Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not a redox reaction? (a) \(\mathrm{MgCO}_{3} \longrightarrow \mathrm{MgO}+\mathrm{CO}_{2}\) (b) \(\mathrm{O}_{2}+2 \mathrm{H}_{2} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{Na}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NaOH}+1 / 2 \mathrm{H}_{2}\) (d) \(\mathrm{SnCl}_{4} \longrightarrow \mathrm{SnCl}_{2}+\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

Reaction (a) \( \mathrm{MgCO}_{3} \longrightarrow \mathrm{MgO}+\mathrm{CO}_{2} \) is not a redox reaction.

Step 1: Identify Redox Reactions

A redox (reduction-oxidation) reaction involves the transfer of electrons between two species. This is usually identified by changes in the oxidation states of the elements involved in the reaction.

Step 2: Analyze Reaction (a)

In reaction (a): \( \mathrm{MgCO}_{3} \longrightarrow \mathrm{MgO}+\mathrm{CO}_{2} \), no change in oxidation states occurs for magnesium, carbon, or oxygen. This suggests it is not a redox reaction.

Step 3: Analyze Reaction (b)

In reaction (b): \( \mathrm{O}_{2}+2 \mathrm{H}_{2} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O} \), the oxidation state of oxygen changes from 0 to -2 and hydrogen changes from 0 to +1. This indicates a redox process.

Step 4: Analyze Reaction (c)

In reaction (c): \( \mathrm{Na} + \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NaOH} + \frac{1}{2} \mathrm{H}_{2} \), sodium goes from oxidation state 0 to +1, and hydrogen in water goes from +1 to 0 in hydrogen gas. This is a redox reaction.

Step 5: Analyze Reaction (d)

In reaction (d): \( \mathrm{SnCl}_{4} \longrightarrow \mathrm{SnCl}_{2}+\mathrm{Cl}_{2} \), the oxidation state of tin changes from +4 to +2 and chlorine from -1 to 0. This is a redox reaction.

Step 6: Final Decision

After analyzing each reaction, it is clear that reaction (a) does not involve any change in oxidation state, meaning it is not a redox reaction.

Key Concepts

Oxidation States

One of the most important concepts in chemistry is the oxidation state, often referred to as oxidation number. It is used to describe the degree of oxidation (loss of electrons) of an atom in a chemical compound. By assigning oxidation states, we can trace how electrons are transferred in a reaction.
A change in the oxidation number of an atom indicates that a redox reaction is occurring. The general rule is:

• An increase in oxidation state suggests oxidation (loss of electrons).
• A decrease suggests reduction (gain of electrons).

In any chemical reaction, the sum of the oxidation states of the elements involved is consistent before and after the reaction, because electrons are conserved. This makes determining the oxidation state a systematic method for identifying redox reactions.
For example, in reaction (b) from our exercise, the oxidation state of oxygen changes from 0 to -2, while that of hydrogen changes from 0 to +1, reflecting a redox reaction.

Electron Transfer

Electrons play a crucial role in chemical reactions, especially in redox processes. Understanding electron transfer is key to grasping how atoms or molecules lose and gain electrons to form new compounds.
In redox reactions, electron transfer occurs between two substances:

• The substance that loses electrons is oxidized.
• The one that gains electrons is reduced.

These reactions can be identified because they involve changes in oxidation states, as shown by the electron flow. For example, in reaction (c) from our exercise, sodium (\(\)Na) loses an electron, moving from an oxidation state of 0 to +1 (oxidation), while hydrogen in water (\(\)H\(_{2}\)O) is reduced as it gains electrons, causing its oxidation state to go from +1 to 0 in hydrogen gas (\(\)H\(_{2}\)) as a product.These processes of electron exchange are what drive the myriad of complex chemical reactions in both living systems and the environment.

Chemical Reactions Analysis

Analyzing chemical reactions involves breaking down the steps and understanding how reactants transform to products. This process requires attention to the balanced equation, oxidation states, electron transfer, and the types of reaction occurring.
Step-by-step analysis can help ensure accurate identification of redox reactions, which is crucial for determining how oxidation and reduction processes are coupled.
When approaching a chemical reaction analysis like the ones in our exercise, follow these key steps:

• Identify the oxidation states of each element involved in the reactants and products.
• Determine whether there are changes in these states, which signals a redox reaction.
• Confirm that even if oxidation states change, the equation should be balanced with respect to both mass and charge.

For example, in reaction (a) ( \mathrm {MgCO}\(_{3}\) \longrightarrow \mathrm {MgO}+\mathrm {CO}\(_{2}\)), no change in oxidation states for magnesium, carbon, or oxygen confirms it is not a redox reaction.
Comprehensive analysis of chemical reactions not only clarifies the mechanisms of redox processes but also enriches our understanding of chemistry as a whole.