What do u mean by H² and 2H

H2 is molecular hydrogen.it is a molecule of hydrogen that consists of two hydrogen atoms bonded together by one single bond. 2H denotes two moles of elemental hydrogen.it should be noted that elemental hydrogen is not bonded to anything.

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.