Thermodynamics explores the connection between energy and the EXTENT of a reaction but does not give information about reaction rates (Kinetics).

Transcription

1 Thermodynamics explores the connection between energy and the EXTENT of a reaction but does not give information about reaction rates (Kinetics). Rates of chemical reactions are controlled by activation energy! lower E a, faster rate. Collision frequency;! collisions,! rate Equilibrium is reached when rate forward = rate reverse, also depends on energy (temp). 2 THERMODYNAMICS Review of Energy and Enthalpy Changes (Ch. 10) Energy Changes: Heat and Work Heat = q = energy transferred due to a difference in temperature. +q means heat is added to the system Work = w = action of force through a distance (often P!V) +w means work is done on the system Energy Changes!E = E final state " E initial state!e (kj/mol) O 2 (g) " 2 O atoms Water " ice at 25 o C "6.0 Si (s) + O 2 (g) " SiO 2 (s) "908 3

2 A STATE FUNCTION is a quantity that does not depend on the process by which the system was prepared Example: Your altitude (height above sea level) does not depend on the route you took to class this morning. State functions are written as uppercase letters (E, H, P, V, T, S#) Changes in state functions are path-independent: reactants 2 1 products!e q and w are not state functions but!e (= q + w) is a state function 4 1 st Law of Thermodynamics: the total energy of the universe is constant. "E universe = 0 "E universe = " E system + " E surroundings "E system = # " E surroundings! Heat and work:!e system = q + w and for PV work at const. pressure,!e system = q P!V 5

3 Enthalpy (H): $H = heat transferred at const. P H is a state function changes are path-independent H = E + PV (sums and products of state functions are also state functions) If "H is + : If "H is # : Classify as endo- or exo-thermic: Ice melting Water boiling Wood burning 6 Standard Enthalpy of Formation: $H o f!h for making 1 mole of a compound from its component elements in their standard states Standard state is the most stable form (pure solid, pure liquid, or gas at P = 1 atm and 298K) For solutes in solution, standard state is usually 1 M. Appendix 2 provides values of $H o f!h o rxn = $!H o f (products) $!H o f (reactants) 7

4 Metals All solids except one (which one?) Standard states of the elements The most stable form of an element at 298 K and 1 atm ( STP ) Nonmetals Atomic gases Noble gases He, Ne, Ar, Kr, Xe, Rn Metalloids All solids Diatomics halogens and H 2, N 2, O 2 H 2, N 2, O 2 (gas) F 2 (gas) Cl 2 (gas) Br 2 (liquid) I 2 (solid) Halogens (group 7) Other nonmetals solids C (graphite), S, P, Se 8 MAIN GROUPS 1A 1 1 H Li Na K Rb Cs Fr [223] 2A 2 4 Be Mg Ca Sr Ba Ra [226] 3B 3 21 Sc Y La* Ac** [227] 4B 4 22 Ti Zr Hf Rf [261] 5B 5 23 V Nb Ta Db [262] 6B 6 24 Cr Mo W Sg [266] PERIODIC TABLE of the ELEMENTS Standard States TRANSITION METALS 7B 7 25 Mn Tc [98] 75 Re Bh [264] 8B 8 26 Fe Ru Os Hs [265] 8B 9 27 Co Rh Ir Mt [268] 8B Ni Pd Pt [269] 1B Cu Ag Au [272] 2B Zn Cd Hg [277] 3A 13 5 B Al Ga In Tl A 14 6 C Si Ge Sn Pb [285] MAIN GROUPS 5A 15 7 N P As Sb Bi A 16 8 O S Se Te Po [209] 116 [289] 7A 17 9 F Cl Br I At [210] 8A 18 2 He Ne Ar Kr Xe Rn [222] 118 [293] * LANTHANOIDS ** ACTINOIDS 58 Ce Th Pr Pa Nd U Pm [145] 93 Np [237] 62 Sm Pu [244] 63 Eu Am [243] 64 Gd Cm [247] 65 Tb Bk [247] 66 Dy Cf [251] 67 Ho Es [252] 68 Er Fm [257] 69 Tm Md [258] 70 Yb No [259] 71 Lu Lr [262]

5 Spontaneous Processes: reaction that is capable of proceeding in the forward direction to a substantial extent under a given set of conditions. Processes that are spontaneous in one direction are nonspontaneous in the reverse direction. 10 Saying a reaction is spontaneous is not the same as saying it will occur if the reactants are mixed. Relationship between kinetics and thermodynamics It means the reaction can occur but may be so slow that nothing seems to happen. In the case of a slow spontaneous reaction it is worthwhile to look for a catalyst, but if we know the reaction is nonspontaneous, there is no point in even mixing the reactants, let alone searching for a catalyst. A nonspontaneous reaction cannot occur of itself without outside intervention. 11

6 A spontaneous reaction (process) can do WORK. Water falling over a dam: Is this process spontaneous? Can this process do work? What is the reverse of this process? Which process can do work? Is the reverse process spontaneous? 2 H 2 + O 2! 2 H 2 O 2 H 2 O! 2 H 2 + O 2 12 Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures. Above 0 C ice melts spontaneously Below 0 C the reverse process is spontaneous. What happens at 0 C? 13

7 Spontaneous processes are irreversible. reversible process: Irreversible processes: 14 Can we predict the spontaneity of a reaction? %H (+ or ") Spontaneous Y or N H 2 O(!)! H 2 O(s) At #10 C 2NaCl(s)! 2Na(s) + Cl 2 (g) 4Fe(s) + 3O 2 (g)! 2 Fe 2 O 3 (s) N 2 (g)! 2N(g) 2H 2 (g) + O 2 (g)! 2H 2 O(g) Hypothesis: 15

