Question

How is enthalpy used to predict whether a reaction is endothermic or exothermic?
When the enthalpy of the reactants is higher than the enthalpy of the products, the reaction is endothermic.
When the enthalpy of the products is higher than the enthalpy of the reactants, the reaction is exothermic.
When the enthalpy change of the reaction is positive, the reaction is exothermic.
When the enthalpy change of the reaction is positive, the reaction is endothermic.

Answer 1

Answer which are correct:

- Statement 1: False. If the enthalpy of the reactants is higher than the enthalpy of the products, ΔH = H_products − H_reactants is negative, so the reaction is exothermic (releases heat).

- Statement 2: False. If the enthalpy of the products is higher than that of the reactants, ΔH is positive, so the reaction is endothermic (absorbs heat).

- Statement 3: False. A positive enthalpy change means the reaction absorbs heat (endothermic), not releases it.

- Statement 4: True. A positive ΔH corresponds to an endothermic reaction.

In short: ΔH = H_products − H_reactants. ΔH < 0 → exothermic (heat released). ΔH > 0 → endothermic (heat absorbed).