1. Enthalpy and Entropy Lesson 2

2. Which way does the reaction go?! At equilibrium, the reaction can go to the product side, and/or the reactant side. What favors one side over the other?

3. TWO THINGS YOU NEED TO KNOW ABOUT REACTIONS IN GENERAL 1. Reactions tend to go toward minimum enthalpy. 2. Reactions tend to go toward maximum entropy

4. 1. Reactions tend to go toward minimum enthalpy Minimum enthalpy or minimum energy is always favoured as molecules want to stay in a state where there is minimum energy ( more stable )

5. Products Favoured Remember Enthalpy (PE) on a PE graph Identify Minimum enthalpy/Minimum PE = This is where the reaction will go.

6. Reactants Favoured

7. Min enthalpy review Min enthalpy favours side with heat term –If rxn is exothermic, prefers products. –If rxn is endothermic, prefers reactants.

8. 2. Reactions tend to go toward maximum entropy Entropy (S) = amount of randomness in a system.

9. Why is MAXIMUM entropy favored? Think about this…

10. If you tossed bricks off of a truck, which pile of bricks are you most likely to produce???

11. Maximum Entropy It is all to do with probability … –The possibility of a very highly organized outcome taking place is much, much lower than a highly random outcome –There are more random outcomes possible than highly organized ones.

12. How do we figure out entropy in a reaction? Ask yourself: what is the most random phase present?

13. How do we figure out entropy in a reaction? Ask yourself: what is the most random phase present? Gases  aqueous solutions  pure liquids  Solids most random (least organized)  least random (most organized)

14. Entropy examples CaC 2 (s) + 2H 2 O (l) C 2 H 2 (g) + Ca(OH) 2 (aq) ↔

15. Entropy Examples A (g) + B (s)  2C (g) + D (s) The side having the most number of particles in the most random phase will have the most randomness and will be favoured.

16. Entropy is randomness or a measure of how spread out, or broken up the system is. Less EntropyMore Entropy H 2 O (s) H 2 O (g) H 2 O (l) NaCl (s) NaCl (aq) N 2 O 4(g) 2NO 2(g) glassbroken glass clean roommessy room H 2 O (l)

17. Entropy summary more moles of gases  gases  aqueous slns  pure liquids  Solids Most random(max entropy)  least random(min entropy)

18. Putting things together Remember, reactions favour: 1- sides with minimum enthalpy 2- sides with maximum entropy.

19. Products are favoured Are the products or Reactants favoured? 0 gases1 gas 1.Zn (s) + 2HCl (aq) H 2(g) + ZnCl 2(aq) ΔH = -252 KJ Goes to Completion For entropy- only look at phase symbols- mainly gases spontaneous ↔ ?

20. ⇌ Products Reactants

21. Which Way? Right or left? Equilibrium spontaneous Are the Products or Reactants favoured? 1 gas2 gases 2.N 2 O 4(g) 2NO 2(g) ΔH = +20 KJ ↔ ?

22. ⇌ reactants Products

23. Which Way? Right or left? Reactants are favoured Does not occur Non spontaneous! Are the Products or Reactants favoured? 3 gases1 gas 3.3C (s) + 3H 2(g) + 45 KJ C 3 H 6(g) ↔ ?

24. ⇌ reactants products

25. More examples: Which way will this reaction go? Zn (s) + 2HCl (aq) → H 2(g) + ZnCl 2(aq) ΔH = -252 KJ

26. Which way will this reaction go? 3C (s) +3H 2(g) + 45 KJ→ C 3 H 6(g)

27. Think… Where would maximum entropy be for an endothermic equilibrium system? Maximum entropy Must be on the opposite side Minimum enthalpy Entropy must increase I’m Entropy

28. Page 47 examples C 2 H 2(g) + 2Cl 2(g)  C 2 H 2 Cl 4(l) + 386 KJ

29. CH 4(g) + O 2(g)  CO 2(g) + 2H 2 O (g) + 394 KJ

30. 4 Au (s) + 3 O 2(g) + 162 KJ  2 Au 2 O 3(s)

31. Summary To find out which side the reaction will go -where is min enthalpy? –If rxn is exothermic, prefers products. –If rxn is endothermic, prefers reactants. -where is max entropy? more moles of gases  gases  aqueous slns  pure liquids  Solids

32. Summary If both min enthalpy and max entropy are at the same side, this is the side the rxn goes to. -products (completion) -reactants side (does not occur) If they are at different sides, it means it is at equilibrium.

33. Homework Hebden: page 48-49 questions 14-16