Question

Out of the following metals, those that cannot be obtained by electrolysis of the aqueous solution of their salts are 1\. \(\mathrm{Ag}\) 2\. \(\mathrm{Mg}\) 3\. Cu 4\. Al (a) 2,4 (b) 2,3 (c) 1,4 (d) all

Short answer

Option (a) 2, 4 is correct, as Mg and Al cannot be obtained by electrolysis of their aqueous salt solutions.

Step-by-step solution

Understanding Electrolysis

To determine which metals cannot be obtained by electrolysis of an aqueous solution of their salts, we need to understand that electrolysis involves using electricity to decompose a compound into its elements. During electrolysis in an aqueous solution, the more reactive metals cannot be deposited at the cathode because water is reduced to form hydrogen gas instead. Therefore, only less reactive metals than hydrogen can be deposited.

Identify Reactivity

Recall the reactivity series: Metals like magnesium (Mg) and aluminum (Al) are highly reactive. Silver (Ag) and copper (Cu) are less reactive than hydrogen.

Apply to the Options

In an aqueous solution, since Mg and Al are higher in the reactivity series than hydrogen, they cannot be deposited by electrolysis because water will preferentially be reduced at the cathode. In contrast, Ag and Cu are able to be deposited as they are less reactive than hydrogen.

Select the Correct Answer

From the given options: - (a) 2, 4: Mg and Al cannot be obtained by electrolysis. - (b) 2, 3: Mg and Cu can be obtained by electrolysis. - (c) 1, 4: Ag and Al can be obtained by electrolysis. - (d) all: This option suggests all metals cannot be obtained by electrolysis which is incorrect. Therefore, the correct answer is (a) 2, 4.

Key concepts

Reactivity Series

The reactivity series is a crucial concept in chemistry, especially when understanding electrolysis. It is a list of metals arranged in order of decreasing reactivity.
The series helps predict how metals will react during electrochemical processes. In the reactivity series, highly reactive metals like potassium and sodium are at the top, and less reactive metals like gold and silver are at the bottom. Metals higher in the series, like magnesium (Mg) and aluminum (Al), react more vigorously with substances such as water and acids.

• Metals at the top (e.g., Mg, Al): Highly reactive.
• Metals at the bottom (e.g., Ag, Cu): Less reactive.

During electrolysis of aqueous solutions, we compare the metal's reactivity to hydrogen. If the metal is more reactive than hydrogen, it won't be deposited at the cathode because water will be reduced to form hydrogen gas instead. This is why knowing the reactivity series is important in predicting whether a metal can be recovered from its salt in solution.

Aqueous Solutions

An aqueous solution is a solution in which water is the solvent. In the context of electrolysis, the behavior of metals in aqueous solutions is key.
The solution contains water molecules, ions from the dissolved metal salts, and possibly other ions from dissolved electrodes. In electrolysis, the presence of water influences what happens at the electrodes. Water can undergo reduction at the cathode, where it splits to form hydrogen gas and hydroxide ions. This reaction competes with the discharge of metal ions, especially if the metal is higher in the reactivity series.

• Water provides hydrogen ions (H⁺) that compete with metal ions at the cathode.
• Only metals less reactive than H⁺ ions will typically deposit from the aqueous solution.

Understanding these dynamics is essential to predicting outcomes of electrolysis for various metals.

Metal Deposition

Metal deposition during electrolysis is an exciting electrochemical process. It’s an essential technique for obtaining pure metals from their compounds. During electrolysis, metal ions in solution move towards the cathode, where they gain electrons and deposit as pure metal.
However, not all metals in solution will deposit. Metals can only deposit if they are less reactive than hydrogen from the reactivity series. As ions at the cathode accept electrons, they convert into solid metal form and build up on the electrode surface. This process depends on the solution's composition and the metal's reactivity.

• Low-reactivity metals like Ag and Cu can deposit.
• High-reactivity metals like Mg and Al do not deposit in aqueous solution electrolysis.

Metal deposition is important in various industrial applications, such as electroplating and metal extraction.

Electrochemical Processes

Electrochemical processes involve the interaction between electricity and chemical change. Electrolysis is a primary electrochemical process. It relies on an electric current to drive a non-spontaneous chemical reaction.
In this process, electrical energy is converted into chemical energy. In electrolysis of aqueous solutions, ions move through the liquid to electrodes: positive ions (cations) move towards the cathode while negative ions (anions) move towards the anode. After reaching the electrode, they participate in redox reactions. At the cathode, reduction occurs, while oxidation happens at the anode.

• Cathode: Reduction occurs, metal ions gain electrons (e.g., Cu²⁺ + 2e⁻ → Cu).
• Anode: Oxidation occurs, non-metal ions lose electrons (e.g., 4OH⁻ → 2H₂O + O₂ + 4e⁻).

Understanding these processes provides insight into how chemicals and ions behave under the influence of electric current, which is fundamental to skilled work in chemistry involving electrolysis.