Problem 62 - A Complete Resource Book in Chemistry for JEE Main

Question

Amongst the following, identify the species with an atom in \(+6\) oxidation state (a) \(\mathrm{MnO}_{4}^{-}\) (b) \(\mathrm{Cr}(\mathrm{CN})_{6}^{3-}\) (c) \(\mathrm{NiF}_{6}{\underline{\phantom{xx}}}^{2-}\) (d) \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

CrO2Cl2 is the species with an atom in +6 oxidation state.

Step 1: Determine Oxidation State in MnO4-

In \( \mathrm{MnO}_{4}^{-} \), let the oxidation state of Mn be \( x \). Oxygen has an oxidation state of \(-2\). The equation is \( x + 4(-2) = -1 \), leading to \( x - 8 = -1 \). Solving for \( x \), we get \( x = +7 \).

Step 2: Determine Oxidation State in Cr(CN)6^3-

In \( \mathrm{Cr}( ext{CN})_{6}^{3-} \), let the oxidation state of Cr be \( x \). Each CN has an oxidation state of \(-1\), so we have \( x + 6(-1) = -3 \). Solving this, we find \( x - 6 = -3 \), and thus \( x = +3 \).

Step 3: Determine Oxidation State in NiF6^2-

In \( \mathrm{NiF}_{6}^{2-} \), let the oxidation state of Ni be \( x \). Each fluorine atom has an oxidation state of \(-1\). The equation is \( x + 6(-1) = -2 \). Solving for \( x \), we get \( x - 6 = -2 \), which means \( x = +4 \).

Step 4: Determine Oxidation State in CrO2Cl2

In \( \mathrm{CrO}_{2} ext{Cl}_{2} \), let the oxidation state of Cr be \( x \). Oxygen has an oxidation state of \(-2\), and chlorine has an oxidation state of \(-1\). Write the equation as \( x + 2(-2) + 2(-1) = 0 \). We get \( x - 4 - 2 = 0 \). Thus, solving for \( x \) gives \( x = +6 \).

Step 5: Identify the species with +6 oxidation state

From the calculations, we see that \( \mathrm{CrO}_{2} ext{Cl}_{2} \) is the compound where the central atom (Cr) has an oxidation state of \(+6\).

Key Concepts

Chromium Compounds

Chromium is a transition metal that is known for forming a variety of compounds, particularly in coordination chemistry. Its versatility stems from its ability to exist in multiple oxidation states. This allows chromium to bind with different ligands, or molecules that donate a pair of electrons. Here are some important points to note about chromium compounds:

• Variety of Oxidation States: Chromium can exhibit oxidation states ranging from +1 to +6. The most common states are +3, +5 and +6.
• Colorful Compounds: Many chromium compounds are brilliantly colored. For instance, potassium dichromate ( K_{2}Cr_{2}O_{7} ) is a vibrant orange.
• Applications: These compounds have applications in different fields, including pigmentation, tanning, and the production of stainless steel.

Understanding chromium's chemistry, particularly its oxidation states, is crucial because it determines how chromium interacts chemically, including the types of compounds it forms.

Oxidation Number Calculation

Determining the oxidation number of an element in a compound is crucial in understanding its reactivity and the type of chemical bonds it forms. It's a hypothetical charge based on the assumption that all bonds are ionic. Here's how you can calculate it:

• Rule 1: The sum of the oxidation states of all atoms in a neutral molecule must be zero. For polyatomic ions, it must equal the charge of the ion.
• Rule 2: Group one metals have an oxidation state of +1, and Group two metals are +2. Fluorine is always -1, oxygen is generally -2, and hydrogen is usually +1.
• Step-by-Step Calculation: Assign known oxidation states first. Then, solve the algebraic equations where unknowns are the oxidation states of the remaining elements.

In the given exercises, we applied these rules to find the correct oxidation states. By considering the charges and known values, we solved for each element in the compound. Practicing these calculations will enhance your ability to approach and interpret complex chemical formulas.

Coordination Chemistry

Coordination chemistry is the study of complex molecules formed by the coordination of metal atoms with ligands. These compounds, known as coordination complexes, play a pivotal role in both biological systems and industrial applications. Here are some core concepts:

• Coordination Number: It is the total number of ligand atoms that bind to the central metal ion. It helps in predicting the geometry of the complex.
• Ligands: These are ions or neutral molecules that donate electron pairs to metal atoms. Common ligands include CN^-, H₂O, and NH₃.
• Examples & Applications: Chlorophyll, responsible for the green color in plants, is a coordination complex with magnesium. Similarly, hemoglobin, which transports oxygen in blood, involves iron coordination.

Coordination chemistry is essential for understanding how metals interact at a molecular level, aiding in the development of novel materials and bioinorganic techniques.