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Entropy and Free Energy. Driving Forces of Reactions So far we have seen that reactions are spontaneous if they give off heat – exothermic There is a.

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Presentation on theme: "Entropy and Free Energy. Driving Forces of Reactions So far we have seen that reactions are spontaneous if they give off heat – exothermic There is a."— Presentation transcript:

1 Entropy and Free Energy

2 Driving Forces of Reactions So far we have seen that reactions are spontaneous if they give off heat – exothermic There is a natural tendency in the universe for systems to get to lowest energy state Why do endothermic reactions happen? –Demo with barium –Ammonium chloride dissolving in H2O - endothermic

3 Natural direction Scientists notice that there is a natural direction for processes –Balls roll down hill, not up hill –Ice melts above 0º C, never refreezes above 0 C –A gas fills its container uniformly, never collects in one area –Heat flows from hot to cold; hotter object doesn’t get hotter when exposed to a colder object –Wood burns spontaneously, but CO2 and H2O don’t form wood when heated

4 What makes these processes irreversible? All of the processes have the following in common: –in all cases you have less information about how the particles are organized than you did before Analogy: Why don’t we have fire drills during lunch? –Less “information” about where students are during lunch compared to when they are in class Measure of “information” about a system = ENTROPY

5 Entropy Measure of the amount of randomness or disorder in a system –Symbol: S –Units: J/K No matter what the process, entropy (of universe) is always increasing… Entropy was introduced in 1865 by Rudolf J. E. Clausius, a German physicist. Clausius said he derived the term from the Greek words en trope, which means “in the transformation” He used it to describe the dissipation or apparent loss of energy available to do work as energy is transformed in a system.

6 Entropy Analogies Throw a card into the air – 2 possible positions (up or down) Throw a deck of cards into the air – how many possible positions? –One possibility is that they land organized in a stack. How probable is that? The more cards = the more entropy MORE MOLES = MORE ENTROPY

7 Given the following reaction, how is entropy changing: N 2 (g) + 3 H 2 (g)  2NH 3 (g) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more information

8 What causes entropy to increase? Statistics… Boltzmann Bucks Demo –At first everyone has $1, we play rock-paper-scissors for awhile. Some people have more money than others, but no one has all the money and Not everyone has exactly $1 –Why not? A.There is only one way for the money to be arranged so that everyone has $1 B.There are only 9 ways for the money to be arranged so that one person has $9 (assuming class size of 9) C.There are many ways for some people to have no money, some to have $1 and some to have $2 or $3. D.So there is a higher probability that the $ will be arranged as described in C Nature spontaneously proceeds to the state that has the highest probability of existing. Highest probability = most disordered

9 Entropy Changes Increase moles Dissolving and mixing Increasing temperature Increase volume Solid to liquid or liquid to gas (or S to G) More complicated molecules have higher S than simpler molecles

10 Which one of the following does not generally lead to an increase in entropy of a system? 1.Increase in total number of moles or particles 2.Formation of a solution 3.Formation of a gas 4.Formation of a solid

11 Given the reaction below, how is entropy changing? Br 2 (l)  Br 2 (g) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more info

12 Given the following reaction, how is entropy changing: Ag +1 (aq) + Cl -1 (aq)  AgCl(s) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more info

13 Given the following reaction, how is entropy changing: 2NO 2 (g)  N 2 O 4 (g) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more info

14 Given the following reaction, how is entropy changing: CO(g) + H 2 O(g)  CO 2 (g) + H 2 (g) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more info

15 Given the following reaction, how is entropy changing: H 2 (g) + F 2 (g)  2HF(g) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more info

16 Given the following reaction, what is the sign for ΔS: NaCl(s)  NaCl(aq) 1.positive 2.negative Need more info

17 Given the following reaction, how is entropy changing: 2OH - (aq) + CO 2 (g)  H 2 O(l) + CO 3 2- (aq) 1.Increasing 2.Decreasing 3.Stays the same 4.Need more info

18 Which of the following has the largest increase in entropy? 1.Pb(NO 3 ) 2 (s)  Pb(NO 3 ) 2 (aq) 2.CaCO 3 (s)  CaO(s) + CO 2 (g) 3.2NH 3 (g)  2H 2 (g) + N 2 (g) 4.H 2 (g) + Br 2 (g)  2HBr(g)

19 Spontaneity Spontaneous change: –Occurs w/o continuous input of energy Spontaneous reactions occur when –Reaction is exothermic ( ΔH < 0) –Increase in entropy for the system (ΔS >0) But which is more important? ΔH or ΔS ?

