Problem 164 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal \(\mathrm{M}\) readily forms water soluble sulphate \(\mathrm{MSO}_{4}\), water insoluble hydroxide \(\mathrm{M}(\mathrm{OH})_{2}\) and oxide MO which becomes inert on heating. The hydroxide is soluble in NaOH. M is (a) \(\mathrm{Mg}\) (b) \(\mathrm{Sr}\) (c) \(\mathrm{Be}\) (d) \(\mathrm{Ca}\)

Step-by-Step Solution

Answer

M is (c) \(\text{Be}\) (beryllium).

Step 1: Identify Characteristics of M

The metal M forms a water soluble sulphate (\(\text{MSO}_4\)), which suggests that it might be similar to group 2 metals known for forming soluble sulphates. The metal also forms a water insoluble hydroxide \((\text{M(OH)}_2)\), which is consistent with group 2 metals.

Step 2: Evaluate Solubility of Hydroxide

It is given that \(\text{M(OH)}_2\) is soluble in NaOH. This property is characteristic of beryllium hydroxide (\(\text{Be(OH)}_2\)), which is amphoteric and can dissolve in sodium hydroxide to form a soluble complex.

Step 3: Consider Oxide Properties

The metal oxide (MO) becomes inert on heating, indicating that it may not react further at high temperatures. Beryllium oxide (\(\text{BeO}\)) is known to become inert when heated, which aligns with this description.

Step 4: Analyze Options and Make Conclusion

Based on the information, metal M should have properties that match those primarily of beryllium: soluble sulphate, insoluble hydroxide that dissolves in NaOH, and an oxide that is inert upon heating. Among the given options, \(\text{Be}\) (beryllium) best fits these criteria.

Key Concepts

Solubility of Sulphates

The solubility of sulphates is an interesting topic within group 2 metals. In general, sulphates of metals like magnesium, calcium, strontium, and barium have varying solubility in water. For instance, magnesium sulphate (\(MgSO_4\)) is quite soluble in water, whereas barium sulphate (\(BaSO_4\)) is notably insoluble. This trend generally shows a decrease in solubility as you move down the group in the periodic table. This is because the lattice energy does not significantly increase compared to the hydration energy decrease, leading to lower solubility.

Beryllium sulphate (\(BeSO_4\)), however, stands out due to its good solubility in water, somewhat similar to magnesium. This makes beryllium unique among its group because beryllium compounds often behave differently than those of other alkaline earth metals. This difference is largely attributed to beryllium's small atomic size and high charge density, which affect its chemical behavior in various ways.

Amphoteric Hydroxides

Amphoteric substances can react both as acids and bases. Beryllium hydroxide (\(Be(OH)_2\)) is a classic example of an amphoteric hydroxide. This property is not commonly observed among other group 2 metal hydroxides, which are typically just basic.

When \(Be(OH)_2\) is mixed with a strong base like sodium hydroxide (\(NaOH\)), it dissolves by forming a complex ion. This occurs because the hydroxide can accept more hydroxide ions to form beryllate ions, \([Be(OH)_4]^{2-}\), showing its acidic side. On the other hand, it can also release ions when in the presence of strong acids, showcasing its basic properties. The ability to react with both acids and bases makes it unique and useful in various chemical processes.

Properties of Beryllium

Beryllium is a fascinating element among the alkaline earth metals. It has distinct characteristics that make it dissimilar to its group members. These unique properties are primarily due to its small atomic and ionic size, leading to high ionization energy and significant electronegativity compared to other group 2 elements, like calcium or strontium.

Some important properties include:

• High melting point and tensile strength, making it valuable in aerospace applications.
• Formation of covalent compounds, unlike the ionic compounds typically formed by other alkaline earth metals.
• Resistance to corrosion, which makes it suitable for use in various harsh environments.

These unique attributes not only highlight beryllium's importance in industrial applications but also its singular behavior in chemical reactions compared to other metals on the periodic table.

Insolubility of Hydroxides

The hydroxides of group 2 metals generally become less soluble as you move down the group. Magnesium hydroxide (\(Mg(OH)_2\)) is only slightly soluble in water, while calcium hydroxide (\(Ca(OH)_2\)) is more soluble, forming lime water. As for strontium (\(Sr(OH)_2\)) and barium hydroxide (\(Ba(OH)_2\)), they are reasonably soluble and used in various applications.

Beryllium hydroxide, \(Be(OH)_2\), diverges from this trend. It is largely insoluble in water, yet it dissolves in excess base due to its amphoteric nature, setting it apart from the other group 2 hydroxides which are primarily basic. This amphoteric behavior results from beryllium's smaller size and higher charge to radius ratio, allowing it to adopt characteristics unusual for its group.