Problem 78 - Chemistry The Molecular Science

Question

Write chemical equations to illustrate the amphoteric behavior of (a) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (b) \(\mathrm{Sn}(\mathrm{OH})_{2}\).

Step-by-Step Solution

Answer

Both Cr(OH)_3 and Sn(OH)_2 can act as either acids or bases depending on the reacting agent.

Step 1: Understanding Amphoteric Behavior

An amphoteric substance can act both as an acid and a base. For each compound, write two reactions: one showing the substance acting as a base, reacting with an acid, and the other showing it acting as an acid, reacting with a base.

Step 2: Writing the Reaction for Cr(OH)_3 with Acid

To illustrate \( ext{Cr(OH)}_3\) acting as a base, we react it with a strong acid like \( ext{HCl}\). The equation is: \[\text{Cr(OH)}_3 + 3 \text{HCl} \rightarrow \text{CrCl}_3 + 3 \text{H}_2\text{O}\] This reaction shows \( ext{Cr(OH)}_3\) accepting protons from \( ext{HCl}\), which is typical behavior for a base.

Step 3: Writing the Reaction for Cr(OH)_3 with Base

Now, show \( ext{Cr(OH)}_3\) acting as an acid by reacting it with a strong base like \( ext{NaOH}\). The equation is: \[\text{Cr(OH)}_3 + \text{OH}^- \rightarrow [\text{Cr(OH)}_4]^−\] This equation shows \( ext{Cr(OH)}_3\) donating a proton to \( ext{OH}^-\), demonstrating its acidic nature.

Step 4: Writing the Reaction for Sn(OH)_2 with Acid

Illustrate the amphoteric behavior of \( ext{Sn(OH)}_2\) by reacting it with an acid like \( ext{HCl}\). The balanced equation is: \[\text{Sn(OH)}_2 + 2 \text{HCl} \rightarrow \text{SnCl}_2 + 2 \text{H}_2\text{O}\] Here, \( ext{Sn(OH)}_2\) acts as a base, accepting protons from \( ext{HCl}\).

Step 5: Writing the Reaction for Sn(OH)_2 with Base

Finally, demonstrate \( ext{Sn(OH)}_2\) acting as an acid by reacting it with a strong base like \( ext{NaOH}\). The balanced equation is: \[\text{Sn(OH)}_2 + 2 \text{OH}^- \rightarrow [\text{Sn(OH)}_4]^{2-}\] This reaction shows \( ext{Sn(OH)}_2\) donating protons to \( ext{OH}^-\), acting as an acid.

Key Concepts

Cr(OH)3 reactions

Chromium(III) hydroxide, \(\text{Cr(OH)}_3\), beautifully illustrates the amphoteric behavior of certain compounds. This compound can both donate and accept protons, enabling it to act either as an acid or a base depending on the surrounding conditions. When \(\text{Cr(OH)}_3\) comes into contact with a strong acid like hydrochloric acid (HCl), it behaves as a base.

• The chemical equation for this reaction is: \[\text{Cr(OH)}_3 + 3 \text{HCl} \rightarrow \text{CrCl}_3 + 3 \text{H}_2\text{O}\].
• In this scenario, \(\text{Cr(OH)}_3\) accepts protons (H⁺) from \(\text{HCl}\).

In contrast, when \(\text{Cr(OH)}_3\) interacts with a strong base like sodium hydroxide (NaOH), it acts as an acid.

• The reaction is represented by: \[\text{Cr(OH)}_3 + \text{OH}^- \rightarrow [\text{Cr(OH)}_4]^−\].
• Here, \(\text{Cr(OH)}_3\) donates a hydroxide ion to the \(\text{OH}^-\).

This dual behavior showcases the versatility of \(\text{Cr(OH)}_3\).

Sn(OH)2 reactions

Tin(II) hydroxide, \(\text{Sn(OH)}_2\), is another remarkable example of an amphoteric compound. It demonstrates both acidic and basic properties, similar to \(\text{Cr(OH)}_3\). When \(\text{Sn(OH)}_2\) meets up with an acid like \(\text{HCl}\), it functions as a base.

• The balanced equation for this reaction is: \[\text{Sn(OH)}_2 + 2 \text{HCl} \rightarrow \text{SnCl}_2 + 2 \text{H}_2\text{O}\].
• In this reaction, \(\text{Sn(OH)}_2\) accepts two protons from \(\text{HCl}\).

Meanwhile, when \(\text{Sn(OH)}_2\) is mixed with a base like \(\text{NaOH}\), it transforms into an acid role.

• The reaction is given by: \[\text{Sn(OH)}_2 + 2 \text{OH}^- \rightarrow [\text{Sn(OH)}_4]^{2-}\].
• Here, it donates protons to form a complex ion, interacting with hydroxide ions.

This dual functionality plays an essential role in chemical processes.

Acid-base reactions

Acid-base reactions involve the transfer of protons between reactants, which can drastically change their chemical nature. An acid is a substance that can donate protons, while a base can accept them.

• In the case of \(\text{Cr(OH)}_3\) and \(\text{Sn(OH)}_2\), these substances can alternatively act as either acids or bases depending on the reacting species.
• Acid-base reactions can lead to neutralization, where the acid and base react to form water and a salt.

Since amphoteric substances contain characteristics of both acids and bases, they allow for unique reaction pathways that can be harnessed in various chemical applications. This behavior is central to understanding the versatility of these complex reactions.

Chemical equations

Chemical equations are symbolic representations of chemical reactions, showing the reactants transforming into products. They follow the law of conservation of mass, ensuring that the number of atoms on both sides of the equation is balanced.

• In reactions showing amphoteric behavior, such as those involving \(\text{Cr(OH)}_3\) and \(\text{Sn(OH)}_2\), chemical equations are a crucial way to represent these transformations.
• The balanced equations illustrate not only the stoichiometry but also the nature of participant reactants—highlighting which compounds are behaving as acids or bases.

By understanding and writing these equations, chemists can predict the outcomes of reactions and explore the reactivity of materials in various environments. These equations are vital for anyone studying chemistry as they form the language in which reactions are described.