Question

Consider the reactions: (a) \(\mathrm{H}_{3} \mathrm{PO}_{2}(\mathrm{aq})+4 \mathrm{AgNO}_{3}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+4 \mathrm{Ag}(\mathrm{s})+4 \mathrm{HNO}_{3}(\mathrm{aq})\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}(\mathrm{aq})+2 \mathrm{CuSO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+2 \mathrm{Cu}(\mathrm{s})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}(\mathrm{l})+2\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}(\mathrm{aq})+3 \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{-}(\mathrm{aq})+2 \mathrm{Ag}(\mathrm{s})+\) \(4 \mathrm{NH}_{3}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}(\mathrm{l})+2 \mathrm{Cu}^{2+}(\mathrm{aq})+5 \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow\) No change observed. What inference do you draw about the behaviour of \(\mathrm{Ag}^{+}\) and \(\mathrm{Cu}^{2+}\) from these reactions?

Short answer

\( \mathrm{Ag}^{+} \) is more effective in reducing agents than \( \mathrm{Cu}^{2+} \) under these conditions.

Step-by-step solution

Analyze Reaction (a)

In reaction (a), \( \mathrm{H}_{3} \mathrm{PO}_{2} \) is reacting with \( \mathrm{AgNO}_{3} \) to produce \( \mathrm{H}_{3} \mathrm{PO}_{4} \), solid silver (Ag), and \( \mathrm{HNO}_{3} \). This indicates that \( \mathrm{Ag}^{+} \) ions are reduced to Ag metal, while \( \mathrm{H}_{3} \mathrm{PO}_{2} \) is oxidized to \( \mathrm{H}_{3} \mathrm{PO}_{4} \).

Analyze Reaction (b)

In reaction (b), \( \mathrm{H}_{3} \mathrm{PO}_{2} \) reacts with \( \mathrm{CuSO}_{4} \) producing \( \mathrm{H}_{3} \mathrm{PO}_{4} \), solid copper (Cu), and \( \mathrm{H}_{2} \mathrm{SO}_{4} \). Here, \( \mathrm{Cu}^{2+} \) ions are reduced to Cu metal, while \( \mathrm{H}_{3} \mathrm{PO}_{2} \) gets oxidized.

Analyze Reaction (c)

In reaction (c), benzaldehyde \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO} \) reacts with diamminesilver \( \mathrm{Ag} \), forming \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{-} \), solid silver (Ag), ammonia, and water. The \( \mathrm{Ag}^{+} \) is reduced to Ag metal, facilitating the oxidation of benzaldehyde to benzoate.

Analyze Reaction (d)

No reaction occurs with \( \mathrm{Cu}^{2+} \) and \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO} \) in the conditions provided. \( \mathrm{Cu}^{2+} \) is not able to oxidize benzaldehyde under these conditions, indicated by no change.

Draw Inference about Ag+ and Cu2+

From these reactions, \( \mathrm{Ag}^{+} \) can easily be reduced to metallic silver, reducing different species in reactions (as seen in reactions a and c). \( \mathrm{Cu}^{2+} \), meanwhile, is capable of reducing only some species, as shown in reaction (b), but not benzaldehyde as seen in reaction (d).

Key concepts

Oxidation

Oxidation is a fundamental concept in chemistry that involves the loss of electrons by a substance. Essentially, when oxidation occurs, the substance being oxidized increases its oxidation state, meaning it gives up electrons.
In the reactions provided, oxidation is seen in the transformation of \( \mathrm{H}_3 \mathrm{PO}_2 \) into \( \mathrm{H}_3 \mathrm{PO}_4 \). This involves the loss of electrons from phosphorus, which results in an increase in its oxidation state. The overall process can be remembered by the acronym "OIL" (Oxidation Is Loss of electrons).
Whenever a substance is oxidized, another substance is reduced to maintain the balance of electron transfer.

Reduction

Reduction is the opposite of oxidation and involves the gain of electrons by a substance, which leads to a decrease in its oxidation state. Think of reduction as gaining more negative charge due to additional electrons.
In reactions (a) and (c), \( \mathrm{Ag}^{+} \) ions are reduced to solid silver \( \mathrm{Ag} \). This shows the gain of electrons, reducing the ionic silver to its metallic form. The term "RIG" (Reduction Is Gain of electrons) helps recall that reduction involves gaining electrons.
Reduction and oxidation always occur together, known as redox reactions, where one species is oxidized while the other is reduced.

Ag+ Ions

\( \mathrm{Ag}^{+} \) ions play a crucial role in redox chemistry, often participating as strong oxidizing agents. They have a tendency to gain electrons and become metallic silver, especially when they react with substances that are easily oxidized. This characteristic is clearly observed in reactions (a) and (c), where \( \mathrm{Ag}^{+} \) ions are reduced and form solid silver.
This reduction showcases their potential as effective oxidizing agents, being able to accept electrons from different species. This property makes silver compounds valuable in various redox reactions, as they readily partake in electron transfer.

Cu2+ Ions

\( \mathrm{Cu}^{2+} \) ions, like \( \mathrm{Ag}^{+} \) ions, are also involved in redox reactions but exhibit different behavior. The key feature of \( \mathrm{Cu}^{2+} \) is its moderate tendency to gain electrons and become metallic copper. However, it cannot always oxidize other substances.
For instance, in reaction (b), \( \mathrm{Cu}^{2+} \) ions are reduced to copper, demonstrating their capability to act as an oxidizing agent in specific scenarios. However, they do not partake in reaction (d) with benzaldehyde. This inability shows their limitations, indicating that certain substances cannot provide \( \mathrm{Cu}^{2+} \) with the necessary electrons under the given conditions.
In essence, \( \mathrm{Cu}^{2+} \) ions are less versatile compared to \( \mathrm{Ag}^{+} \), especially in their ability to be reduced in varying environments and react with different reducing agents.