# 9.3 Strength of Acids and Bases This section explores the concept of acid and base strength according to the Brønsted-Lowry theory, focusing on the relative ability of substances to donate or accept protons. ## Acid Strength The **strength of an acid** is a measure of its ability to donate a proton ($H^+$). - A **strong acid** is one that donates protons to a *high degree*. - A **weak acid** is one that donates protons to a *lesser degree*. Different Brønsted acids donate protons to different extents. The strength is relative; an acid is considered "strong" or "weak" in comparison to another. **Example:** Hydrochloric acid ($\mathrm{HCl}$) is a stronger acid than acetic acid ($\mathrm{CH_3COOH}$). Acetic acid, in turn, is a stronger acid than water ($\mathrm{H_2O}$). This can be represented as: $$ \mathrm{HCl} > \mathrm{CH_3COOH} > \mathrm{H_2O} $$ ## Base Strength The **strength of a base** is a measure of its ability to accept a proton ($H^+$). - A **strong base** is one that accepts protons to a *high degree*. - A **weak base** is one that accepts protons to a *lesser degree*. Similar to acids, the strength of bases is relative. **Example:** Ammonia ($\mathrm{NH_3}$) is a relatively stronger base than water ($\mathrm{H_2O}$) because it has a greater ability to accept a proton. This can be represented as: $$ \mathrm{NH_3} > \mathrm{H_2O} $$ ## The Levelling Effect In aqueous solutions, the strength of strong acids is limited by the basicity of water. This is known as the **Levelling Effect**. Water acts as a base and reacts with any acid stronger than the hydronium ion ($H_3O^+$) to form $H_3O^+$. Consequently, all strong acids like $HCl$, $HNO_3$, and $HClO_4$ appear to have the same strength in water. ## Possible Questions and Answers **Q:** What defines the strength of a Brønsted-Lowry acid? **A:** Its ability or tendency to donate a proton ($H^+$). A stronger acid donates a proton more readily than a weaker acid. **Q:** Why is hydrochloric acid considered a stronger acid than acetic acid? **A:** Hydrochloric acid donates its proton to a much higher degree (it fully ionizes in water), whereas acetic acid only donates its proton to a small extent (it partially ionizes). **Q:** Is water an acid or a base in the examples provided? **A:** Water is *amphoteric*, meaning it can act as both an acid and a base. It is a weaker acid than acetic acid and a weaker base than ammonia. See also for more on conjugate pairs. - **Acid Strength**: Refers to the relative ability of a substance to **donate a proton**. Strong acids are excellent proton donors. - **Base Strength**: Refers to the relative ability of a substance to **accept a proton**. Strong bases are excellent proton acceptors. - Strength is a *comparative* measure. For example: - **Acids**: $\mathrm{HCl} > \mathrm{CH_3COOH} > \mathrm{H_2O}$ - **Bases**: $\mathrm{NH_3} > \mathrm{H_2O}$ See and for detailed discussion of ionization differences.