20 Total Entropy System vs. Surroundings vs. Universe –S universe = S system + S surroundings –S univ is always increasing. Two spontaneous processes: –CaCl 2 (s)  Ca 2+ (aq) + Cl -1 (aq) ΔH = -66kJ S sys is increasing S surr is increasing because heat is released to surroundings –NH 4 Cl(s)  NH Cl - (aq) ΔH = 15 kJ S sys is increasing S surr is decreasing b/c surroundings are losing heat Non-spotaneous –Na(s)  Na (l) ΔH =2.59 kJ S sys = increasing S surr = decreasing

21 A.NH 4 Cl(s)  NH Cl - (aq) ΔH = 15 kJ B.Na(s)  Na (l) ΔH =2.59 kJ C.Na + (g) + Cl - (g)  NaCl(s) ΔH = -771 kJ Why is A spontaneous, but not B? –Entropy is much greater for A Why is C spontaneous? –Enthalpy is large

22 How do you know if enthalpy or entropy will make the reaction more spontaneous? Remember: S universe = S system + S surroundings S surr depends on temperature The lower the surrounding temperature, the more significant adding heat is The higher the surrounding temp, the less significant adding heat is. –Analogy Imagine you give $1 to someone w/ only $10 to their name? Imagine the effect of giving $1 to a millionaire. Who is affected more?

23 Exothermic reactions that release heat to the surroundings are a a stronger driver of reactions when the surrounding temperature is low. At high temperatures, an exothermic reaction isn’t such a strong driving force, entropy is more important. Negative sign b/c ΔH defined in terms of system: exothermic reaction from system’s perspective causes increase in entropy of surroundings. S universe = S system + S surroundings Entropy of system Determined by ΔH of system

24 S univ = S sys + S surr Substitute: - ΔH sys /T for S surr Multiply by (-T) Define new quantity –Free energy: ΔG = -T ΔS univ –Free energy tells whether a rxn will be spontaneous at a given temperature. –Measures the maximum energy available to do useful work –Reactions at equilibrium have ΔG = 0 -T ΔS univ = ΔH sys - T ΔS sys ΔG = ΔH sys - T ΔS sys

25 Based on the previous slides and derivation of ΔG, rxns will be spontaneous if ΔG is 1.Less than 0 2.Greater than 0 3.Equal to 0 4.Spontaneity has nothing to do with ΔG.

26 Under which conditions will reactions ALWAYS be spontaneous? 1.ΔH > 0, ΔS >0 2.ΔH > 0, ΔS < 0 3.ΔH 0 4.ΔH < 0, ΔS <0 ΔG = ΔH sys - T ΔS sys

27 Under which conditions will reactions NEVER be spontaneous? 1.ΔH > 0, ΔS >0 2.ΔH > 0, ΔS < 0 3.ΔH 0 4.ΔH < 0, ΔS <0 ΔG = ΔH sys - T ΔS sys

28 Under which conditions will reactions be spontaneous at high temps? (other than when they are always spontaneous) 1.ΔH > 0, ΔS >0 2.ΔH > 0, ΔS < 0 3.ΔH 0 4.ΔH < 0, ΔS <0 ΔG = ΔH sys - T ΔS sys

29 ΔH > 0, ΔS >0 A large positive value for the term (TΔS) can make ΔG negative if it is bigger than ΔH Reaction is endothermic so the entropy of surroundings is decreasing. At high temps, this won’t make as big of a difference as it would at lower temps. ΔG = ΔH sys - T ΔS sys

