Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

The decreasing order of solubility of alkaline earth metal hydroxides \(\left[\mathrm{M}(\mathrm{OH})_{2}\right]\) is given as (a) \(\mathrm{Ba}>\mathrm{Sr}>\mathrm{Ca}>\mathrm{Mg}>\mathrm{Be}\) (b) \(\mathrm{Be}<\mathrm{Mg}<\mathrm{Ca}<\mathrm{Sr}<\mathrm{Ba}\) (c) \(\mathrm{Be}=\mathrm{Mg}>\mathrm{Ca}>\mathrm{Sr}>\mathrm{Ba}\) (d) \(\mathrm{Mg}<\mathrm{Be}<\mathrm{Ca}<\mathrm{Ba}<\mathrm{Sr}\)

Step-by-Step Solution

Answer

The correct order is option (a): \( \mathrm{Ba} > \mathrm{Sr} > \mathrm{Ca} > \mathrm{Mg} > \mathrm{Be} \).

Step 1: Understand the Solubility Trend

The solubility of alkaline earth metal hydroxides (Mg(OH)_2, Ca(OH)_2, Sr(OH)_2, and Ba(OH)_2) generally increases down the group in the periodic table. This means barium (Ba) will have the highest solubility, followed by strontium (Sr), calcium (Ca), magnesium (Mg), and beryllium (Be) has the lowest solubility.

Step 2: Analyze Given Options

The options are different sequences representing the solubility order of the hydroxides of alkaline earth metals: (a) Ba > Sr > Ca > Mg > Be, (b) Be < Mg < Ca < Sr < Ba, (c) Be = Mg > Ca > Sr > Ba, and (d) Mg < Be < Ca < Ba < Sr. We need to identify the option that correctly follows the described trend: Ba > Sr > Ca > Mg > Be.

Step 3: Match with Solubility Order

Option (a) 'Ba > Sr > Ca > Mg > Be' matches the known solubility order for alkaline earth metal hydroxides. This order follows the increasing solubility of these hydroxides as you move down the group from beryllium to barium.

Key Concepts

Alkaline Earth Metals

Alkaline Earth Metals are a group of elements found in the second column of the Periodic Table. They include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements are known for being shiny, silvery-white, and somewhat reactive metals. Despite their similarities, they have distinct characteristics that vary as you move down the group.

One of the key properties of alkaline earth metals is their ability to form basic (or alkaline) solutions when they react with water. This is partly due to the formation of hydroxides. However, not all alkaline earth metals react with water in the same way, which leads to varying solubility trends in their hydroxides as well.

Hydroxides

Hydroxides are chemical compounds that contain one or more hydroxide ions, (OH^-). In the case of alkaline earth metals, their hydroxides (e.g., Mg(OH)_2, Ca(OH)_2) tend to be moderately soluble in water, forming alkaline solutions. This solubility is influenced by the size of the metal ions and the lattice energy of the compound.

The solubility of hydroxides such as Mg(OH)_2 increases as we move down the group of alkaline earth metals, from magnesium to barium. This is because the atoms become larger, and the lattice energy decreases, making it easier for them to dissolve in water. At the bottom of the group, barium hydroxide (Ba(OH)_2) dissolves fully, making the solution strongly alkaline. In contrast, beryllium hydroxide (Be(OH)_2) is nearly insoluble because of its very high lattice energy.

Periodic Table Trends

The Periodic Table presents elements in a way that highlights the repeating patterns of atomic properties. As we look at the alkaline earth metals, we notice a trend that affects their solubility in water: elements further down the column are generally more soluble.

This solubility trend is due, in part, to increasing atomic size and decreasing lattice energy as you move down the group. For example:

• Atomic size: As you move down the group from Be to Ba, the atoms become larger with more electron shells.
• Lattice energy: This is the energy released when the ions form a solid lattice. Larger ions lower the lattice energy, making it easier for the compound to dissolve in water.

Understanding these trends helps in predicting the behavior of similar compounds, especially when working with different elements in chemistry.