Review of Chapter 6

1. Review of Chapter 6 • Thermodynamics = study of energy and its interconversions • Chemists focus on HEAT energy and the CHANGE IN HEAT of a reaction • Ways to measure Enthalpy of a reaction • Calorimetry (q = mcDT) • Hess’s Law • Standard enthalpy of formation • Bond Energy

2. Spontaneous Reactions • A Spontaneous reaction occurs without outside assistance. • Using enthalpy to predict spontaneity • If the energy of the products is lower than the reactants, reaction is likely to occur • When this is true, the reaction is exothermic • Most exothermic reactions are spontaneous

3. Spontaneous Reactions • Not all spontaneous reactions are exothermic • Nor are all exothermic reactions spontaneous • Enthalpy of a reaction is not the only factor that determines if a reaction is spontaneous

4. Entropy • Entropy (S) = measurement of randomness or disorder of a system • Left alone, systems tend to become more disordered • Chemists study the change in entropy (DS) that accompanies a reaction

5. Entropy and Spontaneity • If the products are in a greater state of disorder (increase in entropy) than the reactants, reaction is likely to be spontaneous. • Entropy’s effect on spontaneity depends on temperature • Unit of entropy = J / K mol

6. Spontaneous Reactions • Most spontaneous reactions are • Exothermic • Increase in Entropy

7. 1st Law Thermodynamics • Total energy of the universe remains constant • This is synonymous with the Law of Conservation of Energy

8. 2nd Law Thermodynamics • Entropy of the universe INCREASES for any spontaneous process. • For any spontaneous process, the sum of the change in entropy of the system and surroundings must be positive DSuniverse (+) = DSsystem + DSsurroundings

9. 2nd Law (con’t) • Reactions are only spontaneous in ONE direction. • If a reaction is spontaneous, it will NOT be spontaneous in the reverse direction. • If a reaction is NOT spontaneous as written, it WILL be spontaneous in the reverse direction.

10. 3rd Law Thermodynamics • The entropy of a perfect crystal at 0 Kelvin is zero. • All other entropy values are positive. • Change in entropy values however, can be positive or negative.

11. Entropy • Change in entropy of a reaction will be positive if • Solids are melted to form pure liquids or dissolved to form solutions. • Liquids or solids are converted to gas. • The number of moles of gas increases. (More moles of gas on product side) • The temperature increases.

12. Entropy • Predict if the following will have a positive change in entropy or negative • 2 C (s) + O2 (g) 2CO (g) • 2 K (s) + Br2 (l)  2KBr (s) • 2 MnO2 (s)  2 MnO (s) + O2 (g) • O (g) + O2 (g)  O3 (g)

13. Entropy • Entropy is a state function • Change in entropy can be calculated just like enthalpy DSoreaction = S npSo products – S nrSo reactants

14. Calculating Entropy • Predict and find D So for the reaction 2 NH3 (g) + CO2 (g) NH2CONH2 (aq) + H2O(l)

15. Entropy at a Phase Change • To calculate entropy at a phase change, use the equation DS = DH / T **change DH to joules • You must know DH and the temperature to do this! • Try the example in the notes.

16. Free Energy • What can tell us if a reaction is spontaneous? • A property called Free energy (G) • Free energy = measurement of amount of energy available to flow out of a system

17. Free Energy • Free energy depends on three things • Change in enthalpy • Change in entropy • Temperature • Change in free energy can be calculated using the equation DGo = DHo – TDSo **This equation is important

18. DGo = DHo – TDSo • DHo tells us the amount of work that could potentially be done by the system • DSotells us the amount of work that needs to be done on the system • DGo is the maximum amount of work that can be done by the system

19. Free Energy Sign • The sign of free energy tells us about the reaction

20. Free Energy • Free energy is a state function • Standard free energy of formation can be calculated and used to determine change in free energy of a reaction • Change in free energy can be found DGoreaction = DGof (products) - DGof (reactants)

21. Calculating Free Energy • Use the Gof values in the book to determine DGo for the reaction C2H5OH (l) + 3 O2 (g) 2 CO2 (g) + 3 H2O (g)

22. Non-Standard Conditions • To find DG under non-standard conditions, you must know • Change in free energy under standard conditions (DGo) • Temperature (T) • Ratio of products to reactants (Q) (Reaction Quotient)

23. Reaction Quotient • Most reactions do not go all the way to completion • Reaction quotient tells us how far a reaction has gone • Molar concentrations or pressures for gases are used to calculate the reaction quotient

24. Reaction Quotient • Equation to calculate reaction quotient Q = molar concentrations products molar concentrations reactant For the reaction aA + bB  qQ + rR Q = [Q]q [R]r [A]a [B]b ** Amounts of pure solids and liquids not used

25. Free Energy Non-Standard • Once you know the • Standard change in free energy • Temperature • Reaction quotient You can use the equation DG = DGo + RT lnQ R = universal gas constant (8.314 J/K mol)

26. Free Energy at Equilibrium • In fact, these reactions can occur in the forward direction and the reverse direction simultaneously • For reactions that do not go all the way to completion, there comes at point at which the rate of reaction in the forward direction equals the rate of reaction in the reverse direction

27. Free Energy at Equilibrium • Ta-Da!!! This point is known as equilibrium • All reactions that come to equilibrium do so at a very specific set of conditions • The ratio of products to reactants at equilibrium is expressed by the equilibrium constant (K) • At equilibrium the reaction quotient is equal to the equilibrium constant (Q = K)

28. Free Energy at Equilibrium • Additionally, a system at equilibrium has no free energy available to flow out of the system. Therefore, at equilibrium, DG = 0 and Q = K. • We can find the standard change in free energy for a system at equilibrium by using the equation: DGo = - RT ln K

29. Sign of DGo K value Favored direction Negative Greater than 1 Products favored (greater concentration of products than reactants) Zero Equal to 1 Equal products and reactants Positive Less than 1 Reactants favored (greater concentration of reactants than products) Free Energy at Equilibrium • The sign of DGo and the value of K can give us information about how far a reaction comes toward completion before equilibrium is reached.