original acid molecule.

• Position of Equilibrium: For weak acids, the equilibrium lies significantly to the left, meaning the concentration of intact molecules is much higher than the concentration of $H^+$ ions.

• Acid Dissociation Constant ($K_a$): The strength of a weak acid is quantified by $K_a$. A higher $K_a$ value indicates a greater degree of dissociation and thus a stronger 'weak' acid.

• Logarithmic pH Scale: The pH scale measures $H^+$ concentration. Because it is logarithmic, a change of 1 pH unit corresponds to a 10-fold change in $[H^+]$ concentration.

Calculating Hydrogen Ion Concentration

• For Strong Monoprotic Acids: Since dissociation is 100%, the concentration of $H^+$ ions is equal to the initial concentration of the acid. If $[HCl] = 0.1 \text{ mol/dm}^3$, then $[H^+] = 0.1 \text{ mol/dm}^3$.

• For Weak Acids: Calculating $[H^+]$ requires the use of the equilibrium constant expression: $K_a = \frac{[H^+][A^-]}{[HA]}$

Determining pH Changes

• To find the factor of change in $H^+$ concentration between two pH values, use the formula: $10^{\Delta pH}$. For example, moving from pH 3 to pH 5 is a change of 2 units, meaning the concentration decreases by a factor of $10^2 = 100$.

• It is critical to distinguish between Strength (how well it dissociates) and Concentration (how much is dissolved per unit volume).

• A dilute strong acid (e.g., $0.0001 \text{ M } HCl$) can have a higher pH than a concentrated weak acid (e.g., $10 \text{ M } CH_3COOH$) because the total number of ions depends on both factors.