Problem 119 - A Complete Resource Book in Chemistry for JEE Main

Question

A solution containing one mole per litre of each \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2} ; \mathrm{AgNO}_{3} ; \mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2} ;\) is being electrolyzed by using inert electrodes. The values of standard electrode potentials in volts (reduction potentials) are \(\mathrm{Ag} / \mathrm{Ag}^{+}=+0.80,2 \mathrm{Hg} / \mathrm{Hg}_{2}^{+4}=+0.79\) \(\mathrm{Cu} / \mathrm{Cu}^{+4}=+0.34, \mathrm{Mg} / \mathrm{Mg}^{+}=-2.37\) With increasing voltage, the sequence of deposition of metals on the cathode will be (a) \(\mathrm{Cu}, \mathrm{Hg}, \mathrm{Ag}\) (b) \(\mathrm{Ag}, \mathrm{Hg}, \mathrm{Cu}, \mathrm{Mg}\) (c) \(\mathrm{Mg}, \mathrm{Cu}, \mathrm{Hg}, \mathrm{Ag}\) (d) \(\mathrm{Ag}, \mathrm{Hg}, \mathrm{Cu}\)

Step-by-Step Solution

Answer

The sequence of deposition is (d): Ag, Hg, Cu.

Step 1: List Given Reduction Potentials

The reduction potential values for the metal ions involved are given as: 1. \( \mathrm{Ag} / \mathrm{Ag}^{+} = +0.80 \text{ V} \)2. \( 2 \mathrm{Hg} / \mathrm{Hg}_{2}^{2+} = +0.79 \text{ V} \)3. \( \mathrm{Cu} / \mathrm{Cu}^{2+} = +0.34 \text{ V} \)4. \( \mathrm{Mg} / \mathrm{Mg}^{2+} = -2.37 \text{ V} \) (Note: Mg is not in the solution) The associated metals in the solution are Ag, Hg (as \( \mathrm{Hg}_{2}^{2+} \)), and Cu.

Step 2: Determine Deposition Order by Reduction Potential

The metals will deposit on the cathode starting with the metal having the highest reduction potential. This is because a higher reduction potential suggests that the metal ion is more readily reduced. Order by potential:1. \( \mathrm{Ag} \) with \( +0.80 \text{ V} \)2. \( 2 \mathrm{Hg} / \mathrm{Hg}_{2}^{2+} \) with \( +0.79 \text{ V} \)3. \( \mathrm{Cu} \) with \( +0.34 \text{ V} \)

Step 3: Ignore Mg From the List

Despite \( \mathrm{Mg} \) being included for reference, the solution does not contain any \( \mathrm{Mg}^{2+} \) ions as the problem lists it separately. Therefore, \( \mathrm{Mg} \) is ignored when sequencing deposition.

Step 4: Select the Correct Option

Match the order of the metal deposition based on their standard reduction potentials with the given choices. Option (d) \( \mathrm{Ag}, \mathrm{Hg}, \mathrm{Cu} \) matches the sequence derived in Step 2.

Key Concepts

Standard Electrode Potentials

Standard electrode potentials are key to understanding electrochemical reactions. Each involves a half-cell reaction, representing either reduction or oxidation. The standard electrode potential is the tendency of a chemical species to acquire electrons, and thus be reduced, when compared to the standard hydrogen electrode, which acts as a reference point with its potential defined as 0 volts.

Such potentials are measured under standard conditions: a temperature of 298K, a pressure of 1 atm, and solutions at a 1 M concentration. The greater the reduction potential of a species, the more likely it is to gain electrons—that is, be reduced—when compared to other species. In the given exercise, the reduction potentials of different metals, like Ag with +0.80 V and Cu with +0.34 V, play crucial roles in determining which metal will deposit first during electrolysis.

Electroplating

Electroplating is a practical application of electrochemical cells. It involves coating an object with a thin layer of metal using an electric current. This process utilizes the principles of standard electrode potentials, where ions in a solution are reduced and deposited onto an object's surface.

The object to be plated serves as the cathode in an electrochemical cell, while the anode is often made of the metal intended for plating. When the electric current passes through the solution, metal ions receive electrons and form a solid metal layer on the cathode, effectively coating it. The ability to coat an object in metal enhances its appearance, resistance to corrosion, and other desirable properties. The exercise focuses on determining the order of metal deposition based on their reduction potentials, which is a critical step in effective electroplating.

Inert Electrodes

Inert electrodes play a pivotal role in electrochemical processes, especially when the electrodes themselves do not participate in the reaction. Materials such as platinum or graphite are typical as they remain unchanged by the electrolytic reactions.

Their primary function is to provide a surface for the electrochemical reaction without influencing the reaction itself. Thereby, they serve as the site for oxidation and reduction reactions, facilitating electron flow between the external circuit and the solution.

In the given exercise, the use of inert electrodes allows the focus to be solely on the redox processes occurring with the solutions' ions. This ensures that the metal ions' deposition potential can be accurately measured and analyzed.

Electrolysis

Electrolysis is the process of causing a non-spontaneous chemical reaction through an external electric source. By applying a current, substances that wouldn't normally change their chemical composition start to do so at each electrode, with distinct oxidation and reduction events.

In the exercise provided, electrolysis is utilized to deposit metal ions such as silver, mercury, and copper onto the cathode.

The process is guided by the standard electrode potentials of the ions involved. Generally, metals with higher reduction potentials are deposited on the cathode first as they are reduced sooner than others with lower potentials. Understanding electrolysis helps us in various applications, including extracting metals, refining them, and producing a range of chemical substances.