Question

Lithium is the strongest reducing agent though it has highest ionisation energy in its group. Which of the following factors is responsible for making Li the strongest reducing agent?(a) Large heat of atomisation (b) Smaller size (c) Large sublimation energy (d) Large amount of hydration enthalpy

Short answer

Lithium is the strongest reducing agent in its group due to the large amount of hydration enthalpy, which compensates for its high ionization energy, making electron loss energetically favorable. The correct answer is (d) Large amount of hydration enthalpy.

Step-by-step solution

Understanding Reducing Agents

A reducing agent is a substance that loses electrons in a chemical reaction, thereby reducing the oxidizing agent. The strength of a reducing agent is often determined by its ability to lose electrons. A strong reducing agent easily loses electrons. Factors that can influence the ease of electron loss include ionization energy, enthalpy of atomization, atomic size, sublimation energy, and hydration enthalpy.

Analyzing the Options

To determine which factor makes lithium the strongest reducing agent, consider each option: (a) Heat of atomization generally impacts the energy required to break metal bonds, which can affect reducing strength, but not as directly as other factors. (b) Smaller size can increase ionization energy, which usually would suggest a weaker reducing agent, contradicting lithium's strong reducing capabilities. (c) Large sublimation energy pertains to the energy required to convert from solid to gas, and while it's a component of the total energy needed to create ions, it is not the direct cause. (d) Large amount of hydration enthalpy significantly enhances lithium's reducing power because it implies a large release of energy when lithium ions are solvated by water molecules, thus compensating for the high ionization energy and fostering the loss of electrons.

Identifying the Correct Factor

Lithium, despite having the highest ionization energy, is the strongest reducing agent in its group because the large amount of energy released during hydration (hydration enthalpy) compensates for its ionization energy. This makes the overall process energetically favorable, hence, lithium easily loses an electron to become a cation in aqueous solution. Therefore, the factor responsible for making Li the strongest reducing agent is the large amount of hydration enthalpy.

Key concepts

Hydration Enthalpy

Hydration enthalpy is a measure of the energy released when ions are solvated by water molecules. It is an exothermic process, which implies a release of energy, bringing stability to the system. When salts dissolve in water, they dissociate into their constituent ions. Each ion is surrounded by water molecules, and this process of hydration is what contributes to the overall solubility of an ionic compound.

For elements like lithium (Li), although they possess high ionization energies, which typically act as a barrier to electron loss, the substantial release of energy through hydration enthalpy counteracts this effect. In particular, smaller ions with high charges tend to have larger hydration enthalpies due to the strong attractions between the ions and the polar water molecules. The energy released can compensate for the energy needed to remove an electron—the essence of ionization energy, thereby reinforcing the element's role as a reducing agent.

The concept of hydration enthalpy is profound because it not only helps us to understand why certain elements are highly reactive but also forms the basis for predicting the solubility of a variety of compounds in water. In the context of the exercise, lithium's large amount of hydration enthalpy is the key factor that makes it the strongest reducing agent, despite having the highest ionization energy within its group.

Ionization Energy

Ionization energy is defined as the amount of energy required to remove an electron from an atom or ion in its gaseous state. It is an indicator of the grip an atom has on its electrons; the higher the ionization energy, the more tightly an electron is held, and vice versa.

As one goes down a group in the periodic table, the ionization energy tends to decrease because the electrons are further from the nucleus, thereby reducing the nuclear pull. Conversely, across a period, ionization energy tends to increase due to the increasing positive charge of the nucleus, which holds onto the electrons more tightly. Thus, ionization energy can predict an element's reactivity, with high ionization energy usually implying low reactivity as a reducing agent.

However, the scenario with lithium is intriguing. With high ionization energy, it would typically be less inclined to lose electrons and act as a reducing agent. But because of the large hydration enthalpy that accompanies the solvation of lithium ions, this high ionization energy is offset, allowing lithium to part with electrons more readily than expected and act as a powerful reducing agent.

Chemical Reactivity

Chemical reactivity refers to the tendency of a substance to undergo chemical change. The reactivity is influenced by several factors, including atomic size, ionization energy, electron affinity, and the nature of the bonds within the substance. The combination of these factors determines how readily a substance will engage in chemical reactions.

In the realm of reducing agents, chemical reactivity is characterized by the ability to donate electrons. A reactive metal, for instance, will readily lose electrons to form positive ions. This is a fundamental aspect of the reducing behavior which is at the core of redox reactions. Reducing agents are themselves oxidized in the process of reducing another species. Therefore, a strong reducing agent is one that is easily oxidized.

Applying this to lithium, despite its reluctance to lose an electron due to high ionization energy, its chemical reactivity is dramatically heightened by the significant release of energy when it undergoes hydration. The substantial hydration enthalpy overcomes the ionization energy barrier, thereby increasing its overall reactivity and making it an exceedingly potent reducing agent. This illustrates the complex interplay of various factors contributing to the chemical reactivity of elements in the periodic table.