Chapter 10: Phenomena

Transcription

1 Chapter 10: Phenomena Phenomena: Below is data from several different chemical reactions. All reaction were started by putting some of every substance in the chemical reaction into an expandable/contractable container at 25 C. If reactants/products were in a gas state, 1 atm of the gas was added to the container. If reactants/products were in an aqueous state, 100 ml of water was added to the container and then the concentrations were adjusted to 1M using solids. If the reactants/products were in a liquid or solid state, 10 g of the substance was added. Therefore, for the reaction: 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) 10 g of Fe, 10 g of Fe 2 O 3, and 1 atm of O 2 would be put into the reaction container. Looking at the thermodynamic data that was gathered see what patterns you can identify. (You do not need to know what ΔG or ΔS are to identify the patterns.) Reaction 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) NH 4 NO 3 (s) NH 4+ (aq) + NO 3- (aq) 4AlCl 3 (s) + 9O 2 (g) 2Al 2 O 3 (s) + 12ClO(g) 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) Direction Reaction Runs Forward Forward Reverse Forward ΔE ΔH ΔG ΔS H 2 O(l) H 2 O(s) Reverse

2 Chapter 10 Spontaneity, Entropy, & Free Energy Big Idea: The change in free energy of a reaction indicates whether a reaction is spontaneous. In any spontaneous process there is always an increase in the entropy of the universe. o Entropy o ΔS of Physical Reactions o Isothermal Processes o 2 nd Law of Thermo o Free Energy o Hess s Law/ 3 rd Law of Thermo o Equilibrium 2

3 Entropy 3 Entropy (S): Entropy is a measure of how energy and matter can be distributed in a chemical system. # of molecules of left side # of ways of arranging (microstates)

4 Entropy In General: Entropy increases from solid to liquid to gas corresponding to an increase in positional probability. Entropy increases when you dissolve a solid in liquid corresponding to an increase in positional probability. The larger the volume the larger the positional probability and the greater the entropy (n constant). The larger the pressure the smaller the positional probability and the lower the entropy (n constant). The larger the molecule the larger the number of relative positions of the atoms resulting in a greater positional probability and a greater entropy. The higher the temperature the greater the range of energies, therefore the larger the entropy. 4

5 Entropy Student Question Predict which of the following reactions has a negative entropy change. I. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) II. NH 3 (g) + HCl(g) NH 4 Cl(s) III. 2KClO 4 (s) 2KClO 3 (s) + O 2 (g) a) II and III b) III c) II d) I e) I and II 5

6 Entropy Phase Change: The condition (for a given pressure, and temperature) at which two different phases are in dynamic equilibrium. Melting/Freezing Evaporation/Condensation Sublimation/Deposition Liquid/Solid Liquid/Gas Solid/Gas 6

7 ΔS of Physical Reactions Calculating ΔS for physical reaction X(s, T i ) X(g, T f ) Step 1: Calculate ΔS to bring to melting point Step 2: Calculate ΔS involved in fusion Step 3: Calculate ΔS to bring to boiling point Step 4: Calculate ΔS involved in vaporization Step 5: Calculate ΔS to bring to final temperature S S nh fus T nh vap T 7

8 ΔS of Physical Reactions Student Question What is ΔS for 88.0 g of CO 2 undergoing the following reaction at constant pressure? CO 2 (s, 150. K) CO 2 (g, 195. K) Helpful Information: 195, 25.2, 1.07 a) 24.9 b) 154 c) 233 d) 283 e) None of the above 8

9 Isothermal Processes 9

10 Isothermal Processes 2 Step Isothermal Expansion 6 Step Isothermal Expansion Pressure Pressure P 1 P 1 2 P 1 4 P 1 P 1 2 V1 1 2V 4V 1 Volume Step Isothermal Expansion Pressure P 1 P 1 2 P 1 4 V1 1 2V 4V 1 Volume Reversible Process: A process that can be reversed by an infinitesimal change in a variable. 10 P 1 4 V1 1 2V 4V 1 Volume Note: In order for a reversible process to occur the system must be at equilibrium during the entire process.

11 Isothermal Processes Student Question Calculate the ΔS associated with a process in which 5.00 mol of gas expands reversibly at constant temperature T = 25 C from a pressure of 10.0 atm to 1.00 atm. a) 28,500 b) 95.7 c) d) -28,500 e) None of the above 11

12 2 nd Law of Thermo 2 nd Law of Thermodynamics: A spontaneous change is accompanied by an increase in the total entropy of the system and its surroundings. Note: The second law of thermodynamics applies to ΔS uni and not ΔS sys. So far we have only discussed ΔS sys. 12

13 Free Energy Can we relate spontaneity to a change in the system instead of the universe assuming we are at constant temperature and pressure? Divide Through by T ΔS univ >0 then ΔG sys <0 spontaneous process Δs univ <0 then ΔG sys >0 non spontaneous process 13

14 Free Energy Student Question Hold the rubber band a short distance from your lips. Quickly stretch it and press it against your lips carefully (don t hurt those delicate lips). Do you experience a warming or cooling sensation? Carefully release the rubber band and experience the sensation. Is stretching a spontaneous or a non-spontaneous process? What are the correct signs for ΔG, ΔH, and ΔS when you allow the rubber band to relax? ΔG ΔH ΔS a) + + b) + c) + + d) + 14

