Question

how is enthalpy used to predict whether a reaction is endothermic or exothermic?
when the enthalpy change of the reaction is positive, the reaction is endothermic.
when the enthalpy change of the reaction is positive, the reaction is 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.

Explanation:

To determine the correct statement, recall enthalpy concepts:

• Endothermic: $\Delta H = H_{products} - H_{reactants} > 0$ (heat absorbed, products have higher enthalpy than reactants).
• Exothermic: $\Delta H < 0$ (heat released, products have lower enthalpy than reactants).

Analyze each option:

1. Top - left: "When the enthalpy change of the reaction is positive, the reaction is endothermic." → Correct (matches $\Delta H > 0$ for endothermic).
2. Top - middle: "Positive $\Delta H$ = exothermic" → Incorrect (exothermic has $\Delta H < 0$).
3. Top - right: "Reactants’ enthalpy > products’ → endothermic" → Incorrect (reactants > products means $\Delta H = products - reactants < 0$, exothermic).
4. Bottom: "Products’ enthalpy > reactants’ → exothermic" → Incorrect (products > reactants means $\Delta H > 0$, endothermic).

Answer:

Top - Left Box (When the enthalpy change of the reaction is positive, the reaction is endothermic.)