Enthalpy is the amount of heat energy absorbed or released during a chemical reaction. The enthalpy of reactants and products can help to predict the nature of the reaction.

When the enthalpy of the reactants is higher than the products, it means that more energy is required to break the bonds between the reactant molecules than that released by forming bonds between the product molecules. This results in a net release of energy during the reaction, which is characteristic of exothermic reactions.

Explanation:

Exothermic reactions are those in which heat is released during the reaction, leading to an increase in the temperature of the surroundings. This occurs because the energy released during the reaction is greater than the energy required to break the bonds between the reactant molecules. Therefore, exothermic reactions have negative enthalpy values, as the enthalpy of the products is lower than that of the reactants.

On the other hand, endothermic reactions are those in which heat is absorbed during the reaction, leading to a decrease in the temperature of the surroundings. This occurs because the energy required to break the bonds between the reactant molecules is greater than the energy released by forming bonds between the product molecules. Therefore, endothermic reactions have positive enthalpy values, as the enthalpy of the products is higher than that of the reactants.

Spontaneity is a measure of the likelihood of a reaction occurring without external intervention. A spontaneous reaction is one that occurs naturally without the need for an input of energy. The spontaneity of a reaction is determined by the change in free energy (ΔG) during the reaction.

Non-spontaneous reactions, on the other hand, require an input of energy to occur. These reactions have a positive ΔG value and are not likely to occur without external intervention.

Conclusion:

In conclusion, when the enthalpy of the reactants is higher than the products, the reaction is exothermic, and it is likely to occur spontaneously without external intervention. Therefore, the correct answer is option D, exothermic.