1. Thermodynamics Will a reaction happen?

2. Energy Substances tend to react to achieve the lowest energy state. Most chemical reactions are exothermic. Doesn’t work for things like ice melting. An ice cube must absorb heat to melt, but it melts anyway. Why?

3. Entropy The degree of randomness or disorder. S Drop a box of marbles. Watch your room for a week.

4. Entropy Entropy of a solid Entropy of a liquid Entropy of a gas A solid has an orderly arrangement. A liquid has the molecules next to each other. A gas has molecules moving all over the place.

5. Entropy increases when... Reactions of solids produce gases or liquids, or liquids produce gases. A substance is divided into parts -so reactions with more reactants than products have an increase in entropy. the temperature is raised -because the random motion of the molecules is increased. a substance is dissolved.

6. Entropy calculations There are tables of standard entropy. Standard entropy is the entropy at 25ºC and 1 atm pressure. Abbreviated Sº, measure in J/K.

7. Spontaneity Will the reaction happen, and how can we make it?

8. Spontaneous reaction spontaneous reactions will happen. nonspontaneous reactions don’t. Two factors influence. Enthalpy (heat) and entropy(disorder).

9. Two Factors Exothermic reactions tend to be spontaneous. -negative  H. -Reactions where the entropy of the products is greater than reactants tend to be spontaneous. Positive  S. -A change with positive  S and negative  H is always spontaneous. -A change with negative  S and positive  H is never spontaneous.

10. Other Possibilities Temperature affects entropy. Higher temperature, higher entropy. For an exothermic reaction with a decrease in entropy (like rusting). Spontaneous at low temperature. Nonspontaneous at high temperature. Entropy driven.

11. Other Possibilities An endothermic reaction with an increase in entropy like melting ice. Spontaneous at high temperature. Nonspontaneous at low temperature. Enthalpy driven.

12. Gibbs Free Energy The energy free to do work is the change in Gibbs free energy.  Gº =  Hº - T  Sº (T must be in Kelvin) All spontaneous reactions release free energy. If:  G <0 for a spontaneous reaction.  G = 0 reaction is at equilbrium  G >0 for a non spontaneous reaction.

13.  G=  H-T  S HH SS Spontaneous? -+- At all Temperatures GG ++? At high temperatures, “entropy driven” --? At low temperatures, “enthalpy driven” +-+ Not at any temperature, Reverse is spontaneous

14. 2H 2 S(g) + O 2 (g)  2H 2 O(l) + 2 S Find ΔG if ΔH is – 531.4 kJ and ΔS is – 412 J/K at 25 °C. Change the temperature to Kelvin 25 + 273 = 298

15. 2H 2 S(g) + O 2 (g)  2H 2 O(l) + 2 S  G =  H - T  S  G = -531.4 kJ - 298K (-412.5 J/K)  G = -531.4 kJ +123000 J  G = -531.4 kJ +123 kJ  G = -408.4 kJ  G = -410 kJ Spontaneous Exergonic- it releases free energy.

16. Find ΔG if ΔH is – 366 kJ and Δ S is 340 kJ/K at a temp of 38°C. Change temp to Kelvin 38 + 273 = 311 K ΔG = ΔH -T ΔS = -366 kJ – (311K) ( 340kJ/K) = -106106 kJ = - 110000 kJ Spontaneous reaction, negative  G

17. What is the ∆H and is the process spontaneous at 382 K, if ∆G is 22 000J and the ∆S is 322 J/K ? Answer: 150 000 J Assignment: p 948 # 488, 489,491 P 548 # 24,25 P 549 # 31