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Thermodynamics B

Thermodynamics B. Thermodynamics Deals with the interconversion of heat an other forms of energy. Energy can be converted from one form to another, it can not be created or destroyed. i.e. The energy of the universe is constant. First Law:.  E = q + w.

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Thermodynamics B

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  1. Thermodynamics B

  2. Thermodynamics • Deals with the interconversion of heat an other forms of energy Energy can be converted from one form to another, it can not be created or destroyed. i.e. The energy of the universe is constant. First Law: E = q + w

  3. Want to be able to predict if process or reaction will occur spontaneously (without outside interference) In many cases, spontaneous reactions are exothermic, but not always. H2O CaCl2(s) Ca+2(aq) + 2 Cl - (aq) H = - 82.8 kJ H2O NH4NO3(s) NH4+(aq) + NO3 - (aq) H = + 25.0 kJ So: can’t use H to determine spontaneity. New term: Entropy: (S) Entropy is a state function Entropy is a measure of the randomness or disorder of a system. The greater the disorder, the greater the entropy e.g. S(s) < S(l) << S(g)

  4. Different from enthalpy, can have standard absolute Entropy (So) So: So = Sof - Soi So always > 0 Second Law of Thermodynamics: Entropy of the universe always increases for an irreversible process or remains constant for a reversible process. Suniverse = Ssystem + Ssurroundings Ssurroundings comes primarily from change in heat energy (q at constant pressure) but also depends on the Temperature of the surroundings.

  5. Third Law of Thermodynamics Entropy of a pure, perfectly crystalline substance is zero at absolute zero of temperature. At 0 K motion ceases, therefore, there is only one possible structure and no randomness (perfect order) Why? • = number of arrangements k = boltzmann constant So, third law gives us Absolute Entropy

  6. Determine if the entropy of the system: A) increases B) decreases C) stays the same 1. Benzene(s) --> benzene (l) A 2. NaNO3(s) --> NaNO3 (aq) A 3. S(s) + O2(g) --> SO2(g) C 4. MgCO3(s) --> MgO(s) + CO2(g) A 5. PCl3(l) + Cl2(g) --> PCl5(s) B 6. 2 HgO(s) --> 2 Hg(l) + O2(g) A

  7. Entropy and Physical Properties • S increases with mass: He < Ne < Ar • S increases with molecular complexity: CH$ < C2H6 < C3H8 • S increases with softness of solid crystals • S is greater in metallic solids than in network solids: C (diamond) < C(graphite) • S increases when a solid melts • S increases when a solid or liquid vaporizes • S usually increases when a solid or liquid dissolves in water • S decreases when a gas dissolves in water 9. S increases as volume increase 10. S increases as temperature increases

  8. Which of the following will have the largest entropy? C3H8 (l) C3H8(g) C3H6(g) C4H10(l) C4H10(g)

  9. In order to determine spontaneity, need to know Suniv > 0 Suniv = Ssys + Ssurr and it is not always possible to determine what is happening in the system and the surroundings. So, it is necessary to have another way to assess spontaneity that depends only on the system

  10. Gibbs Free Energy (G) G = H - TS where all the quantities are for the system G is a state function It is an Extensive property There is NO absolute free energy, like enthalpy, we can only measure G. Gibbs - Helmholtz Equation G = H - T S GT,P < 0 Spontaneous Reaction GT,P > 0 Non - Spontaneous Reaction GT,P = 0 Reaction at Equilibrium

  11. Standard Free Energy change for a reaction Gorxn Reactants and Products all in standard states Standard States Gas 1 atm P Liquid pure liquid Solid pure solid Solution 1 M solutions Element Most stable form at 1 atm. Standard free energy of formation Gof 1 mole of a single product from elements in their standard states By definition: Gof of an element in its standard state = 0

  12. G = H - T S If all reactants and products are standard states then Go = Ho - T So Relationship for spontaneity based on non standard conditions G < 0 spontaneous G = 0 Equilibrium G > 0 non spontaneous How does this relate to standard conditions? Q = Reaction Quotient G = Go + RT ln Q R = gas constant T = temp in K

  13. G = Go + RT ln Q At equilibrium G = 0 Reaction is ? K; the equilibrium constant So: Q = ? And: G = 0 = Go + RT ln K Which gives us a relationship between equilibrium and thermodynamics. Go = - RT ln K Go < 0, K >>1 and the reaction at equilibrium has more products than reactants and had to be spontaneous in the forward direction to get there. Go > 0, K << 1 and the reaction at equilibrium has more reactants than products and had to be spontaneous in the reverse direction to get there.

  14. Since the reactions are temperature dependent, you can find the temperature when the reaction goes from spontaneous to non spontaneous in the forward direction by setting Go = 0 Go = Ho - T So Go = 0 = Ho - T So Ho = T So Units: Remember:  H in kcal/mol or kJ / mol  G in kcal/ mol or kJ / mol  S in cal / mol K or J/ mol K So: watch your units!!!

  15. A: What is Go in kJ for the reaction? = (2(N2O) + O2) - (4(NO)) = (2(103.6) + 0) - (4(86.7) = - 139.6 kJ

  16. B: What is Ho in kJ for the reaction? = (2(N2O) + O2) - (4(NO)) = (2(81.5) + 0) - (4(90.4)) = - 198.6 kJ C: What is So in J/K for the reaction? = (2(N2O) + O2) - (4(NO)) = (2(220.0) + ?) - (4(210.6)) Go = Ho - T So = ((-198.6 kJ) - (-139.6 kJ))/ 298 K = - 0.2000 kJ/K = - 200.0 J/ K

  17. D: What is So in J/K for O2(g)? = (2(N2O) + O2) - (4(NO)) - 200.0 = (2(220.0) + O2) - (4(210.6)) So for O2(g) = 204.4 J/K E: What is Eo for the reaction in kJ? Ho = Eo + PV Ho = Eo + nRT Eo = Ho - nRT = (-198.6) - ((3-4)(8.314/1000)(298)) = - 196.1 kJ/mol

  18. F: Is the reaction as written: A) spontaneous B) non - spontaneous C) can’t tell G: At what temperature will the reaction change direction? = 0 Go = Ho - T So Ho = T So = 993 K H: What is the value of K Go = - RT ln K - 139.6 kJ = - (8.314J/1000J/kJ)(298K)lnK K = 2.95 x 10 24 ln K = 56.34

  19. A certain reaction gives off heat and increases in entropy. The reaction is? A) spontaneous at all temperatures B) non spontaneous at all temperatures C) spontaneous at high temp and non spontaneous at low D) spontaneous at low temp and non spontaneous at high

  20. A certain reaction gives off heat and increases in entropy. The reaction is? A) spontaneous at all temperatures B) non spontaneous at all temperatures C) spontaneous at high temp and non spontaneous at low D) spontaneous at low temp and non spontaneous at high G = H - T S (-) - T(+) - at all temp G = So rxn spontaneous at all Temp

  21. A reaction absorbs heat and increases in entropy. The reaction is? A) spontaneous at all temperatures B) non spontaneous at all temperatures C) spontaneous at high temp and non spontaneous at low D) spontaneous at low temp and non spontaneous at high

  22. A reaction absorbs heat and increases in entropy. The reaction is? A) spontaneous at all temperatures B) non spontaneous at all temperatures C) spontaneous at high temp and non spontaneous at low D) spontaneous at low temp and non spontaneous at high G = H - T S (+) - T(+) + at low temp, - at high temp So: non spontaneous at low temp spontaneous at high temp G =

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