15 Hess s Law / 3 rd Law of Thermo Hess s Law: A reaction enthalpy/free energy/entropy is the sum of the enthalpies/free energies/entropies of any sequence of reactions (at the same temperature and pressure) into which the overall reaction can be divided. Things to remember: If you add reactions together, add ΔH/ΔG/ΔS. If you flip a reaction, flip the sign of ΔH/ΔG/ΔS. If you multiply a reaction by a constant, multiply ΔH/ΔG/ΔS by the same constant. 15

16 Hess s Law / 3 rd Law of Thermo Student Question What is ΔG for SO 2 (g) + ½O 2 (g) SO 3 (g) given the following information? 2S(s) + 3O 2 (g) 2SO 3 (g) ΔG = -742 S(s) + O 2 (g) SO 2 (g) ΔG = a) -71 b) -442 c) -671 d) e) None of the above 16

17 Hess s Law / 3 rd Law of Thermo Substance Thermodynamic Data at 298 K ΔH f ( ) ΔG f ( ) S ( ) C 2 H 4 (g) CH 4 (g) CO 2 (g) C 2 H 6 (g) O(g) O 2 (g) CH 3 CO 2 H(l) CH 3 OH(g) CH 3 CH 2 OH(l) C 6 H 12 O 6 (s) HCl(g) H 2 (g) H 2 O(l) H 2 O(g) Fe(s) Substance Thermodynamic Data at 298 K ΔH f ( ) ΔG f ( ) S ( ) Fe 2 O 3 (s) N 2 (g) NO 2 (g) NO(g) N 2 O 4 (g) NH 3 (g) HNO 3 (l) NH 4 Cl(s) O 2 (g) P 4 O 10 (s) H 3 PO 4 (s) S rhombic (s) H 2 S(g) SO 2 (g) SO 3 (g) * Other ΔH f, ΔG f, and S can be found in appendix 4 in the back of your book. 17

18 Hess s Law / 3 rd Law of Thermo Thermodynamic Data at 298 K Substance ΔH f ΔG f What is for 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(l)? S NH 3 (g) O 2 (g) NO(g) H 2 O(l)

19 Equilibrium Student Question Consider the reaction: C (graphite) + CO 2 (g) 2CO(g) What is ΔG ( ) at 25 C when the pressures are: , 20.. Helpful Information: At 25 C ΔH rxn =172.5 and ΔS rxn =175.9 a) d) -52,290.2 b) c) -44,991.4 e) None of the above 19

20 Equilibrium For the reaction below at equilibrium there are many more products then reactants. What is the sign of ΔG? A(g) B(g) 20

21 Take Away from Chapter 10 Big Idea: The change in free energy of a reaction indicates whether a reaction is spontaneous. In any spontaneous process there is always an increase in the entropy of the universe. Entropy Know how positional probability and energy probability effects entropy (12,19,&40) Be able to predict the sign of ΔS (7,& 43) Be able to calculate S of a system with known number of microstates Ω Be able to calculate (29,30,33,48,&49) (constant temperature) 21 ΔS of Physical Reactions (changing temperature) Know how to calculate ΔS for physical reactions at constant pressure (31) Know that a similar multi step process that is used to calculate ΔS during physical reactions can be used for ΔH. (32) Problems 23 and 25 review from last chapter. Numbers correspond to end of chapter questions.

22 Take Away from Chapter 10 Isothermal Process Know that for an isothermal expansion of an ideal gas ΔE=0 Know that the maximum work is done for the reversible expansion/contraction of an ideal gas. (28) (27) 2 nd Law of Thermo A spontaneous change is accompanied by an increase in the total entropy of the universe. ΔS uni +, spontaneous ΔS uni, non spontaneous (41&50) (at constant pressure) 22 Numbers correspond to end of chapter questions.

23 Take Away from Chapter 10 Free Energy Be able to calculate the change in free energy (ΔG) (51,56,57,& 64) Know implications of the sign of ΔG ΔG sys is +, non spontaneous ΔG sys is, spontaneous Hess s Law / 3 rd Law of Thermo Know that Hess s Law can be applied to ΔG and ΔS as well as ΔH Know how to get ΔS rxn and ΔG rxn from other known ΔS rxn and ΔG rxn (62) Know how to get ΔS rxn and ΔG rxn from table (54,60,61) (59) (44) Know that while absolute values of H and G cannot be calculated, an absolute value of S can. 3 rd Law Thermodynamics: The entropy of a perfect crystal is 0 K is 0 23 Numbers correspond to end of chapter questions.

24 Take Away from Chapter 10 Equilibrium Know that Q and ΔG are related by (68,69,70,& 71) ΔG is +, reverse reaction spontaneous ΔG is -, forward reaction spontaneous ΔG is 0, at equilibrium Know that K and ΔG are related by (74,78,79,84,86,87,88,91,&109) ΔG = 0, (K=1) equal amounts of products and reactants at equilibrium ΔG > 1, (K<1) more reactants than products at equilibrium ΔG < 1, (K>1) more products than reactants at equilibrium 24 Numbers correspond to end of chapter questions.