Question

Which of the following are combustion reactions. This question has multiple correct options
A. \[C{H_4}{\text{ + }}2{O_2}{\text{ }} \to {\text{ }}C{O_2}{\text{ }} + {\text{ }}2{H_2}O\]
B. \[{C_2}{H_5}{\text{OH + }}2{O_2}{\text{ }} \to {\text{ 2}}C{O_2}{\text{ }} + {\text{ 3}}{H_2}O\]
C. \[CO{\text{ }} + {\text{ }}{0_2}{\text{ }} \to {\text{ }}C{O_2}\]
D. \[2C{H_4}{\text{ }} + {\text{ }}{0_2}{\text{ }} \to {\text{ }}2C{H_3}OH\]

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 a stable molecule requires a lot of energy, such as 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 the formation of chemical bonds between molecules need less energy as compared to the breaking of bonds between molecules, which need more energy and results in heat being absorbed from the surroundings.
In a combustion reaction, the compound combines with oxygen molecules and yields carbon dioxide with water. As the formation of bonds occurs, heat is released in the reaction and thus it is an exothermic reaction.
 \[C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O\]
 \[{C_2}{H_5}{\text{OH + }}2{O_2}{\text{ }} \to {\text{ 2}}C{O_2}{\text{ }} + {\text{ 3}}{H_2}O\]
In the case of carbon monoxide, which is inflammable compounds get burns in presence of oxygen and produce carbon dioxide as follows,
 \[CO{\text{ }} + {\text{ }}{0_2}{\text{ }} \to {\text{ }}C{O_2}\]

So, the correct options are, A, B, C.

Additional information:
While in the decomposition of water, the water molecule breaks 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.
 \[2{H_2}O \to 2{H_2} + {O_2}\]

Note:
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.