Problem 11 - Chemistry in 30 days

Question

The oxidation number of C in HCN and HNC respectively are (a) \(+2,+2\) (b) \(+2,+4\) (c) \(+4,+4\) (d) \(-2,-2\)

Step-by-Step Solution

Answer

The correct answer is (a) \(+2,+2\).

Step 1: Understand the Problem

We need to find the oxidation number of carbon in two different compounds: HCN and HNC, and provide an option from the given choices. Oxidation numbers are a way of keeping track of electron transfer in chemical reactions.

Step 2: Apply Known Oxidation Rules

Carbon, hydrogen, and nitrogen need to have specific oxidation states. We know that hydrogen usually has an oxidation state of +1. Nitrogen is more electronegative and typically takes -3 in compounds, especially when bonded to carbon.

Step 3: Calculate Oxidation Number in HCN

For HCN:- Let the oxidation number of carbon (C) be x.- H has an oxidation number of +1.- N has an oxidation number of -3.Using the formula: sum of oxidation numbers = 0 for a neutral molecule,\[ +1 + x - 3 = 0 \]Solving for x gives:\[ x - 2 = 0 \]\[ x = +2 \]

Step 4: Calculate Oxidation Number in HNC

For HNC:- Follow the same approach as HCN.- H has +1 and N has -3.- Let the oxidation number of carbon (C) be y.\[ +1 - 3 + y = 0 \]Solve for y:\[ y - 2 = 0 \]\[ y = +2 \]

Step 5: Compare Results with Options

From the calculations, the oxidation number of carbon in both HCN and HNC is +2. Thus, option (a) "+2,+2" is the correct answer.

Key Concepts

HCN

Hydrogen Cyanide (HCN) is a simple molecule consisting of a hydrogen atom, a carbon atom, and a nitrogen atom. It is a linear molecule with the structure H-C≡N. Understanding its constituents is essential.

• **H (Hydrogen):** Generally has an oxidation state of +1 in compounds.
• **C (Carbon):** The oxidation state can vary and is what needs to be determined.
• **N (Nitrogen):** Usually takes an oxidation state of -3 due to its higher electronegativity compared to carbon.

In this molecule, by applying the rule that the sum of oxidation numbers in a neutral molecule is zero, we deduce that the oxidation number of carbon is found by solving this equation: \[ +1 + x - 3 = 0 \]Solving for \(x\), we find that the oxidation number of carbon is +2.

HNC

Isocyanic acid (HNC) is another form of the cyanide ion and is an isomer of HCN. Even though the elements involved are the same, the arrangement is slightly different, leading to different chemical properties but similar oxidation numbers.

• In **HNC**, hydrogen is still +1.
• Nitrogen, being more electronegative than carbon, remains -3.
• We are to solve for carbon's oxidation state next, denoted as \(y\).

Using the same rule, sum of oxidation numbers equals zero, for HNC:\[ +1 + y - 3 = 0 \]Solving this equation leads to an oxidation number of +2 for carbon, similar to HCN. It reiterates that the oxidation state mechanism through rearrangement maintains a consistent outcome for carbon in both molecules.

oxidation state rules

Oxidation state rules are a set of guidelines to help determine how electrons are distributed in molecules and ions. Here are some basics to remember:

• **For a pure element**, the oxidation state is always 0 (e.g., \(N_2\), \(O_2\)).
• **For a simple ion**, it is the charge of the ion (e.g., Na\(^+\) has an oxidation state of +1).
• **Hydrogen** generally has an oxidation number of +1, except when bonded to metals in hydrides.
• **Oxygen** usually has an oxidation state of -2, except in peroxides where it's -1.
• **The sum of oxidation states** for all atoms in a neutral compound is 0 and equals the charge of the ion in ionic compounds.

These rules are invaluable for identifying how atoms are gaining, losing, or sharing electrons in a reaction.