Summary
Highlights
Acid strength increases with higher Ka values. For example, HF (Ka = 7.2 x 10^-4) is a stronger acid than HCN (Ka = 6.2 x 10^-10) because its Ka value is significantly larger.
For oxyacids, acids with a greater number of oxygen atoms are generally stronger. For instance, HNO3 is stronger than HNO2, and HClO4 is stronger than HClO3, and so on.
Base strength increases with higher Kb values. For example, methylamine (Kb = 4.4 x 10^-4) is a stronger base than ammonia (Kb = 1.8 x 10^-5) due to its larger Kb value.
For binary acids (hydrogen directly attached to an element), acid strength increases as you go down and to the right in the periodic table. Thus, HI is stronger than HBr, which is stronger than HCl, and so on. This is due to atomic size, where larger ions (like iodide) can better stabilize the negative charge.
For oxyacids, acid strength increases with higher electronegativity of the central atom. For example, HClO is stronger than HIO because chlorine is more electronegative than iodine.
It's important to memorize the six common strong acids: HI, HBr, HCl, H2SO4, HNO3, and HClO4. If an acid is not on this list, it is generally considered a weak acid. HF, despite being highly corrosive, is considered a weak acid.
The stronger an acid, the weaker its conjugate base, and vice versa. By comparing the Ka values of conjugate acids like H2S (Ka = 1 x 10^-7) and HNO2 (Ka = 4 x 10^-4), we can determine that H2S is the weaker acid, making its conjugate base, HS-, the stronger base compared to NO2-.
Acid strength decreases as the Pka value increases. Therefore, a lower Pka value indicates a stronger acid. Acetic acid (Pka = 4.74) is a stronger acid than hypochlorous acid (Pka = 7.46) because it has a lower Pka value.