Problem 29 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The standard reduction potentials at \(298 \mathrm{~K}\) for the following half reactions are given against each [1981-1 Mark] \(\mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Zn}(\mathrm{s}) \quad-0.762\) \(\mathrm{Cr}^{3+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Cr}(\mathrm{s}) \quad-0.740\) \(2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g}) \quad 0.000\) \(\mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Fe}^{2+}(\) aq \() \quad 0.770\) which is the strongest reducing agent? (a) \(\mathrm{Zn}(\mathrm{s})\) (b) \(\mathrm{Cr}(\mathrm{s})\) (c) \(\mathrm{H}_{2}(\mathrm{~g})\) (d) \(\mathrm{Fe}^{2+}(\mathrm{aq})\)

Step-by-Step Solution

Answer

The strongest reducing agent is \(\mathrm{Zn}(\mathrm{s})\) (option a).

Step 1: Understanding Reduction Potential

The standard reduction potential indicates a substance's tendency to gain electrons and be reduced. Higher reduction potentials mean a stronger oxidizing ability, whereas lower potentials (more negative) indicate a stronger reducing ability.

Step 2: Analyze the Given Data

The given reduction potentials are:- \(\mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e}^- \rightarrow \mathrm{Zn}(\mathrm{s})\), \(E^\circ = -0.762\)- \(\mathrm{Cr}^{3+}(\mathrm{aq})+2 \mathrm{e}^- \rightarrow \mathrm{Cr}(\mathrm{s})\), \(E^\circ = -0.740\)- \(2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^- \rightarrow \mathrm{H}_{2}(\mathrm{~g})\), \(E^\circ = 0.000\)- \(\mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{e}^- \rightarrow \mathrm{Fe}^{2+}(\mathrm{aq})\), \(E^\circ = 0.770\)

Step 3: Determine the Strongest Reducing Agent

A stronger reducing agent will be oxidized more easily, hence it will have a more negative reduction potential. Compare the reduction potentials: - \(\mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e}^-\) with \(E^\circ = -0.762\) is the most negative.- \(\mathrm{Cr}^{3+}(\mathrm{aq})+2 \mathrm{e}^-\) has \(E^\circ = -0.740\) which is less negative.- \(2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^-\) has \(E^\circ = 0.000\).- \(\mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{e}^-\) with \(E^\circ = 0.770\) is the least negative.Thus, \(\mathrm{Zn}(\mathrm{s})\) is the strongest reducing agent.

Key Concepts

Electrochemistry

Electrochemistry is the branch of chemistry that explores the relationship between chemical reactions and electrical energy. One of its primary concerns is understanding how chemical energy can be converted into electrical energy and vice versa. This is particularly important in devices like batteries and fuel cells, which rely on these principles to function effectively.

Electrochemistry involves various processes, but a key concept is the transfer of electrons between atoms and molecules. This transfer occurs during oxidation-reduction (redox) reactions, where one substance gains electrons (reduction), and another loses electrons (oxidation). Redox reactions are the foundation upon which electrochemical cells, like galvanic cells, are built.

Some crucial components in electrochemical systems include:

• Electrodes: Conductive materials that allow the transfer of electrons to and from a solution.
• Electrolyte: A medium providing ions for the transfer of electrons between electrodes.
• Voltage: The difference in potential energy between two points, driving the flow of electrons.

Understanding these processes helps us predict how reactions proceed and how we can harness them for practical applications.

Half-Reactions

In electrochemistry, reactions can be broken into two half-reactions: oxidation and reduction. Each half-reaction shows either the loss or gain of electrons. This division helps in understanding how electrons flow in an electrochemical cell.

For example, consider a typical battery. Inside, chemical energy is converted into electrical energy through redox reactions. By examining the half-reactions separately, we can better grasp the overall process:

• Reduction half-reaction: This involves the gaining of electrons. For instance, in the supplied exercise, the reaction \( \mathrm{Fe}^{3+} + 2e^- \rightarrow \mathrm{Fe}^{2+}\) shows how iron gains electrons.
• Oxidation half-reaction: This involves the losing of electrons, such as with zinc's reaction \(\mathrm{Zn} \rightarrow \mathrm{Zn}^{2+} + 2e^-\) when reversed.

Comparing reduction potentials of these half-reactions helps determine which is more likely to occur in a given condition and is crucial for predicting the behavior of electrochemical cells.

Reducing Agent

A reducing agent, or reductant, is a substance that induces the reduction of another substance by giving away electrons and, in the process, becomes oxidized itself. The strength of a reducing agent is often indicated by its standard reduction potential.

Standard reduction potentials (\(E^\circ\) values) provide a way to rank substances based on their tendencies to lose electrons. A more negative \(E^\circ\) signifies a stronger reducing agent, meaning it can donate electrons more readily.

In our example, zinc metal \(\mathrm{Zn(s)}\) has the most negative \(E^\circ = -0.762\), making it the strongest reducing agent. It can easily give away electrons compared to others with less negative potentials.

• With the most negative potential, \(\mathrm{Zn(s)}\) undergoes oxidation more willingly.
• When identifying reducing agents, it's helpful to remember that reducing agents themselves get oxidized in the process.

Understanding the concept of reducing agents is key in predicting which substance will act as an electron donor in chemical reactions.