Which of the following are examples of endothermic reactions?

• A. Combustion of methane
• B. Decomposition of water
• C. Dehydrogenation of ethane to the ethylene
• D. Conversion of graphite to diamond

To determine which of the given reactions are endothermic, we first need to understand what an endothermic reaction is. An endothermic reaction is one that absorbs heat from its surroundings, resulting in a decrease in temperature of the environment. Let's analyze each option to see which ones fit this definition.

Analyzing Each Reaction

• A. Combustion of methane: This reaction releases energy in the form of heat and light when methane reacts with oxygen to produce carbon dioxide and water. Since it releases heat, it is an exothermic reaction, not endothermic.
• B. Decomposition of water: This reaction involves breaking down water into hydrogen and oxygen gases. It requires energy input, typically in the form of electricity (electrolysis), to break the bonds in water molecules. Therefore, this is an endothermic reaction.
• C. Dehydrogenation of ethane to ethylene: This reaction involves the removal of hydrogen from ethane to form ethylene. It also requires energy to break the C-H bonds in ethane, making it an endothermic process.
• D. Conversion of graphite to diamond: This transformation requires a significant amount of energy to rearrange the carbon atoms from a planar structure (graphite) to a three-dimensional structure (diamond). Thus, this reaction is also endothermic.

Summary of Findings

From our analysis, we can conclude that the endothermic reactions among the options provided are:

• B. Decomposition of water
• C. Dehydrogenation of ethane to ethylene
• D. Conversion of graphite to diamond

In contrast, option A, the combustion of methane, is exothermic. Understanding these concepts helps in grasping the broader implications of energy changes in chemical reactions, which is fundamental in fields like chemistry, environmental science, and engineering.