Solutions

Dissolution of a solid:

Ions have more entropy (more states)

But,

Some water molecules have less entropy (they are grouped around ions).

Usually, there is an overall increase in S.

(The exception is very highly charged ions that make a lot of water molecules align around them.)

Chemical

Thermodynamics

Entropy Changes

• In general, entropy increases when
• Gases are formed from liquids and solids.
• Liquids or solutions are formed from solids.
• The number of gas molecules increases.
• The number of moles increases.

Chemical

Thermodynamics

Third Law of Thermodynamics

The entropy of a pure crystalline substance at absolute zero is 0.

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Chemical

Thermodynamics

Third Law of Thermodynamics

The entropy of a pure crystalline substance at absolute zero is 0.

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Entropy:

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2004

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Chemical

Thermodynamics

Standard Entropies

• These are molar entropy values of substances in their standard states.
• Standard entropies tend to increase with increasing molar mass.

Chemical

Thermodynamics

Standard Entropies

Larger and more complex molecules have greater entropies.

Chemical

Thermodynamics

Entropy Changes

Entropy changes for a reaction can be calculated the same way we used for ΔH:

S° for each component is found in a table.

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Note for pure elements:

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Chemical

Thermodynamics

Practical uses: surroundings & system

Entropy Changes in Surroundings

• Heat that flows into or out of the system also changes the entropy of the surroundings.
• For an isothermal process:

Chemical

Thermodynamics

Practical uses: surroundings & system

Entropy Changes in Surroundings

• Heat that flows into or out of the system also changes the entropy of the surroundings.
• For an isothermal process:

• At constant pressure, q_sys is simply ΔH° for the system.

Chemical

Thermodynamics

Link S and ΔH: Phase changes

A phase change is isothermal (no change in T).

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Entropy_system

Chemical

Thermodynamics

Entropy Change in the Universe

• The universe is composed of the system and the surroundings.

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Therefore,

ΔS_universe = ΔS_system + ΔS_surroundings

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• For spontaneous processes

ΔS_universe > 0

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Practical uses: surroundings & system

Chemical

Thermodynamics

Practical uses: surroundings & system

Chemical

Thermodynamics

Practical uses: surroundings & system

Make this equation nicer:

Chemical

Thermodynamics

−TΔS_universe is defined as the Gibbs free energy, ΔG.�

For spontaneous processes: ΔS_universe > 0

And therefore: ΔG < 0

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Practical uses: surroundings & system�…Gibbs Free Energy

ΔG is easier to determine than ΔS_universe.

So:

Use ΔG to decide if a process is spontaneous.

Chemical

Thermodynamics

Gibbs Free Energy

1. If ΔG is negative, the forward reaction is spontaneous.
2. If ΔG is 0, the system is at equilibrium.
3. If ΔG is positive, the reaction is spontaneous in the reverse direction.

Chemical

Thermodynamics

Standard Free Energy Changes

Standard free energies of formation, ΔG_f° are analogous to standard enthalpies of formation, ΔH_f°.

ΔG° can be looked up in tables,

or

calculated from S° and ΔH°.

Chemical

Thermodynamics

Free Energy Changes

Very key equation:

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This equation shows how ΔG° changes with temperature.

(We assume S° & ΔH° are independent of T.)

Chemical

Thermodynamics

Free Energy and Temperature

• There are two parts to the free energy equation:
• ΔH°— the enthalpy term
• TΔS° — the entropy term

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• The temperature dependence of free energy comes from the entropy term.

Chemical

Thermodynamics

Free Energy and Temperature

By knowing the sign (+ or -) of ΔS and ΔH,

we can get the sign of ΔG and determine if a reaction is spontaneous.

Chemical

Thermodynamics

Free Energy and Equilibrium

Remember from above:

If ΔG is 0, the system is at equilibrium.

So ΔG must be related to the equilibrium constant, K (chapter 15). The standard free energy, ΔG°, is directly linked to K_eq by:

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Chemical

Thermodynamics

Free Energy and Equilibrium

Under non-standard conditions, we need to use ΔG instead of ΔG°.

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Q is the reaction quotiant from chapter 15.

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Note: at equilibrium: ΔG = 0.

away from equil, sign of ΔG tells which way rxn goes spontaneously.

Chemical

Thermodynamics

Gibbs Free Energy

1. If ΔG is negative, the forward reaction is spontaneous.
2. If ΔG is 0, the system is at equilibrium.
3. If ΔG is positive, the reaction is spontaneous in the reverse direction.

Chemical

Thermodynamics