Enthalpy

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Notes: (23-2)

Chemical reactions will always try to reach their lowest energy state.  This low energy state relates to the chemical's minimum potential energy.  The enthalpy (DH) is the numerical representation of the potential energy of the of the products compared to the potential energy of the reactants.
 DH for an endothermic reaction is positive DH for an exothermic reaction is negative

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After the activation energy has been generated, most spontaneous reactions are exothermic.  The energy required to move the reaction forward is provided by the formation of the new bonds.

reactions that generate a large amount of heat are always spontaneous
 C3H8 (g)+ 5O2(g) g 3CO2(g) + 4H2O (g)       DH = -2043kJ
Not ALL spontaneous reactions are exothermic
 H2O (s) g H2O (s)                                                     DH = +6.01kJ

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Gibbs Free Energy

Because not all exothermic reactions are spontaneous, an American mathematician named J. Willard Gibbs related enthalpy and entropy to determine whether reactions would be spontaneous or not.  The information was related in the Gibbs Free Energy equation.

DG = DH - TDS

 If DG is negative, the reaction is spontaneous If DG is positive, the reaction is not spontaneous and requires additional energy to proceed If DG is zero, the reaction is at equilibrium