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Spontaneity
Recap of Enthalpy
• Describes chemical potential energy stored in matter.
• Can only measure changes in enthalpy.
• Enthalpy is arithmetical.– Reverse equation, reverse sign of H– Multiply equation by some #, multiply H by
that number.– Add two equations, add the H’s.
Spontaneous Processes
• A physical or chemical change that occurs with no outside intervention.
• Some energy may be supplied to get the process started – activation energy
Examples of Exothermic Spontaneous Processes
• Burning methane gas, CH4, in the bunsen burner.
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -891 kJ
• Iron rusting
4Fe(s) + 3O2(g) 2Fe2O3(s) H = -1625 kJ
Some Spontaneous Processes are Endothermic
• Dissolving NH4NO3(s) in water is an example of a spontaneous endothermic process.
• NH4NO3(s) NH4+(aq) + NO3
-(aq)
• Ice melting is another spontaneous, endothermic process.
• H2O(s) H2O(l)
Nature is lazy & disorganized
• 2 driving forces in nature– Get to the lowest energy state– Get to the most chaotic state
• For a system to get lower in energy, it must release energy.
• So nature favors exothermic processes with -H.
• But spontaneous endothermic processes occur, so something else is important, too.
Entropy, S
• A measure of the disorder or randomness of the particles of a system.
• Law of Disorder: spontaneous processes always occur in the direction that increases the chaos of the universe.
Unmelt.mov
Unmix.mov
Unshatr.mov
Chaos
• The more degrees of freedom a system has, the more chaotic it can be.
• Degrees of freedom = ways you can move & places you can be.
• Gases have much more entropy than liquids & liquids have more than solids.
Entropy = 0?
• A pure crystal with no imperfections at 0K.
• Every atom is where it’s supposed to be.
• Nothing is moving.
Changes in Entropy, S
S = Sfinal – Sinitial or Sproducts – Sreactants
• Nature wants to increase S, so Sfinal > Sinitial
• Nature wants S to be positive.
Predicting Changes in Entropy
• Sgas > Sliquid > Ssolid
• SMixture > SPure Substance
• SDissolved Solid > SSolid but
• Sgas > SDissolved Gas
Predicting Entropy
• Which has more entropy, 1 mole of dry ice or 1 mole of CO2 gas?
• 1 mole of CO2 gas
Predicting Changes in Entropy
• 2SO3(g) 2SO2(g) + O2(g) S > 0
• 2 moles of gas on the reactant side.
• 3 moles of gas on the product side.
• Plus, the product side is a mixture.
Temperature & Entropy
• An increase in temperature increases the random motion of the particles, so entropy increases with temperature.
Predicting Spontaneity
• Use Gibbs free energy expressionG = H - TS
• If G is negative, rxn is spontaneous.
• If G is positive, rxn is not spontaneous.
G = H + (-TS)
H S -TS G
+ + - ? G = f(T)
+ _ + Never negative
NOT spont.
- + - Always negative
SPONT.
- - + ? G = f(T)
G = H - TS
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.Always spontaneous.
Never spontaneous.Never spontaneous.
Spontaneity depends on temperature.Spontaneity depends on temperature.
Spontaneity depends on temperature.Spontaneity depends on temperature.
S is neg.
in chaos.
S is pos.
In chaos.
H is neg.
in energy.
Can’t tell.
G = f(T).
G < 0. Always spontaneous
H is pos.
In energy.
G > 0.
Never spontaneous
Can’t tell.
G = f(T).
G = H - TS
G is negative for spontaneous processes.
Summary
• Nature has 2 driving forces
• Tends to minimize enthalpy (potential energy). Wants H to be negative.
• Tends to maximize entropy (chaos). Wants S to be positive.
Summary
• Spontaneity is determined by the combination of the enthalpy change and the entropy change.
Predict the sign of G
• LiBr(s) Li+(aq) + Br-(aq) + 48.83 kJ
• The reaction is exothermic so H is negative. The chaos increases so S is positive. Both enthalpy and entropy are going in the direction preferred by nature. This reaction is always SPONTANEOUS.
Predict the sign of G
• N2(g) + 2 O2(g) + 66.4 kJ 2 NO2(g)
• The reaction is endothermic so H is positive. The chaos decreases (3 moles of a gas to 2 moles of a gas) so S is negative. Neither the enthalpy nor the entropy goes in the direction that nature prefers, so this reaction is NEVER spontaneous.
Predict the sign of G
• 2 H2(g) + O2(g) 2 H2O(l) + 571.6 kJ
• The reaction is exothermic: H is negative.
• Chaos decreases: S is negative.
• One driving force, enthalpy, is with nature.
• Entropy is going against nature.
• CANNOT predict the spontaneity of this rxn.