8 Test the hypothesis: If "H is negative, is the reaction spontaneous? Demonstrations Ba(OH) 2 8H 2 O(s) + 2NH 4 SCN(s)! Ba(SCN) 2 (aq) + 2NH 3 (aq) + 10H 2 O(!) %H =? H 2 O(s)! H 2 O(!) At +10 C %H =? NH 4 Cl(s) + H 2 O(!)! NH 4 Cl(aq) % H =? 16 "H is not the only factor that determines spontaneity There is another factor that also influences spontaneity:! Nature tends to move spontaneously from a state of lower probability to one of higher probability» G.N. Lewis (Nobel Laureate) 17

9 ENTROPY a thermodynamic parameter (S) that is a measure of the disorder or randomness in a system. The more disordered a system, the greater it s entropy. It is related to the various modes of motion in molecules. Like enthalpy, H, entropy S is a state function. It s value depends ONLY on the state of the system (not how it got there!) 18 Second Law of Thermodynamics: the entropy of the universe is increasing. For reversible processes:!s univ =!S system +!S surroundings = 0 For irreversible processes:!s univ =!S system +!S surroundings > 0 19

10 Which processes have "S > 0? Unopened deck of cards! Cards spread out on a table %S Unassembled car parts! Assembled car Seed + CO 2 + H 2 O + Minerals! Tree 20 Which processes have "S > 0? Can we predict "S for molecular level processes? 1 mole of gas confined in % of the container REMOVE the barrier 1 mole of gas has 2x more space. NH 4 Cl(s) " NH 4+ (aq) + Cl # (aq) 21

11 The three LAWS OF THERMODYNAMICS 1st Law: The total energy in the universe is constant. "E universe = 0 "E universe = "E system + "E surroundings "E system = # "E surroundings 2nd Law: The total entropy in the universe is increasing. " S universe > 0 "S universe = "S system + "S surroundings > 0 3rd Law: The entropy of every pure substance at 0 K (absolute zero temperature) is zero. S = 0 at 0 K. 22 Third Law says ABSOLUTE ENTROPY (S) = 0 at T = 0 K. This means we can measure absolute entropy S (not just $S)! At T=0 K the third Law says that there is perfect order in the system (no entropy). Entropy is a state function (its value depends only on the system's initial and final states). $S can then be defined relative to this initial state (where S = 0) Tabulated values are absolute entropies. 23

12 Entropy increases with the number of microstates of the system Entropy on the Molecular Scale Molecules exhibit several types of motion: Translational: Vibrational: Rotational: 24 If the number of possible microstates increases the entropy increases The number of microstates and, therefore, the entropy tends to increase with increases in: Temperature Volume # of molecules (independently moving particles) 25

13 Increasing Temperature increases Entropy 1.. S (1 mole N 2 (g)) at 300K S (1 mole N 2 (g)) at 200K S of Au(s) at 298K S of Au(s) at 1000K solid liquid gas 2. Entropy depends on the STATE. Trends: sol " gas; sol " liq; liq " gas %S = Entropy (S) melting boiling gas " liq; gas " sol; liq " sol %S = "S (+ or #?) H 2 O (l, 25 o C) " H 2 O(g) CaCO 3 (s) " CaO(s) + CO 2 (g) Ag + (aq) + Cl # (aq) " AgCl(s) Temperature (K)! 26 Entropy increases a molecular complexity increases. 3.. E.g. S Ar < S HCl < S H2O There are more possible vibrational modes as the number of atoms increases. Compare F 2 and O 3. 27

14 Entropy increases a molecular complexity increases. There are also more rotational modes as the number of atoms increases Entropy is an extensive property. E.g. 5 molecule " 7 molecules. S(2 moles HCl(g)) S(1 mole HCl(g)) N 2 O 4 (g)! 2 NO 2 (g) 1 moles! 2 moles 29

15 Summary of Molecular Basis of Entropy 1. Adding heat increases entropy. 2. Entropy depends on the state. Entropy (S) solid liquid gas melting boiling Temperature (K)! 3. Entropy increases as # of atoms in a molecule increases. 4. Entropy is an extensive property: more moles, more entropy Entropy is a state function - its value depends only on the system's initial and final states. Absolute Entropy: S = 0 at T = 0 K (Third Law) 30 The Entropy of a substance in its standard state can be defined. S (Standard Molar Entropy): Unit for S o is: S is always positive (>0) for pure substances! These are molar entropy values of substances in their standard states. Standard entropies tend to increase with increasing molar mass. Substance S J/mol-K Gases H 2 (g) N 2 (g) O 2 (g) F 2 (g) H 2 O(g) NH 3 (g) CH 3 OH(g) Liquids H 2 O(!) 69.9 CH 3 OH(!) Solids Li(s) 29.1 Na(s) 51.4 K(s) 64.7 NaCl(s)

16 The entropy change for a reaction ("S rxn ) can be calculated using tabulated values of absolute entropy. "S o FOR REACTIONS "S (rxn) = & S (products) # & S (reactants) What is %S (rxn) for N 2 (g) + 3 H 2 (g) " 2 NH 3 (g) "S > 0 when: Gases form from either liquids or solids Liquids or solutions form from solids The number of molecules of gas increase during a chemical reaction. 32 Take Home Message Reaction The spontaneity of a reaction depends on: Temperature, "H and "S Next we ll combine these to get a new thermodynamic parameter, Gibbs Free Energy ("G) to predict reaction spontaneity. What you should know: How to calculate the "S of a reaction Predict what molecules/reactions will have a greater or smaller entropy according to the factors that affect entropy. 33