1. Spontaneity

2. Spontaneous Processes P/C change that occurs with no outside intervention exothermic chemical rxns are spontaneous energy still must be supplied to get process started = activation energy

3. Examples of Exothermic Spontaneous Processes Burning methane gas, CH 4 : CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  H = -891 kJ Oxidation of iron, Fe (rusting): 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = -1625 kJ

4. Some Spontaneous Processes are Endothermic Dissolving - NH 4 NO 3 (s) in water: spontaneous endothermic process NH 4 NO 3 (s)  NH 4 + (aq) + NO 3 - (aq) Phase change - ice melting: also spontaneous, endothermic process H 2 O(s)  H 2 O(l)

5. Nature is lazy & disorganized 2 driving forces in nature 1. to achieve lowest energy state 2. to achieve most chaotic state for system to lower its energy: must release energy that’s why nature favors exothermic processes (-  H)

6. But: spontaneous endothermic processes can occur too so something else must also be important…

7. Enthalpy: H for heat describes chemical PE stored in matter can only measure changes in enthalpy enthalpy is arithmetical –reverse reaction, reverse sign of  H –if multiply or divide # moles reactants/products then multiply/divide  H by same number

8. Entropy: S measure of disorder or randomness of particles in system Law of Disorder (2 nd Law of Thermodynamics): spontaneous processes always occur in direction that increases chaos of system First Law of ThermodynamicsFirst Law of Thermodynamics: Energy can be changed from one form to another, but it cannot be created or destroyed.

9. Chaos/Randomness more degrees of freedom system has the more chaotic it can be degrees of freedom: ways particles can move & places particles can be found gases MUCH more entropy than liquids & liquids more than solids

10. Entropy = 0? pure crystal with no imperfections at 0K every atom is where it’s supposed to be nothing is moving entropy would be zero

11. Changes in Entropy,  S  S = S final – S initial OR S products – S reactants nature wants to increase S, so S final > S initial nature wants  S to be positive

12. Predicting Changes in Entropy S gas S liquid S solid S mixture S pure substance S dissolved solid S solid S gas S dissolved gas > > > > >

13. Predicting Entropy Which has more entropy? 1 mole dry ice [CO 2 (s)] OR 1 mole CO 2 gas 1 mole of CO 2 gas

14. Temperature & Entropy ↑ T: ↑ random motion of particles, so entropy ↑ with ↑ T

15. Predicting Spontaneity Use Gibbs free energy expression  G =  H - T  S If  G is (-) then rxn is spontaneous If  G is (+) then rxn is not spontaneous

16.  G =  H - T  S  H is negative;  S is positive  H is negative;  S is negative  H is positive;  S is positive  H is positive;  S is negative always spontaneous never spontaneous spontaneity depends on temperature: hi T – yes hi T – yes low T - no hi T – yes low T - no

17. Summary Nature’s 2 driving forces: H & S 1.nature tends to minimize enthalpy (heat PE) wants  H to be negative 2.nature tends to maximize entropy (chaos) wants  S to be positive