International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What would indicate that a reaction is endothermic when considering bond energies?

1. The energy required to break bonds is less than the energy released

2. The energy required to break bonds is equal to that released

3. The energy required to break bonds is more than the energy released

4. The reaction occurs spontaneously without energy input

The correct answer is: The energy required to break bonds is more than the energy released

In an endothermic reaction, the energy absorbed to break bonds in the reactants is greater than the energy released during the formation of bonds in the products. This results in an overall absorption of energy from the surroundings, usually in the form of heat. When we analyze the situation quantitatively, if the energy required to break the bonds exceeds the energy released, it means that the reaction requires an input of energy to proceed. This is characteristic of an endothermic process, as it draws energy from the environment, leading to a decrease in temperature around the reaction site. The other scenarios do not indicate an endothermic reaction. If the energy required to break bonds is less than the energy released, the reaction would be exothermic, releasing energy to the surroundings. If the energy required to break bonds is equal to that released, there would be no net energy change, and the reaction would be considered energetically neutral. Lastly, if a reaction occurs spontaneously without energy input, it also does not align with an endothermic process since such reactions typically require energy. Thus, the correct indication of an endothermic reaction, based on bond energies, is indeed when the energy needed to break bonds exceeds the energy released, confirming the choice