Entropy

1. Entropy

2. 1st Law of Thermodynamics • Energy is neither created nor destroyed • The energy of the universe is constant • Energy just changes from one form to another • This law lets us know the energy of the system, but does not give us any information about the direction of the energy flow

3. Spontaneity • Spontaneous process – a process that occurs without intervention • Spontaneous processes can be fast or slow • Spontaneity tells us the direction of the energy flow • It tells us NOTHING about the speed of the reaction

4. For example… • A ball spontaneously rolls down a hill • It does not spontaneously roll up • If iron is exposed to air, it spontaneously rusts • The rust does not spontaneously turn back into air & iron

5. Entropy • Entropy (s) – the measure of molecular randomness or disorder • Think of entropy as the amount of chaos • The driving force for a spontaneous process is an increase in entropy

6. Entropy • The natural progression of things is from order to disorder • Or from lower entropy to higher entropy • Think of a deck of cards…when you drop one it goes from order to disorder • Think of your room… it goes from order to disorder • Think if entropy as a probability…not a certainty • Nature spontaneously proceeds toward that states that have the highest probability of existing

7. Entropy • Predict which has the highest entropy • CO2 (s) or CO2 (g) CO2 (g) • 1 mol of N2 at 1 atm or 1 mol of N2 at 0.001 atm 1 mol of N2 at 0.001 atm

8. Entropy • Predict the sign of the entropy change for the following… • Sugar is added to water to form a solution + • Iodine vapor condenses on a cold surface to produce a liquid -

9. 2nd Law of Thermodynamics • 2nd Law of Thermodynamics – In any spontaneous process there is always an increase in entropy of the universe • Energy is conserved…entropy is NOT conserved! • The entropy of the universe is always increasing • S univ = + = spontaneous • S univ = - = not spontaneous (would be spontaneous in the opposite direction) • S univ = 0 = no tendency to occur (system is at equilibrium) • S univ = S sys + S surr

10. Temperature & Spontaneity Ssurr = _ H T T must be in Kelvin • H is usually given in KJ/mol • S will be in KJ/K, but is usually changed to J/K

11. Example Sb2S3 (s) + 3Fe (s)  2Sb (s) + 3FeS (s) H = -125 kJ Calculate Ssurr for this reaction at 25C and 1 atm S = - (H/T) S = - (-125KJ/298K) S = 0.419 KJ/K S = 419 J/K

12. Example Sb4O6 (s) + 6C (s)  4Sb (s) + 6CO (g) H = 778 kJ Calculate Ssurr for this reaction at 25C and 1 atm S = - (H/T) S = - (778KJ/298K) S = -2.61 KJ/K S = -2610 J/K

13. 3rd Law of Thermodynamics • The entropy of a perfect crystal at 0K is zero

14. Free Energy • Free energy (G) – a thermodynamic function equal to the enthalpy minus the product of the entropy and the Kelvin temperature • G = H – TS • A process is only spontaneous in the direction where G is negative

15. Example • At what temperatures is the following process spontaneous at 1 atm? • Br2(l)  Br2(g) • H = 31.0 KJ/mol  31000 J/mol • S = 93.0 J/ K mol • G = H – TS • 0 = 31000 – T(93.0) • T = 333K • Above 333K the reaction is spontaneous

16. Dependence of H & S on Spontaneity G = H-TS Spontaneous at all temperatures Spontaneous at high temperatures Spontaneous at low temperatures Not spontaneous at any temperature