Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Consider the following reduction processes: \(\mathrm{Zn}^{2+}+2 \mathrm{e}^{-} \square \mathrm{Zn}(\mathrm{s}) ; \mathrm{E}^{\circ}=-0.76 \mathrm{~V}\) \(\mathrm{Ca}^{2+}+2 \mathrm{e}^{-} \square \mathrm{Ca}(\mathrm{s}) ; \mathrm{E}^{\circ}=-2.87 \mathrm{~V}\) \(\mathrm{Mg}^{2+}+2 \mathrm{e}^{-} \square \mathrm{Mg}(\mathrm{s}) ; \mathrm{E}^{\circ}=-2.36 \mathrm{~V}\) \(\mathrm{Ni}^{2+}+2 \mathrm{e}^{-} \square \mathrm{Ni}(\mathrm{s}) ; \mathrm{E}^{\circ}=-0.25 \mathrm{~V}\) The reducing power of the metals increases in the order: (a) \(\mathrm{Ca}<\mathrm{Zn}<\mathrm{Mg}<\mathrm{Ni}\) (b) \(\mathrm{Ni}<\mathrm{Zn}<\mathrm{Mg}<\mathrm{Ca}\) (c) \(\mathrm{Zn}<\mathrm{Mg}<\mathrm{Ni}<\mathrm{Ca}\) (d) \(\mathrm{Ca}<\mathrm{Mg}<\mathrm{Zn}<\mathrm{Ni}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{Ca} < \mathrm{Mg} < \mathrm{Zn} < \mathrm{Ni}\)

Step 1: Understand the Problem

The problem provides standard reduction potentials (\(E^{\circ}\)) for four different half-reactions. We need to determine the order in which the metals exhibit reducing power.

Step 2: Recall Reducing Power Concept

Reducing power refers to the ability of a species to donate electrons. The reducing agent itself is oxidized. A more negative standard reduction potential indicates a stronger reducing agent.

Step 3: Analyze the Given Potentials

Let's write down the standard reduction potentials for the metals: \(\mathrm{Zn}\): \(-0.76\,\mathrm{V}\); \(\mathrm{Ca}\): \(-2.87\,\mathrm{V}\); \(\mathrm{Mg}\): \(-2.36\,\mathrm{V}\); \(\mathrm{Ni}\): \(-0.25\,\mathrm{V}\).

Step 4: Determine the Reducing Order

To determine reducing power order, arrange metals from the most negative to the least negative potential: \(\mathrm{Ca} < \mathrm{Mg} < \mathrm{Zn} < \mathrm{Ni}\). This means \(\mathrm{Ca}\) has the highest reducing power while \(\mathrm{Ni}\) has the lowest.

Step 5: Match with Given Choices

Compare your arrangement with the options provided: the correct order \(\mathrm{Ca} < \mathrm{Mg} < \mathrm{Zn} < \mathrm{Ni}\) corresponds to option (d).

Key Concepts

Standard reduction potential

Standard reduction potential is a key concept in electrochemistry that represents the tendency of a chemical species to be reduced, or gain electrons. It is important to remember that these potentials are measured under standard conditions, which usually means at 25°C, 1 atmosphere of pressure, and 1 M concentration for all solutions involved.

Each species involved in an electrochemical reaction has its distinct standard reduction potential, denoted as \(E^{\circ}\). To find this value, the reduction reaction is compared to a reference electrode, commonly the Standard Hydrogen Electrode (SHE), which has a potential of 0.00 V.

A more negative \(E^{\circ}\) value suggests a greater likelihood of the species losing electrons, acting as a reducing agent. Conversely, a more positive \(E^{\circ}\) value implies a species that does not easily give up electrons and hence is a weaker reducing agent. Understanding this concept helps in predicting the outcome of a redox reaction.

Reducing agents

Reducing agents are substances that donate electrons to another species in a chemical reaction, which means they themselves are oxidized. In simpler terms, they 'reduce' the amount of positive charge on another substance by giving it electrons.

In an electrochemical context, the effectiveness of a reducing agent is indicated by the standard reduction potential. The more negative the \(E^{\circ}\) value, the stronger the reducing agent. This is because a negative potential implies a greater tendency to lose electrons.

For instance, in the provided reactions, calcium (Ca) has a very negative reduction potential of -2.87 V, making it a particularly strong reducing agent. This means it is very willing to donate electrons compared to other metals like nickel (Ni), which has a potential of -0.25 V. Being an effective reducing agent is crucial in processes like metal extraction and electroplating.

Electrochemical series

The electrochemical series, sometimes referred to as the activity series, is a list of elements ordered by their standard reduction potentials. This series is extremely helpful in predicting the direction of redox reactions and is essential for understanding which metals can displace others in a reaction.

The series is arranged from the most positive reduction potentials at the top to the most negative at the bottom. Metals at the bottom have a higher tendency to act as reducing agents, while those at the top tend to act as oxidizing agents.

In practical terms, this means a metal higher up in the series can displace any metal below it from its compounds. For example, according to the electrochemical series from the exercise data, calcium (Ca) can displace magnesium (Mg), zinc (Zn), and nickel (Ni) ions from their solutions. Understanding this ranking allows chemists to predict and manipulate many chemical processes effectively.