Enthalpy is a very energetic topic to talk about!
Why? Well Enthalpy is the heat energy that is stored in a chemical system. As this can't be directly measured we instead measure an enthalpy change.
So what is an enthalpy change? Well the enthalpy change is the heat energy exchange with the surroundings at constant pressure.
If you've study physics or chemistry then you'll know that energy must be conserved at all time. In fact the first rule of thermodynamics states that energy may be exchanged between a chemical system and the surroundings but the total energy remains constant.
Exothermic vs. endothermic
Every reaction is either exothermic or endothermic. This means that the reacting chemicals either gain or lose energy during the reaction.
If the reacting chemicals lose energy then the reaction is exothermic (gives off heat) this energy loss is balenced by the energy gain to the surroundings which causes a rise in temperature. The enthalpy change for these reactions are negative.
However if the reacting chemicals gain energy then the reaction is endothermic (takes in heat). The energy gain is ballenced by the energy lost from the surroundings thus why there is a fall in temperature. The enthalpy change for these reactions are positive…