Question

Transition metals despite having higher values of standard reduction potentials, are poor reducing agents. This is due to (a) low heat of hydration (b) high ionization energies (c) low ionization energies (d) high enthalpy of vapourization

Short answer

(b) high ionization energies.

Step-by-step solution

Understanding the Role of Standard Reduction Potentials

Transition metals generally have higher standard reduction potentials, which means they are more likely to gain electrons and be reduced rather than lose electrons and oxidize another substance. Therefore, high reduction potential typically indicates weaker reducing ability, as reducing agents donate electrons.

Evaluating Reducing Agent Properties

A strong reducing agent tends to have a low ionization energy because it can easily lose electrons. Transition metals, although having a high standard reduction potential, could still act as poor reducing agents due to having high ionization energies. This high ionization energy makes it difficult for them to lose electrons.

Analyzing the Given Options

Analyzing the given options, "(a) low heat of hydration," "(b) high ionization energies," "(c) low ionization energies," and "(d) high enthalpy of vaporization," the one that relates to the poor ability to lose electrons and thus to be a reducing agent is (b) high ionization energies.

Drawing the Conclusion

Since high ionization energies inhibit the metal's ability to lose electrons, leading to less effective reducing agents, the correct explanation for why transition metals are poor reducing agents in spite of having high standard reduction potentials is (b) high ionization energies.

Key concepts

Standard Reduction Potential

When discussing transition metals, it is crucial to understand the concept of standard reduction potentials. This value measures the tendency of a chemical species to acquire electrons and be reduced. Metals with high standard reduction potentials, like transition metals, are more inclined to gain electrons. As a result, they are more likely to undergo reduction themselves rather than donating electrons to other substances and causing their oxidation.
This behavior might initially seem confusing. Typically, you would expect a material with a high reduction potential, indicating readiness to gain electrons, to effectively act as a reducing agent. However, the opposite is true. A high standard reduction potential signifies a strong likelihood of reduction, which typically makes them less effective at being reducing agents.
Understanding this balance helps clarify why transition metals, despite their high standard reduction potentials, are not strong reducing agents.

Reducing Agents

Reducing agents are substances that promote the reduction of another species by donating electrons to it. To do so effectively, a reducing agent must easily part with its electrons. This property is often associated with low ionization energies. When a reducing agent has a low ionization energy, it requires little energy to remove electrons, facilitating this electron donation process.
Transition metals generally struggle in this capacity due to their inclination to hold onto their electrons tightly. Despite their potential to reduce other substances, their high ionization energies make it difficult for them to give away electrons freely.
This electron retention behavior is the reason why transition metals don't perform as well as other metals with lower ionization energies. Therefore, their reducing power is limited compared to metals that find it easy to lose electrons due to lower ionization energy.

Ionization Energy

Ionization energy is a fundamental concept when understanding the properties of elements, especially transition metals. It refers to the amount of energy necessary to remove an electron from a gaseous atom or ion. Higher ionization energy means that more energy is required to remove an electron from an atom, indicating a strong hold on its electrons.
Transition metals often exhibit high ionization energies. This makes them less willing to lose electrons, hence poorer at acting as reducing agents. While they may have desirable properties like a high melting point or tensile strength, their electricity retention capability makes electron donation challenging.
In essence, their high ionization energies, while contributing to some notable physical properties, inhibit their ability as reducing agents since they retain their electrons more strongly than other groups of metals.