Question

Indicate which of the following species could, in theory, undergo autoionization: (a) \(\mathrm{NH}_{3}\); (b) \(\mathrm{NH}_{4}^{+} ;\) (c) \(\mathrm{OH}^{-}\); (d) \(\mathrm{O}^{2-}\); (e) \(\mathrm{HF} ;\) (f) \(\mathrm{F}^{-}\)

Short answer

Only HF (e) could undergo autoionization due to its ability to both donate and accept protons.

Step-by-step solution

Understanding Autoionization

Autoionization is a process where a molecule in its neutral form can simultaneously donate and accept a proton, effectively acting as both an acid and a base at the same time. Since it involves proton transfer, we're looking primarily for substances that can both donate and accept a proton (hydrogen ion, H⁺). This often means the substance should have both acidic and basic qualities.

Analyzing Ammonia, \\( \mathrm{NH}_{3} \\\)

Ammonia (\( \mathrm{NH}_{3} \)) is a weak base. It can accept a proton to form \( \mathrm{NH}_{4}^{+} \), but it does not typically donate a proton in its molecular form to another \( \mathrm{NH}_{3} \) molecule, so autoionization is unlikely.

Exploring Ammonium Ion, \\( \mathrm{NH}_{4}^{+} \\\)

Ammonium ion (\( \mathrm{NH}_{4}^{+} \)) is the conjugate acid of \( \mathrm{NH}_{3} \). It can donate a proton to revert to ammonia, but lacks the ability to accept a proton in its fully protonated form, making autoionization impossible.

Considering Hydroxide Ion, \\( \mathrm{OH}^{-} \\\)

Hydroxide ion (\( \mathrm{OH}^{-} \)) is already a strong base and can accept a proton to form water, \( \mathrm{H_{2}O} \). Additionally, water (\( \mathrm{H_{2}O} \)) can further react to form \( \mathrm{H_{3}O}^{+} \) and \( \mathrm{OH}^{-} \), so autoionization is impossible due to lack of proper proton transfer leading to ion generation between only \( \mathrm{OH}^{-} \) ions.

Evaluating Oxide Ion, \\( \mathrm{O}^{2-} \\\)

The oxide ion (\( \mathrm{O}^{2-} \)) is a very strong base and can readily accept a proton to form hydroxide ion, \( \mathrm{OH}^{-} \). However, in its current form, it cannot donate a proton, ruling out autoionization.

Reviewing Hydrofluoric Acid, \\( \mathrm{HF} \\\)

Hydrofluoric acid (\( \mathrm{HF} \)) is a weak acid that can donate a proton to become fluoride ion, \( \mathrm{F}^{-} \). It can also accept a proton, forming \( \mathrm{H_{2}F}^{+} \), suggesting potential for autoionization if both reactions occur with neighboring molecules.

Investigating Fluoride Ion, \\( \mathrm{F}^{-} \\\)

Fluoride ion (\( \mathrm{F}^{-} \)) is the conjugate base of \( \mathrm{HF} \). It can accept a proton to revert to \( \mathrm{HF} \), but cannot donate a proton, which negates the possibility of autoionization.

Key concepts

Proton Transfer

Proton transfer is a fundamental concept in chemistry, particularly in understanding acid-base reactions. In simple terms, a proton transfer occurs when a hydrogen ion (\( \mathrm{H}^+ \)) moves from one molecule to another. This process is essential for many chemical reactions across both biological systems and industrial processes.
In acid-base reactions, proton transfer is crucial:

• An acid is a substance that can donate a proton to another substance.
• A base is a substance that can accept a proton.
• When an acid donates a proton, it becomes its conjugate base.
• Conversely, when a base accepts a proton, it forms its conjugate acid.

The ability of molecules to undergo proton transfer defines much of their chemical reactivity and properties. It's important to note that for autoionization, a molecule must be capable of both donating and accepting protons, acting as either an acid or a base depending on the situation.

Acid-Base Reactions

Acid-base reactions are a type of chemical process that involve the transfer of protons between reactants. These reactions are integral to countless chemical processes, including digestion in living organisms and industrial manufacturing. There are several key components to understanding these reactions:

• **Definition of Acids and Bases:** Acids are substances that increase the concentration of hydronium ions (\( \mathrm{H_3O}^+ \)) in a solution, whereas bases increase hydroxide ions (\( \mathrm{OH}^- \)).
• **Conjugate Pairs:** Each acid has a corresponding base, known as its conjugate base, which forms when the acid donates a proton. Similarly, each base has a conjugate acid, which forms when it gains a proton.
• **Reversibility:** Most acid-base reactions are reversible, meaning they can go both ways depending on conditions.

In theory, for a substance to undergo autoionization (where it acts as both an acid and a base), it must have the capability to donate and accept a proton. A good example of an autoionizable substance is water, which can act as both an acid and base.

Ammonia Chemistry

Ammonia (\( \mathrm{NH_3}\)) is a simple molecule with significant importance in chemistry, primarily due to its basic properties. It plays a crucial role in various chemical industries, including the production of fertilizers and cleaning products.

• **Basic Nature:** Ammonia is a weak base. It can accept a proton to form the ammonium ion (\( \mathrm{NH_4^+}\)), which is its conjugate acid.
• **Lack of Proton Donating Ability:** Despite its ability to accept protons, ammonia does not typically donate protons. This characteristic is why it is unlikely to undergo autoionization, as it lacks the dual ability to function as both an acid and a base in a self-contained manner.
• **Role in Acid-Base Chemistry:** Ammonia is often used in acid-base chemistry to demonstrate the formation of conjugate acid-base pairs: \( \mathrm{NH_3}\) gains a proton to become \( \mathrm{NH_4^+}\).

Understanding these properties helps to explain why ammonia, although a participant in proton transfer, cannot autoionize like some other molecules such as water.