Question

During the reaction of some metals with dilute hydrochloric acid, following observations were made. The temperature of the reaction mixture rises when aluminium $(Al)$ is added mainly because:
A) Heat is evolved
B) Due to the catalyst
C) It is an endothermic reaction
D) It is an exothermic reaction

Answer 1

To solve this question, we must first understand the main reason behind the increase in temperature. Then we need to assess the logic in which type of reaction the temperature of mixture rises, and it rises due to evolution of heat during the reaction and then only we can conclude the correct answer.

Complete solution:
Before we move forward with the solution of this given question, let us first understand some basic concepts:
Exothermic Reaction: is a reaction in which energy is released in the form of light or heat. Thus in an exothermic reaction, energy is transferred into the surroundings rather than taking energy from the surroundings as in an endothermic reaction. In an exothermic reaction, change in enthalpy $(\Delta H)$ will be negative.
Therefore, it can be understood that the net amount of energy required to initiate an exothermic reaction is less than the net amount of energy released by the reaction. When a calorimeter, a device used to measure the heat released by a chemical reaction, the net amount of heat energy that flows through the device is equal to the negative of the total energy change of the system.
And hence we can say that The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.

So, clearly we can conclude that the correct answer is Option D.

Note: Uncontrolled exothermic reactions, those leading to fires and explosions, are wasteful because it is difficult to capture the released energy. Nature affects combustion reactions under highly controlled conditions, avoiding fires and explosions, in aerobic respiration so as to capture the released energy, e.g. for the formation of ATP.