Which of the following statements about endothermic reaction is correct?

Step 1: Analyze the question We need the statement that is always true of an endothermic reaction.

Step 2: The concept An endothermic reaction takes in heat from its surroundings. The products have more energy than the reactants, so ΔH is positive (ΔH > 0). The surroundings usually feel cooler, for example when ammonium nitrate dissolves in water.

Step 3: Check each option

• A. Activation energy is high. Not always true. Activation energy depends on the reaction, and many exothermic reactions have a high activation energy too. It does not define an endothermic reaction.
• B. A catalyst is required. False. A catalyst only speeds up a reaction and has nothing to do with whether it is endothermic.
• C. It occurs reversibly. Not a defining feature. Some endothermic reactions are reversible, but many are not, and many exothermic reactions are reversible as well.
• D. Heat energy is absorbed. True. This is the definition of an endothermic reaction.

Step 4: Answer

Tip: "Endo" means "into", so an endothermic reaction takes heat in. "Exo" means "out of", so an exothermic reaction gives heat out. In exams, choose the option that gives the definition rather than one that only sometimes applies.