A Level Chemistry OCR Practice Exam

What is an endothermic reaction?

1. A reaction that releases heat to the surroundings

2. A reaction where enthalpy of products is less than reactants

3. A reaction that absorbs heat from the surroundings

4. A reaction that occurs at high temperature only

The correct answer is: A reaction that absorbs heat from the surroundings

An endothermic reaction is characterized by the absorption of heat from the surroundings, leading to a decrease in the temperature of the nearby environment. During this process, the system requires energy, typically in the form of heat, to proceed. This heat absorption is a fundamental aspect of endothermic reactions, which is essential for breaking bonds or overcoming activation energy barriers that facilitate the transformation of reactants into products. In the context of the other options, the first choice defines an exothermic reaction rather than an endothermic one, as it describes a process that releases heat. The second option describes a situation commonly associated with exothermic reactions, where the total enthalpy of the products is lower than that of the reactants. The last choice suggests that endothermic reactions can only happen at high temperatures, which is misleading—endothermic reactions can occur at various temperature conditions, depending on the specific reactions and their reactants. Understanding that endothermic reactions absorb energy is critical for grasping how many chemical and physical processes take place in nature and industry. This knowledge is particularly relevant in fields like thermodynamics, where the energy changes during reactions are central concepts.