Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not an endothermic reaction? (a) combustion of methane (b) decomposition of water (c) dehydrogenation of ethane or ethylene (d) conversion of graphite to diamond

Step-by-Step Solution

Answer

The combustion of methane (option a) is not an endothermic reaction.

Step 1: Understanding Endothermic Reactions

Endothermic reactions are chemical reactions that absorb energy from their surroundings. In these reactions, the system takes in energy, usually in the form of heat, resulting in a product that is higher in energy than the reactants.

Step 2: Analyzing Combustion of Methane

Combustion reactions typically release energy, making them exothermic. Specifically, when methane ( ext{CH}_4) combusts with oxygen, it produces carbon dioxide and water, releasing energy in the process. Hence, methane combustion is not endothermic.

Step 3: Exploring Decomposition of Water

Decomposition of water into hydrogen and oxygen requires energy input, usually in the form of electrolysis. This process absorbs energy, making it an endothermic reaction.

Step 4: Investigating Dehydrogenation of Ethane or Ethylene

Dehydrogenation involves removing hydrogen molecules from ethane or ethylene, which generally requires an input of energy to break the molecular bonds. This absorption of energy indicates that the process is endothermic.

Step 5: Examining Conversion of Graphite to Diamond

The conversion of graphite to diamond requires energy as it involves rearranging carbon atoms into a different, more ordered lattice structure. This process absorbs energy, exemplifying an endothermic reaction.

Step 6: Identifying the Non-Endothermic Reaction

Among the given options, the combustion of methane is the only reaction that releases energy instead of absorbing it, thus being the non-endothermic reaction.

Key Concepts

Combustion of Methane

The combustion of methane is a chemical reaction where methane (\( \text{CH}_4 \)) reacts with oxygen to produce carbon dioxide and water. This reaction is well-known for its energy-releasing property.
When methane combusts, it combines with oxygen, undergoing oxidation. The energy released during this reaction comes from breaking the carbon-hydrogen bonds in methane and forming new bonds in the carbon dioxide and water. This release of energy as heat is why combustion reactions are classified as exothermic.

• Reaction: \( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \)
• Energy Outcome: Releases energy

Despite the release of energy, uncontrolled methane combustion can be hazardous, leading to explosions. Hence, understanding its exothermic nature helps control and harness the energy safely in applications like cooking and heating.

Decomposition of Water

The decomposition of water is a fascinating endothermic process, which involves breaking down water into hydrogen and oxygen gases.
To achieve this, energy input is necessary, usually derived from electrical energy, a process known as electrolysis. During electrolysis, electricity is passed through water, breaking the bonds between hydrogen and oxygen atoms.

• Reaction: \( 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2 \)
• Energy Outcome: Absorbs energy

This process is considered endothermic because the energy needed to separate the molecules is greater than the energy initially present in the water. Thus, it demonstrates how energy can be absorbed to facilitate a chemical change, which is crucial in applications like hydrogen fuel production.

Dehydrogenation of Ethylene

Dehydrogenation of ethylene is an endothermic process that involves the removal of hydrogen molecules from ethylene (\( \text{C}_2\text{H}_4 \)).
Removing hydrogen atoms requires an energy supply to break the strong carbon-hydrogen bonds. This energy requirement is why dehydrogenation is classified as an endothermic reaction.

• Reaction: \( \text{C}_2\text{H}_4 \rightarrow \text{C}_2\text{H}_2 + \text{H}_2 \)
• Energy Outcome: Absorbs energy

This process is significant in the chemical industry for producing acetylene and other important compounds, emphasizing the need to understand how energy input can drive molecular transformations.

Conversion of Graphite to Diamond

The transformation from graphite to diamond is an intriguing example of an endothermic reaction. Both graphite and diamond are allotropes of carbon, but they differ in atomic structure.
In this reaction, carbon atoms need to be rearranged from the layered structure of graphite into the more compact, tetrahedral arrangement found in diamonds. This structural conversion necessitates a significant input of energy.

• Process: Graphite \( \rightarrow \) Diamond
• Energy Outcome: Absorbs energy

The conversion is typically achieved under high temperature and pressure conditions, akin to those found deep within the Earth. This energy absorption required for structural reformation underlines why this process is categorized as endothermic.