30 Under which conditions will reactions be spontaneous at low temps? (other than when they are always spontaneous) 1.ΔH > 0, ΔS >0 2.ΔH > 0, ΔS < 0 3.ΔH 0 4.ΔH < 0, ΔS <0 ΔG = ΔH sys - T ΔS sys

31 ΔH < 0, ΔS < 0 A small negative value for the term (TΔS) can still make ΔG negative if it is smaller than the absolute value of ΔH Reaction is exothermic so the entropy of surroundings is increasing. At low temps, this will make a bigger difference than it would at higher temps. ΔG = ΔH sys - T ΔS sys

32 Calculating ΔG For a reaction at 25 C, ΔH = 100 kJ and ΔS = 80 J/K, determine if the reaction is spontaneous. For a reaction with a ΔH = 100 kJ and a ΔS of 80 J/K, at what temperature will the reaction become spontaneous? Watch your units!! Put temps in Kelvin and make sure you aren’t trying to add Joules to Kilojoules!

33 100 KJ – (298*80/1000) = 76.2 kJ = not spontaneous 0 = (x*80/1000)  100 = x(. 08) x = 1250 K, reaction becomes spontaneous at temperatures above 1250 K

34 Calculations of ΔS° rxn, ΔH° rxn and ΔG° rxn Standard Entropy of Formation Tables (ΔS° f ) –Σ n(ΔS° f ) products - Σ n(ΔS° f ) reactants Standard Gibbs Free Energy of Formation Tables (ΔG° f ) –Σ n(ΔG° f ) products - Σ n(ΔG° f ) reactants Standard Gibbs Free Energy of Formation Tables (ΔG° f ) –Σ n(ΔG° f ) products - Σ n(ΔG° f ) reactants –Only good for standard conditions! –For ΔG at non-standard conditions, use ΔG = ΔH - TΔS

35 Calculate the free-energy change,  G°, for the oxidation of ethyl alcohol to acetic acid using standard free energies of formation. CH 3 CH 2 OH(l) + O 2 (g)  CH 3 COOH(l) + H 2 O(l)

36 CH 3 CH 2 OH(l) + O 2 (g)  CH 3 COOH(l) + H 2 O(l)  G f °, kJ/mol– –392.5 –237.2 n, mol1111 n  G f °, kJ–174.80–392.5–237.2 –174.8 kJ–629.7 kJ  G° = –454.9 kJ  G° = –629.7 – (–174.8)

37 Sodium carbonate, Na 2 CO 3, can be prepared by heating sodium hydrogen carbonate, NaHCO 3 : 2NaHCO 3 (s)  Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) Estimate the temperature at which the reaction proceeds spontaneously at 1 atm. See Appendix C for data.

38 2NaHCO 3 (s)  Na 2 CO 3 (s) + H 2 O(g) + CO(g)  H f °, kJ/mol–947.7–1130.8–241.8–393.5 n, mol2111 n  H f °, kJ–1895.4–1130.8–241.8–393.5 – kJ– kJ  H° = kJ S f °, J/mol  K n, mol2111 nS f °, J/K J/K541.4 J/K  S° = J/K

39

40 Use ΔG to get K Equilibrium position represents the lowest free energy value available to a particular reaction system

41 ΔG and K Standard free energy change is related to the thermodynamic equilibrium constant, K, at equilibrium. –IF a reaction is NOT at equilibrium, it is proceeding in some direction (forward or reverse) depending on Q, reaction quotient. –That means there exists energy to do work (make reaction proceed) ΔG = Δ G° + RT ln Q At equilibrium: –Δ G = 0, because there is no ability to do any more useful work –and Q = K So we get: –Δ G° = –RT ln K

42 Calculate the value of the thermodynamic equilibrium constant at 25°C for the reaction N 2 O 4 (g) 2NO 2 (g) The standard free energy of formation at 25°C is kJ/mol for NO 2 and kJ/mol for N 2 O 4 (g).

43  G° = 2 mol(51.30 kJ/mol) – 1 mol(97.82 kJ/mol)  G° = kJ – kJ  G° = 4.78 kJ


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