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) is not a redox reaction.

Step 1: Define Redox Reaction

Redox reactions are reactions where oxidation and reduction occur simultaneously. Oxidation involves the loss of electrons, whereas reduction involves the gain of electrons.

Step 2: Analyze Reaction (a)

Reaction (a) is \( \mathrm{MgCO}_{3} \longrightarrow \mathrm{MgO}+\mathrm{CO}_{2} \). Check the oxidation states: \( \mathrm{Mg} \) remains \( +2 \) in \( \mathrm{MgO} \), and \( \mathrm{C} \) in \( \mathrm{CO}_3^{2-} \) is \( +4 \), and remains \(+4\) in \( \mathrm{CO}_2 \). No change indicates no redox occurs.

Step 3: Analyze Reaction (b)

Reaction (b) is \( \mathrm{O}_{2} + 2 \mathrm{H}_{2} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O} \). \( \mathrm{O}_2 \) changes from \( 0 \) to \( -2 \) (reduction), and \( \mathrm{H}_2 \) from \( 0 \) to \( +1 \) (oxidation), indicating a redox reaction.

Step 4: Analyze Reaction (c)

Reaction (c) is \( \mathrm{Na} + \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NaOH} + \frac{1}{2} \mathrm{H}_{2} \). \( \mathrm{Na} \) changes from \( 0 \) to \( +1 \) (oxidation) and \( \mathrm{H} \) in water changes from \( +1 \) to \( 0 \) (reduction), confirming a redox reaction.

Step 5: Analyze Reaction (d)

Reaction (d) is \( \mathrm{SnCl}_{4} \longrightarrow \mathrm{SnCl}_{2} + \mathrm{Cl}_{2} \). \( \mathrm{Sn} \) changes from \( +4 \) to \( +2 \) (reduction) and \( \mathrm{Cl} \) from \( -1 \) to \( 0 \) (oxidation), so it's a redox reaction.

Step 6: Identify Non-Redox Reaction

Among the given reactions, only reaction (a) does not display any changes in oxidation states, hence it is not a redox reaction.

Key Concepts

Oxidation

Oxidation refers to the process where an atom, ion, or molecule loses electrons. This leads to an increase in its oxidation state. When a substance undergoes oxidation, it often becomes more positively charged or less negatively charged.
For instance, in the reaction of sodium with water (reaction c), sodium ( Na ) loses an electron to form Na^+ . This change from an oxidation state of 0 to +1 is a classic example of oxidation.

• The oxidation process is crucial in many chemical reactions, especially redox reactions where oxidation and reduction occur simultaneously.
• Keep in mind that oxidation doesn't always involve oxygen, despite the term 'oxidation' being derived from reactions involving oxygen.

Understanding oxidation is key in determining whether a reaction is a redox reaction.

Reduction

Reduction is the gain of electrons by an atom, ion, or molecule, resulting in a decrease in its oxidation state. In other words, reduction often results in an increase in negative charge or a decrease in positive charge.
In reaction (b), oxygen reacts with hydrogen to form water. The oxygen molecule ( O_2 ) gains electrons, changing from an oxidation state of 0 to -2, indicating it undergoes reduction.

• Reduction is as essential as oxidation in redox reactions, leading to a balance of electron transfer.
• Unlike oxidation, reduction usually involves a decrease in the amount of oxygen in a compound.

When analyzing chemical reactions, identifying reduction processes can help determine if the reaction is of the redox type.

Oxidation States

Oxidation states, also known as oxidation numbers, indicate the degree of oxidation of an atom in a compound. They serve as a helpful tool to determine whether oxidation or reduction has occurred.
Oxidation states are assigned based on specific rules, such as:

• An atom in its elemental form always has an oxidation state of 0.
• The oxidation state of a monoatomic ion is equivalent to its charge.
• In compounds, hydrogen typically has an oxidation state of +1, and oxygen usually has an oxidation state of -2.

An easy way to identify redox reactions is by tracking changes in these states. Considering reaction (a), the lack of change in oxidation states of the elements involved signifies it is not a redox reaction.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products through the breaking and forming of chemical bonds. They can be classified into several types, including synthesis, decomposition, combustion, and redox reactions.
Redox reactions are particularly significant because they involve the transfer of electrons, leading to changes in oxidation states.

• The energy changes associated with redox reactions are fundamental for processes such as energy production in cells (respiration) and burning fuels.
• Not all reactions are redox, so identifying changes in oxidation states is crucial to classifying the type of chemical reaction occurring.

In the problem discussed, differentiating a non-redox reaction from redox ones required analyzing the electron transfer and oxidation state changes for each reaction.