Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Zn(s) is the strongest reducing agent; option (c).

Step 1: Understand Reduction Potentials

Reduction potential indicates a substance's ability to gain electrons (be reduced). A more negative reduction potential suggests that the substance is a stronger reducing agent.

Step 2: Analyze Given Half-Reactions

Identify the given reduction potentials: - Zn(s): -0.762 V - Cr(s): -0.740 V - H2(g): 0.000 V - Fe2+(aq): 0.770 V.

Step 3: Determine Strongest Reducing Agent

The strongest reducing agent is the species that most readily gives away electrons, which is indicated by the most negative standard reduction potential. From the given potentials, Zn(s) with a potential of -0.762 V is the most negative.

Step 4: Choose the Correct Option

Review the options: (a) H2(g), (b) Cr(s), (c) Zn(s), (d) Fe2+(aq). The strongest reducing agent corresponding to the most negative reduction potential is Zn(s). Select option (c).

Key Concepts

Standard Reduction Potential

Standard reduction potential is a measure of the tendency of a chemical species to acquire electrons and be reduced. It is measured in volts (V) and is usually determined under standard conditions: a concentration of 1 M for solutions, a pressure of 1 atm for gases, and a temperature of 298 K (25°C).

When we talk about standard reduction potentials, we're essentially discussing how likely a substance is to gain electrons. A more positive potential indicates a greater likelihood of a substance undergoing reduction. On the other hand, a negative standard reduction potential means the substance has less tendency to gain electrons.

• Standard reduction potential values allow us to predict the direction of electron flow in electrochemical cells.
• These values are crucial for calculating the overall cell potential, which determines if a reaction will be spontaneous under standard conditions.
• Knowing the potential can help in understanding how electrons will move in a redox reaction.

Thus, when attempting to identify the strongest reducing agent, we look for the most negative standard reduction potential, as this indicates a greater tendency to lose electrons.

Reducing Agents

A reducing agent, also known as a reductant, is a chemical component that donates electrons to another substance in a chemical reaction, thereby reducing that other substance’s oxidation state. Essentially, the reducing agent itself gets oxidized in the process.

The effectiveness of a reducing agent is inversely related to its standard reduction potential. A more negative standard reduction potential typically signifies a stronger reducing agent. This is because a strong reducing agent wants to give up its electrons readily.

• They are crucial in redox reactions, where reduction refers to the gain of electrons by a molecule.
• Many metals, such as zinc and chromium, exhibit strong reducing properties due to their tendency to be oxidized.
• Effective reducing agents will have more negative standard reduction potentials compared to the species they are reducing.

This concept is highlighted when comparing metals like zinc and iron, where zinc often serves as a more effective reducing agent due to its lower standard reduction potential.

Electrochemical Cells

Electrochemical cells, often called galvanic or voltaic cells, are devices that convert chemical energy into electrical energy through redox reactions. They are instrumental in our day-to-day life, powering everything from batteries in our gadgets to large-scale energy storage systems.

Electrochemical cells operate by separating the oxidizing and reducing agents in different compartments. These compartments are connected by a wire and a salt bridge, which facilitates the flow of ions and electrons. Each half of the cell has a different half-reaction occurring, one involving reduction and the other oxidation.

• The anode is where oxidation happens, usually involving the reducing agent, which loses electrons.
• The cathode is the site for reduction, where electrons are gained.
• The flow of electrons from the anode to the cathode through the external circuit generates electrical power.

The efficiency and potential of an electrochemical cell depend significantly on the standard reduction potentials of the reactions occurring at the anode and cathode. By understanding these potentials, we can predict the feasibility and voltage of the electrochemical reaction.