Question

Which of the following are examples of endothermic reactions?
(Multiple Choice)
A. Combustion of methane
B. Decomposition of water
C. Dehydrogenation of ethane to the ethylene
D. Conversion of graphite to diamond

Answer 1

Chemical reactions in which the reactants absorb heat energy from the surroundings to form products are called endothermic reactions. Since they absorb heat, the temperature of the surroundings is lowered, thereby creating a cooling effect.

Complete step by step answer:
Decomposition of water – The decomposition of water in given by the equation:
${{H}_{2}}O\to {{H}_{2}}+\dfrac{1}{2}{{O}_{2}}$

The decomposition of water requires 285.8 kJ energy per mole of water decomposed. It is an endothermic reaction. Heat is required to break the chemical bonds between hydrogen and oxygen molecules of water.
Dehydrogenation of ethane to ethylene – The dehydrogenation of ethane is given by the equation:
${{C}_{2}}{{H}_{6}}\to {{C}_{2}}{{H}_{4}}+{{H}_{2}}$

Here, energy is required to break the C-H bond in ethane to convert it to ethylene. Since energy is absorbed, this is an example of endothermic reaction.
Conversion of graphite to diamond – Graphite is more stable than diamond. Hence, energy is required for graphite to convert to diamond. Hence, this is an example of endothermic reaction.
So, the correct answer is “Option B,C and D”.

Note: Combustion of methane – Combustion of methane is an exothermic reaction and is given by the following equation:
$C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O$

Here, methane burns in the presence of oxygen to give carbon dioxide and water. This process of combustion releases energy. Hence, exothermic reactions are those chemical reactions that release energy. So, option A is an exothermic reaction.