Question

Alkali metals cannot be extracted by reduction of their oxides and other compounds because (a) alkali metals are strong reducing agents (b) alkali metals have low ionisation enthalpy (c) alkali metals have high lattice enthalpy (d) alkali metals are strongly basic in nature.

Short answer

The correct answer is (a) alkali metals are strong reducing agents, which means that they are more likely to be reduced (gain electrons) than to act as reducers for their own oxides and other compounds.

Step-by-step solution

Understanding the Properties of Alkali Metals

Acknowledge that alkali metals, which are located in Group 1 of the periodic table, possess specific chemical properties due to their one valence electron. These properties include high reactivity and a strong tendency to lose their single valence electron to form cations. The properties listed in the options impact their extraction process.

Analyze Option (a) - Alkali Metals as Reducing Agents

Consider that reducing agents donate electrons in chemical reactions. Since alkali metals easily lose their single valence electron due to low ionization energies, they serve as good reducing agents. As strong reducing agents, they cannot be used to reduce their own oxides because they are more prone to reduction themselves than to act as reducers for their own compounds.

Analyze Option (b) - Low Ionisation Enthalpy

Low ionisation enthalpy means it requires less energy to remove the outermost electron. This property explains why alkali metals are good reducing agents but does not directly explain why their oxides and other compounds cannot be reduced by them.

Analyze Option (c) - High Lattice Enthalpy

High lattice enthalpy indicates that the ionic compounds of alkali metals, such as their oxides, have strong ionic bonds due to the substantial attraction between the ions. These strong ionic bonds require a large amount of energy to break, making the chemical reduction of these compounds challenging with conventional reducing agents.

Analyze Option (d) - Strongly Basic Nature

Being strongly basic relates to the reaction of alkali metals with water and their oxides' basicity. However, this characteristic does not explain the difficulty in their extraction via reduction of their oxides and other compounds.

Conclusion

Recognize that the key reason alkali metals cannot be extracted by the reduction of their oxides and other compounds is due to their nature as strong reducing agents themselves. They cannot reduce their own compounds because in a redox reaction, they act as the species being oxidized rather than the species causing reduction.

Key concepts

Reducing Agents

A reducing agent is a substance that donates electrons in a chemical reaction, thereby reducing another substance. In the context of alkali metals, these elements are particularly potent reducing agents because they readily lose their single valence electron.

This propensity to donate an electron comes from their low ionisation enthalpy, which allows them to achieve a stable electronic configuration with minimal energy input. Since alkali metals are themselves easily reduced (lose electrons), they cannot reduce their own oxides or other compounds. This is akin to asking someone who only has a single dollar to pay for another's expenses; they simply don't have enough to spare.

Moreover, their reducing ability is not just a theoretical aspect but deeply impacts their real-world applications. For instance, lithium, an alkali metal, is used as a reducing agent for the production of certain metals from their oxides.

Practical Implications of Reducing Behavior

Due to their reducing nature, alkali metals are also used in batteries and other applications where electron donation is critical.

Ionisation Enthalpy

Ionisation enthalpy refers to the energy required to remove the most loosely bound electron from an isolated gaseous atom to form a cation. When we discuss alkali metals, which sit at the leftmost corner of the periodic table, we are talking about elements with just one electron in their outermost shell.

This single electron can be removed with less energy compared to elements with more electrons in their outer shell—largely because of the 'shielding effect' from the inner electrons and the distance of the outer electron from the nucleus.

In practical terms, the lower ionisation enthalpy of alkali metals explains their extreme reactivity and their common existence in nature as compounds rather than free elements.

Understanding Reactivity Patterns

Understanding ionisation enthalpy helps explain trends in the periodic table, such as why cesium and francium are more reactive than lithium and sodium, as their ionisation enthalpies decrease down the group.

Lattice Enthalpy

Alkali metals form ionic compounds, wherein the lattice enthalpy is a measure of the strength of the forces holding the ions together in a crystal lattice. High lattice enthalpy means these ionic bonds are particularly strong for alkali metals, requiring a significant amount of energy to break.

Given this high lattice enthalpy, it becomes energetically unfavorable to reduce alkali metal compounds like oxides. The energy required to disrupt the ionic lattice is not provided for by the simple presence of a reducing agent, especially when that reducing agent is the alkali metal itself.

Consequently, alternative methods such as electrolysis are typically used to extract alkali metals from their natural compounds. This process involves passing an electric current through the compound to disrupt the ionic bonds, separating the metal from its counterpart without relying on the concept of chemical reduction.