Question

Which of the following statements is not correct?

• A. Carbonates of alkaline earth metals are insoluble in water
• B. Beryllium halides are covalent in nature
• C. The super oxides of alkali metals are colourless
• D. Alkali metal halides have high negative enthalpies of formation

Hint:

Remember the unique nature of the first element in each group (e.g., Beryllium's high polarizing power leading to covalent compounds). For colored compounds, look for transition metal ions, complex ions, or species with unpaired electrons or delocalized $\pi$ systems. The superoxide ion ($\text{O}_2^-$) has an unpaired electron, making alkali metal superoxides colored and paramagnetic.

Answer 1

The Correct Option is C

Step 1: Analyze statement (A) - Carbonates of Alkaline Earth Metals.
The carbonates of alkaline earth metals (Group 2, $\text{MCO}_3$) are generally insoluble in water. The solubility decreases down the group ($\text{MgCO}_3$ is slightly soluble, but $\text{CaCO}_3, \text{SrCO}_3, \text{BaCO}_3$ are insoluble). The statement is generally accepted as correct in the context of general chemistry.

Step 2: Analyze statement (B) - Beryllium Halides.
Beryllium (Be) is the first and smallest element of Group 2. Due to its very small size and high polarizing power, the compounds it forms, such as its halides ($\text{BeCl}_2, \text{BeF}_2$, etc.), have significant covalent character. $\text{BeCl}_2$ exists as a polymeric chain in the solid state, consistent with covalent bonding. The statement is correct.

Step 3: Analyze statement (C) - Superoxides of Alkali Metals.
Alkali metals (Group 1) form superoxides, $\text{MO}_2$ (e.g., $\text{KO}_2, \text{RbO}_2, \text{CsO}_2$). Superoxides contain the superoxide ion ($\text{O}_2^-$), which has one unpaired electron. This unpaired electron makes the superoxides paramagnetic and colored (usually yellow or orange).
For example, Potassium superoxide ($\text{KO}_2$) is a pale yellow solid.
The statement "The super oxides of alkali metals are colourless" is incorrect.

Step 4: Analyze statement (D) - Enthalpies of Formation of Alkali Metal Halides.
Alkali metal halides ($\text{MX}$) are typical ionic compounds formed by the combination of a highly electropositive metal (Group 1) and a highly electronegative non-metal (Group 17).
The formation of these compounds is highly exothermic, driven by the large negative lattice energy of the resulting ionic crystal. This results in a very high negative enthalpy of formation. The statement is correct.

Step 5: Conclude the final answer.
The incorrect statement is (C).