Which one of the following is not an endothermic reaction?
A.Combustion of methane
B.Decomposition of water
C.Conversion of graphite to diamond
D.Dehydrogenation of ethane

Hint: Combustion of alkane produces a significant amount of energy and this energy is widely used in daily households work too. On the other hand, reactions involving treating reactants at high temperature are endothermic reactions.

Complete step by step answer:
- The reactions in which heat is released are known as exothermic reactions and the reaction in which heat is utilized are known as endothermic reactions.
- Fuel like hydrocarbons such as alkane when made to react with oxygen yields carbon dioxide and water. This process is exothermic. Energy is released during combustion of hydrocarbons.
- Combustion of methane or in general we can say burning of methane is carried out by treating methane with oxygen in air which results in formation of carbon dioxide and water and great deal of energy. The reaction is as follow: \[{\text{C}}{{\text{H}}_4} + 2{{\text{O}}_2} \to {\text{C}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\].
- Decomposition reaction is the process in which one compound is broken down to form a simpler substance. This breaking of bonds of complex compounds requires energy input. So decomposition reactions are endothermic reactions. For example decomposition of water to hydrogen and oxygen is an endothermic process.
- Graphite and diamond both are the form of carbon. When graphite is subjected to high temperature and high pressure such as more than \[100{\text{Kbar}}\] and \[ > 2000^\circ {\text{C}}\] respectively, graphite is converted to diamond. As this process involves high temperature; this is an endothermic process.
- The energy efficient and safe conversion of ethane to ethylene or dehydrogenation of ethane is possible only by simultaneously endothermic treatment of ethane with steam and oxygen and along with exothermic oxidative dehydrogenation. So the overall process becomes endothermic.

Thus, the correct option is A.

Note:
In biogas plants, methane is produced by the microorganisms which is transported to households and used as fuel, because combustion of alkane such as methane is an exothermic process.