When I studied chemistry in high school, I learnt that if the change of enthalpy of a reaction ΔH > 0 , the reaction is endothermic, and if ΔH<0, it is exothermic. However in thermodynamic class, I learnt: $$ ΔG = ΔH - TΔS $$ For a reaction of a battery, the data reads ΔG = -394kJ/mol. (which is also the electrical work by the battery), ΔH = -316kJ/mol. The book then said that the difference (78kJ/mol.) comes from absorbing heat from the environment. So is this an endothermic reaction? However, ΔH<0. Is the system in fact absorbing heat or releasing heat? Thanks!