When a reaction is endothermic,

Analyze the question: It asks what is true of an endothermic reaction, which is one that absorbs heat from its surroundings.

Concept:

• Endothermic: heat is absorbed, so the system gains energy.
• ΔH = H(products) − H(reactants)
• For an endothermic reaction, the products hold more heat energy than the reactants, so ΔH is positive.
• For an exothermic reaction, the opposite is true: ΔH is negative.

Checking each option:

• A. ΔH is negative. Wrong. This describes an exothermic reaction. Endothermic ΔH is positive.
• B. Heat content of a product is less than that of a reactant. Wrong. This also describes an exothermic reaction, where energy is released.
• C. Heat content of reactants is less than the heat content of products. Correct. The reactants absorb heat, so the products end up with more heat content, which gives ΔH > 0.
• D. The reaction is non-spontaneous. Not necessarily true. Some endothermic reactions are spontaneous, for example dissolving ammonium nitrate in water or the evaporation of water. Spontaneity depends on ΔG = ΔH − TΔS, not on ΔH alone.

Tip: Remember "ENdothermic = ENergy ENters." The reactants start low, absorb energy, and finish with the products higher, so ΔH is positive. On an energy profile diagram, the products line sits above the reactants line.