Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Amongst the following, identify the species with an atom in \(+6\) oxidation state: \(\quad[\) Main Online April 19, 2014] (a) \(\left[\mathrm{MnO}_{4}\right]^{-}\) (b) \(\left[\mathrm{Cr}(\mathrm{CN})_{6}\right]^{3-}\) (c) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The species with a +6 oxidation state is (d) \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\).

Step 1: Understanding the question

The question asks us to identify which species contains an atom with a +6 oxidation state among the given options. We need to calculate the oxidation state of the central metal ion for each compound.

Step 2: Calculating oxidation state for \\([\\mathrm{MnO}_{4}]^{-}\\)

In \([\mathrm{MnO}_{4}]^{-}\), let the oxidation state of \(\mathrm{Mn}\) be \(x\). Oxygen has an oxidation state of \(-2\). The sum of oxidation states should equal the charge of the ion, which is \(-1\):\[ x + 4(-2) = -1 \] \[ x - 8 = -1 \] \[ x = +7 \]Thus, \(\mathrm{Mn}\) is in a +7 oxidation state.

Step 3: Calculating oxidation state for \\([\\mathrm{Cr}(\\mathrm{CN})_{6}]^{3-}\\)

In \([\mathrm{Cr}(\mathrm{CN})_{6}]^{3-}\), let the oxidation state of \(\mathrm{Cr}\) be \(y\). Cyanide (\(\mathrm{CN}\)) is a neutral ligand, its charge is \(-1\). The sum must equal \(-3\):\[ y + 6(-1) = -3 \] \[ y - 6 = -3 \] \[ y = +3 \]Thus, \(\mathrm{Cr}\) is in a +3 oxidation state.

Step 4: Calculating oxidation state for \\(\\mathrm{Cr}_{2} \\mathrm{O}_{3}\\)

In \(\mathrm{Cr}_{2} \mathrm{O}_{3}\), let the oxidation state of \(\mathrm{Cr}\) be \(z\). Oxygen has an oxidation state of \(-2\):\[ 2z + 3(-2) = 0 \] \[ 2z - 6 = 0 \] \[ 2z = +6 \] \[ z = +3 \]Thus, \(\mathrm{Cr}\) is in a +3 oxidation state.

Step 5: Calculating oxidation state for \\(\\mathrm{CrO}_{2} \\mathrm{Cl}_{2}\\)

In \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\), let the oxidation state of \(\mathrm{Cr}\) be \(w\). Oxygen is \(-2\) and chlorine is \(-1\):\[ w + 2(-2) + 2(-1) = 0 \] \[ w - 4 - 2 = 0 \] \[ w - 6 = 0 \] \[ w = +6 \]Thus, \(\mathrm{Cr}\) is in a +6 oxidation state.

Step 6: Conclusion

Only \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) contains an atom in a +6 oxidation state. Therefore, the correct answer is option (d).

Key Concepts

Transition Metals

Transition metals are a group of elements found in the center of the periodic table, specifically in the d-block. They are known for their ability to form compounds with a variety of oxidation states, which is due to their d-electrons. This characteristic provides them flexibility in bonding with other atoms in chemical compounds. Common features of transition metals include:

• The capability to form complex ions.
• Multiple oxidation states.
• Colored compounds due to electron transitions between d-orbitals.
• Catalytic properties that are significant in industrial chemical reactions.

Transition metals such as chromium (\(\mathrm{Cr}\)) and manganese (\(\mathrm{Mn}\)) are typically involved in redox reactions because they can readily undergo changes in their oxidation states. This ability plays a crucial role in processes such as biological systems, where metalloproteins containing these metals are active in enzymatic functions.

Chemical Compounds

Chemical compounds are substances formed when two or more elements combine chemically in fixed proportions. They can be simple, like water (H2O), or complex, like \([\mathrm{Cr}(\mathrm{CN})_{6}]^{3-}\). The nature of compounds heavily depends on the types of elements involved and the bonds that form between them. Here are some key aspects of chemical compounds related to transition metals:

• Coordination compounds: Formed when transition metals bond with organic or inorganic ligands, creating complex structures.
• Oxidation states: Determine the compound's reactivity, stability, and color.
• Ionic and covalent character: Transition metals can form both ionic and covalent bonds contributing to diverse compound formation.

Understanding the oxidation states in chemical compounds can provide insights into the compound's overall charge, geometry, and potential reactivity, as seen in the exercise where each compound was analyzed to find the oxidation state of a specific atom.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are chemical reactions involving the transfer of electrons between two substances. These processes are foundational in chemistry, with one substance being oxidized (losing electrons) and another being reduced (gaining electrons). Essential characteristics of redox reactions include:

• Changes in oxidation states: Indicates which substance is oxidized and which is reduced.
• Electron transfer: The core of redox reactions, enabling energy and material transformations in chemical processes.
• Applications in electrochemistry: Such as batteries and electroplating.

In the original exercise, identifying the correct oxidation state of transition metals within compounds involves recognizing which metal atoms participate in redox changes. Chromium (\(\mathrm{Cr}\)), with its multiple oxidation states, acts as a key player in various redox reactions, reflecting its wide applications in catalysis and material synthesis.