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: When the enthalpy change of the reaction is positive, the reaction is endothermic.

Explanation: Exothermic reactions are defined as the reactions in which energy of the product is less than the energy of the reactants. The total energy is released in the form of heat and
`\Delta H` for the reaction comes out to be negative.

Endothermic reactions are defined as the reactions in which energy of the product is greater than the energy of the reactants. The total energy is absorbed in the form of heat and
`\Delta H` for the reaction comes out to be positive.

`\Delta H=H_(products)-H_(reactants)`

Answer 2

The correct option is this: WHEN THE ENTHALPY CHANGE OF THE REACTION IS POSITIVE, THE REACTION IS ENDOTHERMIC.
All chemical reactions are divided into two classes, these are exothermic and endothermic reaction. An exothermic reaction is a type of reaction in which energy is lost to the environment, thus the enthalpy change of the reaction is negative. An endothermic reaction is a type of reaction in which energy is absorbed from the environment, thus the enthaply change of the reaction is positive.