What is a double displacement reaction?

A double displacement reaction is a type of chemical reaction where two compounds react, and positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

Na2S+2HCl→2NaCl+H2S

hope it's helpful :)

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.