b) Limitations of Acid-Base Theories:

Here are the answers to the questions:

b) Limitations of Acid-Base Theories:

• i) Arrhenius theory: This theory is limited to reactions occurring in aqueous solutions and cannot explain acid-base behavior in non-aqueous solvents or reactions that do not involve the formation of $\text{H}^+$ or $\text{OH}^-$ ions. For example, it cannot explain why ammonia ($\text{NH}_3$) is a base, as it does not contain $\text{OH}^-$.

• ii) Brønsted-Lowry theory: This theory requires the transfer of a proton ($\text{H}^+$) for a reaction to be classified as acid-base. It cannot explain reactions where no proton transfer occurs, such as the reaction between boron trifluoride ($\text{BF}_3$) and ammonia ($\text{NH}_3$), which is an acid-base reaction according to the Lewis theory.

• iii) Lewis theory: While the broadest theory, it can be more complex to apply and does not easily predict the relative strengths of acids and bases quantitatively. The concept of electron pair donation/acceptance can also be less intuitive for some reactions compared to proton transfer.

2. Amphiprotic Species:
An amphiprotic species is a molecule or ion that can act as both a Brønsted-Lowry acid (by donating a proton) and a Brønsted-Lowry base (by accepting a proton).

• Illustration with equations: Water ($\text{H}_2\text{O}$) is a common example of an amphiprotic species.

  • When acting as an acid (donating a proton to a base like ammonia):

    $$\text{H}_2\text{O}(l) + \text{NH}_3(aq) \rightleftharpoons \text{OH}^-(aq) + \text{NH}_4^+(aq)$$

  • When acting as a base (accepting a proton from an acid like hydrochloric acid):

    $$\text{H}_2\text{O}(l) + \text{HCl}(aq) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{Cl}^-(aq)$$

3. Neutralization Reaction and Conjugate Acid-Base Pairs:

The neutralization reaction between hydrochloric acid ($\text{HCl}$) and magnesium hydroxide ($\text{Mg(OH)}_2$) is:

$$2\text{HCl}(aq) + \text{Mg(OH)}_2(s) \rightarrow \text{MgCl}_2(aq) + 2\text{H}_2\text{O}(l)$$

The conjugate acid-base pairs are:

• $\text{HCl}$ (acid) and $\text{Cl}^-$ (conjugate base)
• $\text{H}_2\text{O}$ (acid, formed from $\text{Mg(OH)}_2$) and $\text{OH}^-$ (base, from $\text{Mg(OH)}_2$)