Topic: Reaction SpontaneityDo Now:

Spontaneous Processes=physical or chemical change that occurs with no outside intervention

• However, some energy may be supplied to get process started = activation energy

Iron rusting4Fe(s) + 3O2(g) 2Fe2O3(s)

H = -1625 kJ

Combustion CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -891kJ

Because both reactions are spontaneous and exothermic, some 19th century scientist

thought that all spontaneous reactions are exothermic…until

they one day…

H2O(s) H2O(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.

•

ENTROPY

Entropy, S• measure of disorder or randomness of

particles in system

• Law of Disorder (aka 2nd law of thermodynamics): spontaneous processes always occur in direction that increases chaos of system

Changes in Entropy, S

S = Sfinal – Sinitial or Sproducts – Sreactants

Spontaneous reactions need +S

Nature is lazy & disorganized

• For system to lower energy: must release energy– nature favors exothermic processes with -H

• Nature wants to increase S, so Sfinal > Sinitial

• Nature wants S to be positive• But:

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

Low energy(- ΔH)

Chaos(+ ΔS)

Affecting S• Changes in state

Sgas > Sliquid > Ssolid

• Dissolving• CO2(g) CO2(aq) Sgas > SDissolved Gas

• NaCl(S) NaCl(aq) SDissolved Solid > SSolid SMixture > SPure Substance

• Entropy increases as you increase the number of gas molecules• 2SO3(g) 2SO2(g) + O2(g) S > 0

• Temperature• KE increases, Entropy increases

Predicting Entropy• Which has more entropy:

– 1 mole dry ice [CO2(s)]– 1 mole CO2 gas1 mole of CO2 gas

• Which has more entropy: – 2 mole NaCl(s)– 1 mole NaCl(aq)

1 mole NaCl(aq)

Predicting Spontaneity• Use Gibbs free energy expression G = H - TS– G = Free energy (available to do work)– H = enthalpy (heat) of reaction– T= temperature– S = change in entropy (disorder)

• If G is (-) then rxn is spontaneous• If G is (+) then rxn is not spontaneous

G = H - TS+ S - S

+ H 1 2

-H3 4

Sometimes, more likely at high

temp

Never spontaneous

Always spontaneous

Sometimes, more likely at low temp

If G is (-) then rxn is spontaneousIf G is (+) then rxn is not spontaneous

SummaryNature is lazy and disorganized

wants -H and +S

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