Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following salts will form colourless beads in borax bead test? (a) \(\mathrm{CoCl}_{2}\) (b) \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{K}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) (d) \(\mathrm{CoC}_{2} \mathrm{O}_{4}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{Ba(NO}_3)_2\) and (c) \(\mathrm{K}_2 \mathrm{C}_2 \mathrm{O}_4\) form colorless beads.

Step 1: Understanding the Borax Bead Test

In the borax bead test, a salt is heated with borax, Na₂B₄O₇, which forms a borate complex. This test is used to identify certain metal cations based on the color of the bead formed. Transition metals typically form colored beads, while non-transition metals tend to form colorless beads.

Step 2: Identifying Transition Metals

Look at each compound and identify if the salt contains a transition metal. Transition metals are typically those in the d-block of the periodic table. - \(\mathrm{CoCl}_2\) contains cobalt, a transition metal.- \(\mathrm{Ba(NO}_3)_2\) contains barium, not a transition metal.- \(\mathrm{K}_2 \mathrm{C}_2 \mathrm{O}_4\) contains potassium, not a transition metal.- \(\mathrm{CoC}_2 \mathrm{O}_4\) contains cobalt, a transition metal.

Step 3: Predicting Bead Colors

Compounds containing transition metals, such as cobalt, typically produce colored beads:- \(\mathrm{CoCl}_2\) will produce colored beads because of cobalt.- \(\mathrm{Ba(NO}_3)_2\), containing barium, is expected to form colorless beads.- \(\mathrm{K}_2 \mathrm{C}_2 \mathrm{O}_4\) will form colorless beads as it contains no transition metal.- \(\mathrm{CoC}_2 \mathrm{O}_4\) will produce colored beads because of cobalt.

Step 4: Determine the Correct Answer

Based on the borax bead test and the components of each compound, the salts that will form colorless beads are those without transition metals. Thus, \(\mathrm{Ba(NO}_3)_2\) and \(\mathrm{K}_2 \mathrm{C}_2 \mathrm{O}_4\) will form colorless beads.

Key Concepts

Transition Metals

Transition metals are a unique group of elements found in the d-block of the periodic table. They are known for their ability to form various oxidation states and complex ions. This is because these metals have partially filled d-orbitals.
Characteristics of transition metals:

• They are typically very dense and possess high melting and boiling points.
• Many transition metals act as good conductors of electricity and heat.
• These metals usually form coloured compounds due to d-d electron transitions.
• Transition metals can form multiple oxidation states, which is why they're often used in catalysis.

Understanding whether an element is a transition metal is important in tests like the borax bead test, where the colors of the beads primarily result from the presence of these metal ions.

Colorless Beads

In the borax bead test, colorless beads are formed by compounds that do not contain transition metal ions. Transition metals, as mentioned, tend to produce colorful complexes or beads.
The reason for the formation of colorless beads:

• Non-transition metals do not have readily available d-electrons for transitions, meaning they don't typically participate in electronic transitions that manifest as color.
• Such compounds often comprise s-block elements or certain p-block elements, which contribute to a lack of color.
• These metals form simpler complexes, usually white or colorless, as there are no d-d transitions available to absorb visible light.

When predicting the color of the beads formed in this test, a student needs to know whether the metal cation involved is a transition metal or not.

Borate Complex

During the borax bead test, the borate complex plays a significant role. Borate, a derivative of borax (\(\text{Na}_2\text{B}_4\text{O}_7\)), becomes crucial when it forms a complex with the metal salts being tested.What happens in the borax bead test:

• The borate undergoes a reaction upon heating, which initially produces a glassy bead.
• When a metal cation is introduced, it can incorporate into the bead, forming a metal borate complex.
• The color of the bead is influenced by the metal ion present. Complexes derived from transition metals usually exhibit vibrant colors due to electronic transitions in the d-orbitals.

Understanding the formation and role of borate complexes helps in identifying not just the presence of metal ions, but their specific type as well.