Question

Give reasons for the following: (a) Be does not react with water. (b) Be halides are polymeric. (c) Lime water turns milky on passing \(\mathrm{CO}_{2}\), through it but becomes clear when excess of \(\mathrm{CO}_{2}\) is passed.

Short answer

(a) Beryllium does not react with water due to its high ionization energy. (b) Beryllium halides are polymeric due to the small atomic size and high charge to radius ratio of Be, allowing it to form covalent bonds with halogens. (c) Lime water turns milky when CO2 is passed through due to the formation of calcium carbonate, but becomes clear with excess CO2 due to the formation of soluble calcium hydrogen carbonate.

Step-by-step solution

Answer for (a)

Beryllium (Be) is an exception to the typical properties of its group (Group 2: alkaline earth metals) because it has a very high ionization energy. This high ionization energy is due to its small atomic size, and it prevents Be from readily donating its electrons to react with water.

Answer for (b)

Beryllium halides (BeX2, where X represents a halogen) are known to be polymeric. This is because Be has a small size and a high charge to radius ratio, these factors enable it to form covalent bonds with the halogens and cause polymeric structures to form. The Be atom is small enough to accommodate four pairs of electrons in its outer shell, which allows it to form a bridge with two halogen atoms, creating a polymeric structure.

Answer for (c)

This phenomenon occurs due to a reaction of the calcium hydroxide in the lime water with the carbon dioxide (CO2). Initially, when CO2 is passed through lime water, it reacts to form calcium carbonate, which turns the solution milky. However, if an excess of CO2 is introduced, it reacts with the calcium carbonate to form calcium hydrogen carbonate, which is soluble in water and causes the solution to become clear again.

Key concepts

Alkaline Earth Metals

Alkaline earth metals belong to Group 2 of the periodic table and include six elements: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These metals are so named because their oxides form alkaline solutions when dissolved in water, and they occur naturally in various minerals within the Earth's crust.

What makes alkaline earth metals distinctive is their reactivity with water, although this reactivity varies within the group. For example, while magnesium and calcium readily react with water at room temperature, beryllium (Be), due to its unique properties, does not. One reason beryllium is different from its counterparts is its exceptionally high ionization energy that stems from its small atomic radius. This characteristic hinders beryllium from losing electrons to react with water, making it less reactive than the other alkaline earth metals.

Another notable trait of alkaline earth metals is their tendency to form +2 ionic charges. However, beryllium is more inclined to covalent bonding because of its high charge density, contributing to differences in the compounds it forms, such as beryllium halides.

Beryllium Halides Polymerization

When exploring the world of inorganic chemistry, one encounters the fascinating structure of beryllium halides, such as beryllium chloride (BeCl2). Unlike typical ionic halides, beryllium halides tend to form polymeric structures. This is due to the small ionic radius and high charge density of the beryllium ion, which enables it to form covalent bonds with halogen atoms.

In these polymers, a single beryllium atom can form bridges with two halogen atoms, allowing a network-like structure to manifest. This bridging is a result of beryllium's ability to accommodate four electron pairs around it, contrary to what might be expected for a metal with only two valence electrons. This unique feature leads to the formation of chains or lattices, rather than discrete, individual molecules.

The polymerization of beryllium halides can be influenced by factors such as the presence of donor solvents or the temperature of the environment. In a solution, the polymeric chains of beryllium halides may break down, showcasing a dynamic aspect of their chemical behavior.

Carbon Dioxide and Lime Water Reaction

The reaction between carbon dioxide (CO2) and lime water is a classic demonstration in chemistry of how gases can react with aqueous solutions to form precipitates. Lime water is a common term for a diluted solution of calcium hydroxide (Ca(OH)2), which is clear and colorless.

When carbon dioxide is introduced into lime water, it reacts to form calcium carbonate (CaCO3), a white insoluble compound that creates a milky appearance in the solution. This immediate reaction can be represented by the equation: \[\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}\].

This process is used as a simple test for the presence of CO2. However, the reaction doesn't stop there. When excess CO2 is passed through the milky lime water, it leads to the formation of calcium hydrogen carbonate (Ca(HCO3)2), which is soluble in water: \[\mathrm{CaCO_3(s) + CO_2(g) + H_2O(l) \rightarrow Ca(HCO_3)_2(aq)}\].

This secondary reaction clears the solution, as the calcium hydrogen carbonate formed does not precipitate but stays dissolved, demonstrating the reversible nature of the reaction and the dynamic equilibrium between these compounds in aqueous solutions.