Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The oxidation number of carbon in \(\mathrm{CH}_{2} \mathrm{O}\) is (a) \(-2\) (b) \(+2\) (c) 0 (d) \(+4\)

Step-by-Step Solution

Answer

The oxidation number of carbon in \(\mathrm{CH}_{2}\mathrm{O}\) is 0.

Step 1: Recall the Oxidation Number Rules

For organic compounds like formaldehyde, use the following rules to determine oxidation numbers: - Hydrogen is typically assigned an oxidation number of +1. - Oxygen is typically assigned an oxidation number of -2. - The sum of oxidation numbers for a neutral compound is zero.

Step 2: Write the Compound Formula and Set up Equation

The molecular formula for formaldehyde is \(\mathrm{CH}_{2}\mathrm{O}\). Let the oxidation number of carbon be \(x\). The total oxidation number equation based on the formula is: \[x + 2(+1) + (-2) = 0.\]

Step 3: Simplify the Equation

Substitute the known oxidation numbers into the equation:\[x + 2 - 2 = 0.\]Simplify the equation:\[x = 0.\]

Step 4: Identify the Answer

The oxidation number of carbon, which we represented with \(x\), is calculated to be 0. Therefore, the correct answer from the options is (c) 0.

Key Concepts

Oxidation Number Rules

Understanding oxidation number rules is essential when identifying the oxidation state of elements in compounds.
These rules help decipher the electron distribution within the compound structure. Here are some key rules you need to remember:

• Hydrogen usually has an oxidation number of +1 unless it's bonded to metals in binary compounds, where it can be -1.
• Oxygen commonly has an oxidation number of -2. However, in peroxides, it can be -1.
• The oxidation number of a free element (not combined with other elements) is always 0.
• In a neutral compound, the sum of oxidation numbers of all atoms must be zero.

With these rules, we can determine the distribution of electrons among the atoms and how the atoms are bonded.

Formaldehyde

Formaldehyde, with the chemical formula \(\mathrm{CH}_{2}\mathrm{O}\), is one of the simplest aldehydes.
Characterized by a carbon atom double bonded to an oxygen atom and singly bonded to two hydrogen atoms, its structure makes it an ideal candidate for learning about oxidation states in organic compounds. In formaldehyde:

• The carbon atom is bonded to two hydrogens (each contributing an oxidation state of +1).
• The oxygen atom is bonded to the carbon with an oxidation state of -2.

Knowing the molecular structure of formaldehyde helps in assigning oxidation numbers accurately using the rules discussed.
This structure illustrates typical bonding patterns in organic compounds, especially the relationship between carbon and more electronegative elements like oxygen.

Neutral Compound Oxidation Sum

The concept that the sum of oxidation numbers in a neutral compound equals zero is vital in understanding chemical formulas.
This principle ensures that the overall charge of the compound is balanced. For formaldehyde \(x + 2(+1) + (-2) = 0\), we set the oxidation number of carbon to be x:

• Adding the known values of hydrogen and oxygen results in 2(1) + (-2), which simplifies to 0.
• Thus, the equation becomes \(x + 0 = 0\), inevitably showing that x equals 0 for carbon.

This sum-zero rule assists in maintaining charge balance, making it easier to solve for unknown oxidation states in complex molecules.