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

Short answer

The combustion of methane is not an endothermic reaction.

Step-by-step solution

Understanding Endothermic Reactions

Endothermic reactions are chemical reactions that absorb energy from the surroundings, often in the form of heat. This kind of reaction requires energy input to proceed.

Analyzing Option (a): Combustion of Methane

Combustion reactions, like the combustion of methane, release energy in the form of heat and light. This process converts methane and oxygen into carbon dioxide and water, releasing energy, hence it is exothermic.

Analyzing Option (b): Decomposition of Water

Decomposition of water into hydrogen and oxygen requires energy input, typically in the form of electricity, making this reaction endothermic.

Analyzing Option (c): Dehydrogenation of Ethane or Ethylene

Dehydrogenation reactions involve the removal of hydrogen from a molecule, which typically requires an input of energy to break the chemical bonds, qualifying them as endothermic reactions.

Analyzing Option (d): Conversion of Graphite to Diamond

The conversion of graphite to diamond involves re-arranging carbon atoms into a different chemical structure. This process requires significant energy input, making it endothermic.

Conclusion

Based on the analysis, only option (a) involves the release of energy rather than absorption.

Key concepts

Combustion Reaction

Combustion reactions are typically characterized by their exothermic nature, meaning they release energy. These reactions involve a fuel, like methane, reacting with oxygen to produce carbon dioxide, water, and energy in the form of heat and often light. For example, the combustion of methane can be expressed through the chemical equation: \[\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{Energy}\]It's crucial to note that while combustion reactions are primarily known for their energy release, they also support important applications in heating and powering engines. The energy release in combustion reactions is what makes them suitable for such applications, as they provide both heat and mechanical energy efficiently. Thus, combustion reactions are a key focus in studying exothermic processes.

Decomposition Reaction

Decomposition reactions break down a compound into simpler substances. One of the most common examples is the decomposition of water into hydrogen and oxygen. This process is represented by the chemical equation:\[2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2\]Such reactions require an input of energy, making them endothermic. This energy is often supplied in the form of electricity, as seen in the electrolysis of water. During this process, electrical energy helps break the chemical bonds within water molecules. Decomposition reactions are integral to various scientific and industrial processes, including the extraction of metals from ores and the generation of oxygen for medical and industrial uses. The requirement for energy input distinguishes them from exothermic reactions.

Dehydrogenation Reaction

Dehydrogenation reactions involve the removal of hydrogen from a molecule, necessitating energy to break chemical bonds. These reactions are usually endothermic. For instance, the dehydrogenation of ethane can be represented as:\[\text{C}_2\text{H}_6 \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\]In this process, energy is required to separate hydrogen atoms from the parent molecule, typically supplied as heat. Dehydrogenation reactions are crucial in producing unsaturated hydrocarbons from saturated ones, a fundamental process in petrochemical industries. They facilitate the creation of important materials and chemicals by introducing or increasing unsaturation in molecules. This makes dehydrogenation a vital method in chemical synthesis and production.

Conversion of Graphite to Diamond

The conversion of graphite to diamond is a fascinating example of an endothermic process. This transformation involves reorganizing carbon atoms from a hexagonal to a more tightly packed cubic structure. The process demands a significant input of energy, usually in the form of high pressure and temperature. This energy input allows carbon atoms to overcome the energy barrier needed to achieve the stable and ordered diamond structure. The equation describing this conversion under extreme conditions is not simple but reflects a substantial thermodynamic challenge. Graphite's atoms realign to create diamond, providing insight into materials science and helping improve synthetic diamond production. Understanding this conversion is vital in the fields of geology and synthetic materials, where such reactions influence the creation and manipulation of materials with unique properties.