Question

The following is (are) endothermic reaction(s):
A.Combustion of methane
B.Decomposition of water
C.Dehydrogenation of ethane to ethylene
D.Conversion of graphite to diamond

Answer 1

Hint: The reactions which are endothermic in nature have positive enthalpy change and heat is absorbed in the process due to bond breaking. Breaking a bond of stable molecules requires a lot of energy, such as in decomposition and dehydrogenation reactions.

Complete step by step answer:
In a chemical reaction, chemical bonds are either formed or broken. When chemical bonds are formed, heat is released and when chemical bonds break, they require energy so heat is supplied or absorbed. Molecules tend to want to stay together, so formation of chemical bonds between molecules needs less energy as compared to breaking of bonds between molecules, which need more energy and results in heat being absorbed from the surroundings.
Enthalpy of a reaction is an important term here. It is defined as heat energy change that takes place when reactants convert to products. If heat is absorbed during a reaction, its enthalpy change will be positive and it is an endothermic reaction but if heat is released during a reaction, its enthalpy change will be negative and it is an exothermic reaction.
In combustion reaction, methane combines with oxygen molecules and yields carbon dioxide with water. As formation of bonds occurs, heat is released in the reaction and thus it is an exothermic reaction. While in decomposition of water, water molecules break down into hydrogen and oxygen. Breaking down of a molecule requires energy, so heat is absorbed in the process and it is an endothermic reaction.
\[C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O\]
\[2{H_2}O \to 2{H_2} + {O_2}\]
Let us check what happens when ethane is dehydrogenated to ethylene. A molecule of hydrogen is removed and a double bond is formed. It is a complex reaction and the process of dehydrogenation is endothermic in nature. When graphite is converted to diamonds, bonds breaking require a high amount of heat as graphite is stable, and thus it is also an endothermic reaction.
\[{C_2}{H_6} \to {C_2}{H_4} + {H_2}\]
\[{C_{graphite}} \to {C_{diamond}}\]

Hence, the correct options are (B), (C) and (D).

Note:
There are two factors which determine whether a gaseous reaction will be endothermic or not: first, the relative strengths of the bonds i.e., bond enthalpies, and second the relative number of bonds broken and formed.