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Thermodynamics

Thermodynamics. General Information. "Thermodynamics" , is the study of the energy changes or transfers accompanying physical and chemical processes helps us predict whether a rxn will occur under certain conditions DO NOT ASSUME THAT ALL REACTIONS HAPPEN AUTOMATICALLY!. Spontaneity.

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Thermodynamics

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  1. Thermodynamics

  2. General Information • "Thermodynamics", is the study of the energy changes or transfers accompanying physical and chemical processes • helps uspredict whether a rxn will occur under certain conditions • DO NOT ASSUME THAT ALL REACTIONS HAPPEN AUTOMATICALLY!

  3. Spontaneity • "spontaneous" - a process can occur (not necessarily fast) • "non-spontaneous" - a process cannot occur

  4. General Information • many spontaneous rxns release heat energy = "exothermic"; • products have less E than reactants ("downhill") (see diagram) • some rxns absorb energy = "endothermic" = products have more energy than the reactants ("uphill") (see diagram)

  5. Exothermic Endothermic Ea = Activation Energy a = Ea b = Ea for reverse reaction c = enthalpy change

  6. General Information • not all exothermic rxns are spontaneous; not all spontaneous rxns are exothermic, so some other factor must influence spontaneity

  7. Four Laws of Thermodynamics

  8. Enthalpy • DH • DH = heat content • DHf = heat of formation • DHrxn = heat of reaction (which is the sum of DHf) • DHo= the "o" means that conditions are at standard, which for thermodynamics is at 25oC and 101.3 kPa

  9. Hess’ Law The sum of… • DHrxn= [SnDHf]products - [SnDHf]reactants • Consult a Thermodynamics chart; be careful of the units and phase (s, cr, l, aq, g) • ie: 2NO(g) + O2(g) 2NO2(g) • 2(+90.25kJ/mol) 0 2(+33.18kJ/mol) • 2(-90.25kJ/mol) + 2(+33.18kJ/mol) • -180.5kJ/mol + 66.36kJ/mol • = -114.1kJ/mol = DHrxn Coefficients from the BCE Why is this zero? And since this is a negative value, it is an EXOTHERMIC REACTION

  10. Sign of DH

  11. Entropy • DS • DS = a measure of the amount of disorder in a system (randomness) • it is natural for things to become more disordered, and takes energy to make things ordered again • DSrxn= [SnDS ]products - [SnDS ]reactants • solids have low DS; gases have high DS

  12. Entropy

  13. Sign of DS

  14. Entropy Problem • Again, DSrxn= [SnDS ]products - [SnDS ]reactants • N2H4(l) + 2H2O2(l) N2(g) + 4H2O(g) • (-121.2J/K) + 2(-109.6J/K) + 191.61J/K + 4(188.8J/K) NOTE: Since we know it’s Products Minus Reactants, we’ve already changed the signs of the reactants. S = +606.4 J/K A high positive value would be expected when solids turn to liquids or liquids turn to gas since +DS means more disorder

  15. THE EFFECT OF THE SIGNS OF DH and DS ON SPONTANEOUS CHANGE:

  16. Gibbs Free Energy • DG • the sign of the value for Gibbs Free Energy tells us: • +DG reaction is not spontaneous • -DG reaction is spontaneous • DGrxn= [SnDG]products - [SnDG]reactants • DG = DH - TDS (temp in kelvins) Notice the language “is not spontaneous” and “is spontaneous”. This is definitive!

  17. Gibbs Free Energy Problem • ie: Pb(s) + ½ O2(g) PbO(s)DHorxn = -215 kJ DSorxn = -0.092 kJ/K • Is the rxn spontaneous? • DG = -215 - [(298 K) (-0.092kJ/K] • DG = -188 kJ; • yes, the rxn is spontaneous • Again, it doesn’t just say “favors” Remember: This means at “standard”, which for temp, is 25oC or 298 K

  18. Sign of DG

  19. Bond Energy • the change in enthalpy when a chemical rxn occurs is due primarily to the energy required to break the chemical bonds in reactants and the energy produced by forming the chemical bonds in the products (breaking of bond = "bond dissociation") • all bond dissociations are ENDOthermic because energy must be supplied to break bonds; therefore, all BE's are + values

  20. Reactions and Bond Energy • BErxn= [SnDBE ]reactants - [SnDBE]products • the sign of the value for Bond Energy are related to enthalpy of formation, so the same information is given

  21. Factors that Contribute to Bond Strength: • 1) polarity - a polar bond is stronger than nonpolar - in general, the > the diff in e.n. b/w bonded atoms, the more polar the bond & the > the BE • 2) # of e- pairs b/w bonded atoms - multiple bonds have > BE than single bonds • 3) Effects related to sizes of atoms - If nuclei cannot come very close together, shared e- pair will not be effective in bonding them. Far from nucleus bond will be weak

  22. Calculate Enthalpy with B.E.’s • STEP 1 – You must know the B.C.E. • 2 ClF3(g) + 2O2(g) Cl2O(g) + 3OF2(g) • STEP 2 – Draw the molecules involved, with the correct numbers and types of bonds. • STEP 3 – Look up Bond Energies for each. Remember, the BE formula has Reactants before products, not like DH, DS, DG formulas! • 2(3*255) + 2(498) + 2 (-205) + 3(2*-184) • = 1012 kJ = DHrxn

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