1. Topic: Reaction Spontaneity Do Now:

2. Spontaneous Processes no outside intervention =physical or chemical change that occurs with no outside intervention activation energyHowever, some energy may be supplied to get process started = activation energy Iron rusting 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = -1625 kJ Combustion CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (l)  H = -891kJ

3. Because both reactions are spontaneous and exothermic, some 19 th century scientist thought that all spontaneous reactions are exothermic…until they one day… H 2 O (s)  H 2 O (l) endothermic

4. Some Spontaneous Processes are endothermic, some are exothermic ΔH plays a role in determining whether a chemical process occurs spontaneously, but there is another piece to the puzzle. ENTROPYENTROPY

5. Entropy, S measure of disorder or randomnessmeasure of disorder or randomness of particles in system Law of Disorder (aka 2 nd law of thermodynamics)Law of Disorder (aka 2 nd law of thermodynamics) : spontaneous processes always occur in direction that increases chaos of system

6.  S Changes in Entropy,  S  S = S final – S initial or S products – S reactants Spontaneous reactions need +  S

7. lazy & disorganized Nature is lazy & disorganized system to lower energy: must release energyFor system to lower energy: must release energy –nature favors exothermic processes with -  H Nature wants to increase S, so S final > S initial Nature wants  S to be positiveNature wants  S to be positive But: –spontaneous endothermic processes can occur –so something(s) else important, too Low energy (- ΔH) Chaos (+ ΔS)

8.  S Affecting  S Changes in state >> S gas > S liquid > S solid Dissolving > CO 2(g)  CO 2(aq) S gas > S Dissolved Gas > NaCl (S) NaCl (aq) S Dissolved Solid > S Solid > S Mixture > S Pure Substance Entropy increases as you increase the number of gas molecules Entropy increases as you increase the number of gas molecules  S > 0 2SO 3(g)  2SO 2(g) + O 2(g)  S > 0 Temperature Temperature KE increases, Entropy increases KE increases, Entropy increases

9. Predicting Entropy Which has more entropy: –1 mole dry ice [CO 2 (s)] –1 mole CO 2 gas 1 mole of CO 2 gas Which has more entropy: –2 mole NaCl(s) –1 mole NaCl(aq) 1 mole NaCl(aq)

10. Predicting Spontaneity  G =  H - T  SUse Gibbs free energy expression  G =  H - T  S –  G = Free energy (available to do work) –  H = enthalpy (heat) of reaction –T= temperature –  S = change in entropy (disorder)  G is (-)spontaneousIf  G is (-) then rxn is spontaneous  G is (+)not spontaneousIf  G is (+) then rxn is not spontaneous

11.  G =  H - T  S  S  S HHHH 1 2  H 34 Sometimes, more likely at high temp Never spontaneous Always spontaneous Sometimes, more likely at low temp  G is (-)spontaneous If  G is (-) then rxn is spontaneous  G is (+)not spontaneous If  G is (+) then rxn is not spontaneous

12. Summary Nature is lazy and disorganized  H and +  S wants -  H and +  S Because the universe equals the system and the surrounding, spontaneous reactions will occur if  H and +  S always Because the universe equals the system and the surrounding, spontaneous reactions will occur if -  H and +  S always

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