Problem 1 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Astudent made the followingobservations in the laboratory: (I) Clean copper metal did not react with \(1 \mathrm{M}-\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) solution. (II) Clean lead metal dissolves in \(1 \mathrm{M}\) \(-\mathrm{AgNO}_{3}\) solution and crystals of Ag metal appeared. (III) Clean silver metal did not react with \(1 \mathrm{M}-\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) solution. The order of decreasing reducing character of the three metals is (a) \(\mathrm{Cu}>\mathrm{Pb}>\mathrm{Ag}\) (b) \(\mathrm{Cu}>\mathrm{Ag}>\mathrm{Pb}\) (c) \(\mathrm{Pb}>\mathrm{Cu}>\mathrm{Ag}\) (d) \(\mathrm{Pb}>\mathrm{Ag}>\mathrm{Cu}\)

Step-by-Step Solution

Answer

The correct order of decreasing reducing character is (Pb>Cu>Ag).

Step 1: Analyze the reactivity of copper

Observe that clean copper did not react with solutions of lead and silver nitrates. Copper is less reactive than lead and silver in this context, meaning it has a lower tendency to lose electrons (lower reducing character).

Step 2: Analyze the reactivity of lead

Note that lead reacts with silver nitrate solution, forming silver metal. This indicates that lead has a higher reducing character than silver, as it loses electrons more readily to silver ions, reducing them to silver metal.

Step 3: Analyze the reactivity of silver

Recognize that silver did not react with copper nitrate solution, which suggests that silver has a lower reducing character than copper.

Step 4: Determine the order

Putting the observations together, the order of decreasing reducing character is: Pb (reacts with Ag+ and forms Ag), Cu (does not react with Pb2+ or Ag+), and Ag (does not react with Cu2+). Therefore, the correct order is Pb > Cu > Ag.

Key Concepts

Reactivity Series

To understand the order of reducing character in metals, it's essential to begin with the concept of the reactivity series. The reactivity series is a list of metals arranged according to their ability to displace other metals from their compounds, with the most reactive metals placed at the top and the least reactive at the bottom. This order reflects the relative ease with which a metal can lose electrons to form positive ions, a characteristic known as its reducing power. Metals higher in the series, therefore, act as better reducing agents as they more readily lose electrons compared to metals lower in the series.

When analyzing the exercise where copper doesn't react with lead nitrate or silver nitrate, whereas lead reacts with silver nitrate, we can infer that lead is higher on the reactivity series than copper and silver, with silver being the least reactive. This understanding of relative reactivity gives us insights into predicting possible reactions and product formations in displacement reactions among metals.

Chemical Reactivity

Chemical reactivity refers to the tendency of a substance to engage in chemical reactions and transform into new substances. For metals, this usually involves the loss of electrons to form positive ions, a process known as oxidation. Metals have differing reactivities based on their electron configurations, which influence their ability to undergo oxidation.

For example, in the exercise provided, lead's ability to dissolve in silver nitrate solution and form silver indicates lead's greater chemical reactivity. It is more inclined to lose electrons and form lead ions, while simultaneously causing the reduction of silver ions to silver metal. Acknowledging the concept of chemical reactivity enables students to understand why certain metals will react with some substances and not others, making it a crucial concept for accurately predicting the outcomes of chemical reactions involving metals.

Metal Displacement Reactions

Metal displacement reactions are a type of redox reaction where a metal in a compound is replaced by another metal with a greater reducing character. These reactions are directly tied to the relative reactivity of the metals involved. A fundamental rule of thumb is that a more reactive metal can displace a less reactive metal from its compounds.

In the context of the given exercise, lead displacing silver from silver nitrate is a classic example of a metal displacement reaction. Lead, being more reactive, donates electrons to the silver ions, reducing them to silver metal, and in turn, lead is oxidized to lead ions. Such reactions are not only central to understanding chemical reactivity among metals but are also harnessed in industrial processes like metal purification and electroplating.