Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Of the following metals, those that cannot be obtained by the electrolysis of the aqueous solution of their salts are (a) \(\mathrm{Ag}\) and \(\mathrm{Mg}\) (b) \(\mathrm{Ag}\) and \(\mathrm{Al}\) (c) \(\mathrm{Mg}\) and \(\mathrm{Al}\) (d) \(\mathrm{Cu}\) and \(\mathrm{Cr}\)

Step-by-Step Solution

Answer

(c) Mg and Al are the metals that cannot be obtained by electrolysis of their aqueous salts.

Step 1: Understand Electrolysis

In electrolysis, an electric current is passed through a solution to cause a chemical change, typically the decomposition of the liquid into elements or compounds. Metals that can be displaced by hydrogen in the reactivity series can be easily deposited by electrolysis. However, metals higher in the activity series than hydrogen cannot be obtained from an aqueous solution because water will be preferentially reduced.

Step 2: Identify Metal Reactivity

Consider the reactivity of metals: Metals like Ag (Silver) are lower in the reactivity series than hydrogen, while metals like Mg (Magnesium) and Al (Aluminium) are much higher. This means that Mg and Al cannot be deposited by electrolysis from their aqueous salts as water will be reduced instead.

Step 3: Evaluate Each Choice

(a) Ag and Mg: Ag can be deposited; Mg cannot. (b) Ag and Al: Ag can be deposited; Al cannot. (c) Mg and Al: neither can be deposited. (d) Cu and Cr: Cu can be deposited; Cr cannot.

Step 4: Choose Correct Metals

The metals that cannot be obtained by the electrolysis of the aqueous solution of their salts are those higher in the reactivity series than hydrogen. Thus, Mg and Al fit this description.

Key Concepts

Reactivity Series

The reactivity series is a list of metals ranked in order of their reactivity from highest to lowest. This concept is quite essential in predicting the likelihood of a metal to participate in chemical reactions, especially during processes like electrolysis. Metals that are high in the reactivity series, such as potassium, sodium, calcium, magnesium, and aluminum, tend to lose electrons easily and are more reactive than those at the bottom of the series, like silver, gold, and platinum.
Metals higher in the series can displace those lower from their compounds. During electrolysis, metals lower than hydrogen in the reactivity series can be deposited from their aqueous salt solutions because water will not preferentially be reduced to produce hydrogen. Conversely, metals that are higher than hydrogen are not easily deposited because water is reduced instead, releasing hydrogen gas.

Aqueous Solution

An aqueous solution is a solution where water is the solvent. Many chemical reactions, including those involving electrolysis, occur in aqueous solutions. When you dissolve a salt in water, you create an aqueous solution where ions freely move about. These ions play a crucial role in the conduction of electricity.
In the context of electrolysis, an aqueous solution allows ions present in the compound to be separated when electricity is applied. Positively charged ions (cations) are attracted to the cathode (negative electrode), and negatively charged ions (anions) are attracted to the anode (positive electrode). This movement of ions enables the process of electrolysis to result in chemical changes.

Metal Extraction

Metal extraction is the process of obtaining metals from their ores. This is often done to gain pure metal for industrial and commercial use and can be executed through various methods including mining, chemical methods, and electrolysis.
In electrolysis, metal ions in a solution or molten compound are reduced at the cathode, often resulting in the deposition of the pure metal. However, not all metals can be easily extracted via electrolysis from aqueous solutions. As previously mentioned, metals like magnesium and aluminum, due to their higher reactivity, cannot be readily extracted using this method as the water in the solution will preferentially be reduced, producing hydrogen gas instead of the metal.

Chemical Change

Chemical change involves the transformation of substances into different chemical entities. This process often involves the making or breaking of chemical bonds. In the context of electrolysis, a chemical change is driven by an electric current which prompts reactions like decomposition, reduction, or oxidation.
During electrolysis of an aqueous solution, the electric current causes ions to migrate towards the electrodes. This ion migration leads to chemical reactions where new substances are formed. For example, at the cathode, reduction might occur wherein metal ions gain electrons, leading to the deposition of the metal, while at the anode, oxidation might happen, such as the release of oxygen gas or other anionic elements.