Concept:The letter y on the reaction profile shows the activation energy of the reaction.Explanation:On an energy profile diagram, the reactants start at a lower energy level.The reaction must first climb to a maximum energy peak, called the transition state.y is the vertical energy gap between the reactants and this peak.This energy gap is the minimum energy needed for the reaction to occur, which is the activation energy.It is not the overall heat change, so it does not represent an endothermic or exothermic reaction.Answer:B. activation energy
