Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Alkali metals are characterized by 1\. good conductor of heat and electricity 2\. high oxidation potentials 3\. high melting point 4\. solubility in liquid ammonia (a) 1,2 (b) \(2,3,4\) (c) \(1,2,4\) (d) all of these

Step-by-Step Solution

Answer

Option (c) 1,2,4.

Step 1: Identify Alkali Metals

Alkali metals are elements found in Group 1 of the periodic table. These include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These metals are known for certain characteristic properties.

Step 2: Assess Conductivity

Alkali metals are known to be good conductors of heat and electricity due to their free-moving electrons in the outermost shell. This supports statement 1.

Step 3: Evaluate Oxidation Potentials

The term 'oxidation potential' may be used interchangeably with 'ionization energy'. Alkali metals have relatively low ionization energies compared to other elements, which allow them to lose electrons easily and conduct electricity. This does not support statement 2.

Step 4: Examine Melting Points

Alkali metals do not have high melting points; they actually have relatively low melting points compared to most other metals. Therefore, statement 3 is not correct.

Step 5: Consider Solubility in Liquid Ammonia

Alkali metals dissolve in liquid ammonia, forming a deep blue solution due to the solvated electrons. This supports statement 4.

Step 6: Select Correct Statements

After evaluating each statement, we determine that alkali metals are characterized by being good conductors of heat and electricity and solubility in liquid ammonia. Therefore, statements 1 and 4 are correct.

Key Concepts

Group 1 elements

The Group 1 elements, known as alkali metals, occupy the first column of the periodic table. This group includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These metals are unified by the presence of a single electron in their outermost electron shell, which makes them highly reactive and eager to participate in chemical reactions.

• They are shiny and have a silvery appearance.
• Being highly reactive, they are often stored in oil to prevent reactions with air or moisture.
• Their reactivity increases as you move down the group, from lithium to francium.

Despite their common properties, the alkali metals show gradations in behavior; for instance, lithium is less reactive in comparison to cesium, which is situated further down in the group, owing to the outer electron being further from the nucleus.

Properties of alkali metals

Alkali metals are fascinating due to their distinctive properties. Being placed in Group 1 of the periodic table, they possess unique characteristics that distinguish them from other metals.

• They have low density; some like lithium and sodium are light enough to float on water.
• Their melting and boiling points are relatively low compared to other metals, making them soft and easily cuttable with a knife.
• Due to their single valence electron, they readily lose it to form positive ions, engaging in vigorous reactions with water and air.

Their reaction with water produces hydrogen gas and a hydroxide of the respective metal, which is highly exothermic. For example, adding sodium to water results in sodium hydroxide and hydrogen gas, often producing flames.

Conductivity of alkali metals

Alkali metals are exceptional conductors of heat and electricity, attributable to their atomic structure. The presence of a single electron in their outer shell facilitates free electron flow, allowing them to conduct efficiently.

• These free electrons are able to move freely within the metallic structure, translating to high electrical conductivity.
• The availability of free-moving electrons also permits excellent thermal conductivity.

Consequently, their capacity to transfer these electrons easily places them in the category of good electrical conductors, often used in applications requiring such properties.

Solubility in liquid ammonia

An interesting property of alkali metals is their solubility in liquid ammonia, which results in a unique and visually striking deep blue solution. This process is significant for several reasons.

• When dissolved, the alkali metals liberate electrons into the solution, leading to the deep blue color.
• The solvated electrons are responsible for the conductivity of the resulting solution.
• At higher concentrations of metal, the solution may develop a metallic bronze color and become less stable.

This interaction with liquid ammonia is utilized in various chemical syntheses and processes, as it provides a medium for reactions that require free electrons, underscoring the alkali metals' role in chemical industries.