A reaction is endothermic if the

Step 1: Analyze the question We need to identify the statement that correctly describes an endothermic reaction.

Step 2: The concept

• An endothermic reaction absorbs heat from its surroundings. ΔH is positive.
• An exothermic reaction releases heat to its surroundings. ΔH is negative.

Because an endothermic reaction takes in heat, the surroundings (including the reaction vessel) lose heat and feel cool.

Step 3: Check each option

• A. Reaction vessel feels cool during the reaction. Correct. Heat flows from the surroundings into the reaction, so the vessel feels cold. Dissolving ammonium nitrate in water is a familiar example.
• B. Enthalpy change is negative. Wrong. A negative ΔH means exothermic.
• C. Bond forming energy exceeds bond breaking energy. Wrong. If more energy is released forming bonds than is absorbed breaking them, the net result is heat released, which is exothermic. For endothermic reactions, bond breaking energy exceeds bond forming energy.
• D. Heat of formation of reactants exceeds heat of formation of products. Wrong. ΔH = ΣΔHf(products) − ΣΔHf(reactants). If the reactants' heat of formation is larger, ΔH is negative, which is exothermic.

Step 4: Answer The correct option is A.

Exam tip: Remember "ENdothermic = ENters" (heat enters the system, ΔH is +, surroundings feel cold). Exothermic means heat exits, ΔH is −, and the surroundings feel hot.