16-4-2011

1. 16-4-2011

2. Thermodynamics

3. Introduction Thermodynamics examines the heat and work as well as spontaneity of a reaction. Spontaneity is the notion of whether or not a process can take place unassisted. Entropy is a mathematical concept describing disorder and randomness in a system. Free energy is a thermodynamic function that relates enthalpy and entropy to spontaneity, and can also be related to equilibrium constants.

4. Why Study Thermodynamics? • With a knowledge of thermodynamics and by making a few calculations before embarking on a new venture, scientists and engineers can save themselves a great deal of time, money, and frustration. • Thermodynamics tells us what processes are possible. • (Kinetics tells us whether the process is practical.)

5. Law of Conservation of Energy • energy cannot be created or destroyed • First Law of Thermodynamics • energy can be transferred between objects • energy can be transformed from one form to another

6. Spontaneous processes and entropy As far as we are concerned, we would like to be able to predict whether a chemical reaction proceeds spontaneously under a specified set of conditions or not. A chemical reaction is said to be spontaneous if it occurs under a given set of conditions. If it does not occur under this set of conditions it is said to be non-spontaneous. If a reaction is spontaneous in one direction, it is non-spontaneous in the other, and vice versa.

7. Examples of spontaneous processes • A waterfall runs downhill, but never uphill. • A lump of sugar spontaneously dissolves in a cup of tea, but never dissolved sugar lumps back spontaneously. • Heat transfers from a hot object to colder surroundings, but never heat from a cold surrounding transfers to heat a body of higher temperature • Iron exposed to water and oxygen rusts, but never the opposite.

8. Spontaneous Processes • Spontaneous processes are those that can proceed without any outside intervention. • The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not accumulate in one only.

9. Many exothermic processes are Spontaneous In general, product-favored reactions are exothermic and spontaneous E.g. thermite reaction Fe2O3(s) + 2 Al(s)  2 Fe(s) + Al2O3(s) H = - 848 kJ H+ + OH-g H2O H = - 56.2 kJ

10. Many endothermic processes are also spontaneous NH4NO3(s) + heat  NH4+ (aq) + NO3- (aq) Hsol = +25.7 kJ/mol H2O (s) g H2O (l) Ho = +6.01 kJ/mol

11. Conclusion: enthalpy alone is not a sufficient criterion for prediction of spontaneity. Water falling (higher to lower potential energy) is a spontaneous process. H2 and O2 combine spontaneously to form water (exothermic) … liquid water vaporizes spontaneously at room temperature; an endothermic process.

12. In some cases, endothermic reactions that are non-spontaneous at room temperature may become spontaneous when the temperature is raised. Example: 2HgO(s) g 2 Hg(l) + O2(g) Ho = +90.7 kJ/mol Conclusion: Exothermicity favors spontaneity of a reaction but does not guarantee it. In addition, endothermic reactions can be spontaneous. Therefore, we can not judge spontaneity considering DH only.

13. Entropy • Entropy, S, is a measure of the disorder of a system. • Spontaneous reactions proceed to lower energy or increase entropy. • In ice, the molecules are very well ordered because of the H-bonds, therefore, ice has a low entropy. • Melting ice will increase entropy, a spontaneous process although endothermic.

14. Entropy • Entropy is a state function. • For a system, S = Sfinal - Sinitial • If S > 0 the randomness increases • if S < 0 the order increases • Ssolid < Sliquid << Sgas

15. Entropy • There is a balance between energy and entropy considerations. • When an ionic solid is placed in water two things happen: • the water organizes into hydrates about the ions (so the entropy decreases), and • the ions in the crystal dissociate (the hydrated ions are less ordered than the crystal, so the entropy increases).

16. The Molecular Interpretation of Entropy • A gas is less ordered than a liquid which is less ordered than a solid • Any process that increases the number of gas molecules leads to an increase in entropy • When NO(g) reacts with O2(g) to form NO2(g), the total number of gas molecules decreases, and the entropy decreases 2NO(g) + O2(g) g 2NO2(g)

17. Boiling corresponds to a much greater change in entropy than melting. • Entropy will increase when: • Liquids or solutions are formed from solids • Gases are formed from solids or liquids • The number of gas molecules increases • The temperature is increased • The number of atoms in a molecule increases without change in number of moles of gas molecules • The molar mass increases

18. Which of the following processes produces a decrease in the entropy of the system? A. Boiling water to form steam B. Dissolution of solid KCl in water C. Mixing of two gases into one container D. Freezing water to form ice E. Melting ice to form water

19. Consider a pure crystalline solid that is heated from absolute zero to a temperature above the boiling point of the liquid. Which of the following processes produces the greatest increase in the entropy of the substance? A. melting the solid B. heating the liquid C. heating the gas D. heating the solid E. vaporizing the liquid

21. Predict whether each of the following leads to an increase or decrease in the entropy of a system. (a) The synthesis of ammonia: N2(g) + 3 H2(g)  2 NH3(g) (b) Preparation of a sucrose solution: C12H22O11(s) g C12H22O11(aq) (c) Evaporation to dryness of a solution of urea, CO(NH2)2, in water: CO(NH2)2(aq)  CO(NH2)2(s)