The Three Laws of Thermochemistry

The Three Laws of Thermochemistry

Energy can be changed but not created or destroyed. • Entropy increases for a spontaneous reaction; no change in equilibrium. • Entropy of a pure crystal at 0 K = 0. 3 LAWS of THERMODYNAMICS

1) YOU CANNOT WIN! (you can’t get something for nothing bc matter and energy are conserved) • 2) YOU CANNOT BREAK EVEN! (you cannot return to the same energy state bc there is always an increase in entropy) • 3) YOU CANNOT GET OUT OF THE GAME! (bc absolute zero is unattainable) CP Snow’s version

DH is ENTHALPY: heat energy - exothermic DG is GIBBS FREE ENERGY: work energy - spontaneous DS is ENTROPY: system disorder + disordered STATE FUNCTIONS

DH rxn = SHf products - SHf reactants • If -, exothermic • DG rxn = SGproducts - SG reactants • If -, spontaneous • DS rxn = SS products - SS reactants • If +, disordered DH DG DS and Spontaneity

Characterize the chemical reaction using “Thermo” • The combustion of CH4 DH, DS, DG

DH and S do not change much with temperature changes. • DG does vary and determines if a reaction occurs spontaneously at a specific temperature. • DG = DH - T DS {T in Kelvin} Gibbs-Helmholtz equation

DG = 0 at equilibrium—a point where the reaction is neither proceeding forward nor backward. This is the critical temperature for a reaction to occur spontaneously. • 0 = DH - T DS, solve for T • If T is negative, this reaction will never occur! Temperature of Spontaneity

Characterize: CO + 2H2 CH3OH • Calculate DG at 500K (DH, DS don’t change). • Calculate the minimum temperature this reaction will occur. (make DG = 0) DH, DS, DG

DGo = DH - T DS • DGo = -RT lnK{K is equilibrium constant} • DG = DGo + RT lnQ • Q = [products]coefficients [reactants]coefficients Free Energy Relationships

DH = -249 kJ and DS = -278 J/K (units!) • DH = 460 kJ and DS = -275 J/K • DH = 85 kJ and DS = 198 J/K • Were they spontaneous at 25oC? Determine temperature of Spontaneity

The Three Laws of Thermochemistry