Question

\(\mathrm{AlCl}_{3}\) is (1) anhydrous and covalent. (2) anhydrous and ionic. (3) covalent and basic. (4) co-ordinate and acidic.

Short answer

Option (4): co-ordinate and acidic.

Step-by-step solution

- Identify Compound Type

Determine if \(\text{AlCl}_{3}\) is covalent or ionic. Aluminum chloride (\(\text{AlCl}_{3}\)) is a compound formed between aluminum and chlorine. In its anhydrous form, \(\text{AlCl}_{3}\) is known to be covalent due to the polarization effects of the aluminum cation on the electron cloud of the chloride anion.

- Determine Acidity or Basicity

Evaluate whether the substance is acidic or basic. Anhydrous \(\text{AlCl}_{3}\) is acidic; it reacts with water to form hydrochloric acid (HCl) and aluminum hydroxide (Al(OH)3). Thus, it is not basic.

- Consider Co-ordination

Identify the nature of \(\text{AlCl}_{3}\) in terms of co-ordination. Anhydrous \(\text{AlCl}_{3}\) exists as a dimer (\(\text{Al}_2\text{Cl}_6\)) in the gas phase and involves co-ordinate covalent bonds where a shared pair of electrons comes from one atom.

- Choose the Correct Description

Combine all the characteristics: \(\text{AlCl}_{3}\) is covalent (not ionic or basic), it forms co-ordinate bonds, and it is acidic. Hence, the best fit is option (4): co-ordinate and acidic.

Key concepts

covalent bonding

Covalent bonding involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms forms a covalent bond. In the case of \(\text{AlCl}_3\), the aluminum and chlorine atoms share electrons. Chlorine atoms have 7 electrons in their outer shell and need 1 more to complete the octet, while aluminum has 3 electrons in its outer shell and can accept more to reach a stable state. The sharing of electrons makes the bond covalent.

Covalent bonds typically occur between non-metal elements. Aluminum, although a metal, forms covalent bonds with chlorine because of the polarizing effect it has on chlorine's electron cloud. This type of bond ensures the compound can exist stably in solid and gaseous states.

Key characteristics of covalent bonds include:

• Low melting and boiling points.
• Poor electrical conductivity.
• Often exist as gases, liquids, or soft solids.

The \(\text{AlCl}_3\) molecule exemplifies covalent bonding, providing an excellent real-world example for students to understand this concept.

coordinate covalent bonds

Coordinate covalent bonds, also known as dative bonds, occur when both electrons in the bond come from the same atom. This bond is a special type of covalent bond. Within \(\text{AlCl}_3\), coordinate covalent bonds play a significant role.

In the gas phase, \(\text{AlCl}_3\) often exists as a dimer, \(\text{Al}_2\text{Cl}_6\). This dimer formation involves coordinate covalent bonds where one chlorine atom donates both electrons to form the bond with an aluminum atom. This type of bonding is essential in stabilizing the dimer structure and is crucial to understanding the compound's chemical behavior.

Key points about coordinate covalent bonds include:

• Similar to regular covalent bonds in terms of strength and chemical properties.
• Formation often involves a lone pair of electrons from one atom.
• Common in transition metals and complex ions.

Understanding coordinate covalent bonds gives insight into more complex molecular structures and reactions, highlighting the versatility of covalent interactions.

acidic compounds

Acidic compounds are substances that release protons (\text{H}^+) when dissolved in water. Anhydrous \(\text{AlCl}_3\) is considered acidic. Upon exposure to water, it hydrolyzes to form hydrochloric acid (HCl) and aluminum hydroxide (Al(OH)3). This reaction confirms its acidic nature.

Acidity in compounds like \(\text{AlCl}_3\) is due to the ability of aluminum ions to attract and polarize water molecules, facilitating the release of \text{H}^+ ions. This behavior is typical of Lewis acids, which accept electron pairs.

Characteristics of acidic compounds:

• Lower pH when dissolved in water.
• Can react with bases to neutralize and form salts.
• Often corrosive and can affect indicators like litmus paper (turning them red).

By identifying \(\text{AlCl}_3\) as an acidic compound, students learn about its reactivity and applications, further solidifying the concept of acidity in chemistry.