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.