Topic: Reaction Spontaneity Do Now:

Spontaneous Processes no outside intervention =physical or chemical change that occurs with no outside intervention Sometimes energy (referred to as activation energy) is supplied to get the process started Iron rusting 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = kJ Combustion CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (l)  H = -891kJ

Because both reactions on the previous page are spontaneous and exothermic, so 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

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

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

Spontaneous processes occur in the direction to increase chaos because that’s how nature works!

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

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 Low energy (- ΔH) Chaos (+ ΔS)

 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

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)

But: spontaneous endothermic processes can occur so something(s) else important, too

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

 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

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

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

Thermodynamics is the study of entropy and enthalpy changes that accompany chemical reactions Thermodynamics Tells us if a reaction will occur