Question

Which one of the following is an endothermic reaction?
A. Combination of carbon and oxygen to form carbon monoxide.
B. Combination of nitrogen and oxygen to form nitrogen monoxide.
C. Combination of glucose and oxygen to form carbon dioxide and water.
D. Combination of zinc and hydrochloric acid to form zinc chloride and hydrogen.

Answer 1

Endothermic reactions can be defined as the reactions in which reactants absorb heat from the surroundings for the formation of products. In this type of reaction, there is the formation of the chemical bond. The value of entropy ($\Delta S$) is considered to be decreasing in the surroundings. If we talk about the enthalpy ($\Delta H$), its value is positive.

Complete answer:

Combination of carbon and oxygen to form carbon monoxide.
When there is the combustion of carbon and oxygen, it can form two products that are carbon -dioxide and carbon monoxide. But carbon dioxide is formed when there is complete combustion of carbon whereas carbon monoxide is formed when combustion is incomplete.
The reaction can be represented as follows:
$C(s) + {O_2}(g) \to C{O_2}(g) + heat$
$2C(s) + {O_2}(g) \to 2CO$
Here, the heat is released.
Combination of nitrogen and oxygen to form nitrogen monoxide
The reaction can be represented as:
${N_2} + {O_2} \to 2NO$
This reaction occurs when nitrogen and oxygen are heated at very high temperatures like ${3000^\circ }C$. We can say that heat is absorbed in this reaction.
Combination of glucose and oxygen to form carbon dioxide and water.
The above mentioned combination is considered to be the photosynthesis process. It is a natural process. The reaction can be represented as:
${C_6}{H_{12}}{O_6} + 6{O_2} \to 6C{O_2} + 6{H_2}O + Heat$
Thus, heat is released in the process.
Combination of zinc and hydrochloric acid to form zinc chloride and hydrogen.
The reaction can be written as follows:
$2Zn + 2HCl \to 2ZnC{l_2} + {H_2}$
In this reaction, heat is released.
Thus, we can conclude that the combination of nitrogen and oxygen to form nitrogen monoxide is an example of an endothermic reaction.

So, the correct answer is Option B.

Note:
- The reactions are classified based on heat released and heat evolved into the two categories as:

Endothermic reaction
Exothermic reaction
- We already discussed the endothermic reaction. Now, exothermic reactions are the reactions in which heat is released into the surroundings. The value of entropy increases whereas the value of enthalpy is negative in this case. For example Combustion or burning of fuel.