Endothermic and Exothermic Changes

Objective: Distinguish net energy change from reaction rate and barriers.

An exothermic reaction transfers energy to surroundings under the stated conditions, while an endothermic process absorbs energy. An energy diagram can show reactant and product energy levels and a higher transition-state barrier; the barrier height relates to activation energy, not the net difference. Catalysts lower activation energy for a pathway and can increase reaction rate without changing the equilibrium energy difference between starting and ending states. Temperature, concentration, surface area, and catalysts can change rates in context. A faster reaction is not necessarily more exothermic. Energy conservation includes reacting system plus surroundings, and a temperature change alone needs a controlled context for interpretation.

Objective: Apply endothermic and exothermic changes to calculations, evidence, and specified situations.

At constant pressure, heat exchanged by the reacting system is associated with enthalpy change under the stated conditions. Exothermic reactions have negative ΔH for the system; endothermic reactions have positive ΔH. Calorimetry estimates heat using q=mcΔT for a calibrated surrounding solution, with the reaction’s heat approximately the opposite sign if other transfers are negligible. Bond-energy estimates compare energy absorbed breaking bonds with energy released forming bonds. Molar reaction energy scales with the reaction amount as written.