Question

Among \( \text{Fe}^{2+}, \text{Fe}^{3+}, \text{Cr}^{2+} \) and \( \text{Zn}^{2+} \), the ion that shows positive borax bead test and with highest ionisation enthalpy is:

• A. \( \text{Fe}^{2+} \)
• B. \( \text{Zn}^{2+} \)
• C. \( \text{Cr}^{2+} \)
• D. \( \text{Fe}^{3+} \)

Answer 1

The Correct Option is C

Step 1: Understanding Borax Bead Test.
The Borax bead test is used to detect the presence of metal ions by heating them in a flame and observing the color change in the borax bead. Positive results generally occur for transition metals with certain oxidation states, especially \( Fe^{3+} \), which is known to form a colored complex in the Borax bead test.
Step 2: Ionization Enthalpy.
Ionization enthalpy refers to the energy required to remove an electron from an atom or ion. \( Fe^{3+} \) has the highest ionization enthalpy among the given ions, as removing an electron from a highly charged ion like \( Fe^{3+} \) requires more energy compared to other ions.

Step 3: Comparison of ions.
- \( Fe^{2+} \): Shows a positive Borax bead test but not as intense as \( Fe^{3+} \).
- \( Zn^{2+} \): Does not show a positive Borax bead test as it does not form a colored complex in the test.
- \( Cr^{2+} \): Also does not show as strong a Borax bead reaction as \( Fe^{3+} \).
- \( Fe^{3+} \): Shows a strong positive result in the Borax bead test and has the highest ionization enthalpy among the given ions.
Final Answer: \[ \boxed{\text{(D) } Fe^{3+}} \]