First Law of Thermodynamics:
1. Heat is a flow of NRG
2. NRG is conserved.
Consequently, if there is a flow of heat during a rxn, the NRG of the reacting substances must change
If heat flows in, NRG increases (delta H is +), endothermic. If heat flows out (warms things up), delta H is -, exothermic, energy decreases.
In any rxn at constant pressure, the enthalpy change is equal to the heat of rxn
enthalpy is extensive (proportional to the amount) 36 g has twice the enthalpy of 18 g.