Question

A reaction that occurs only when heat is added is best described as:
A. Exothermic
B. Endothermic
C. An equilibrium process
D. Spontaneous
E. Non-spontaneous

Answer 1

Hint: Chemical reaction is a process in which one or more substances are converted to more different substances. Substances may be either chemical elements or compounds. Some reactions may involve energy changes, like absorption of heat energy etc.

Complete step by step answer:
An endothermic reaction is a chemical reaction in which more energy is needed to break bonds in the reactants than the energy released when new bonds are formed in the form of products. These are basically the reactions in which the reactants absorb heat energy from the surroundings to form products. This reaction occurs only when the heat is added.
Moreover, these reactions lower the temperature of their surrounding area, thereby creating a cooling effect.

Now, in the given picture, the energy tied up in the molecular bonds is greater in the products than in the reactants. However, there is a need for the activation energy to commence the reaction but, once that hurdle is overcome, the reaction proceeds resulting in the formation of product C and the resulting product has more bond energy than the reactants. Some examples of endothermic reactions are melting of ice to form water, sublimation of solid $C{O_2}$ , baking of bread, evaporation of liquid water to form water vapor etc.

Hence, option B is correct.

Note: For exothermic reactions, the potential energy of the product is generally lower than that of the reactants. On the other hand, the potential energy of the product in an endothermic reaction is higher than that of the reactants.