*When a reaction is reversed, the magnitude of ΔH stays the same, but the sign changes.
*When the balanced equation for a reaction is multiplied by an integer, the corresponding value of ΔH must be multiplied by that integer as well.
*The change in enthalpy for a reaction can be calculated from the enthalpies of formation of the reactants and the products
*Elements in their standard states make no contribution to the enthalpy calculations for the reaction, since the enthalpy of an element in its standard state is zero. Allotropes of an element other than the standard state generally have non-zero standard enthalpies of formation.