ThermodynamicsThermodynamicsChapter 19Chapter 19

Liquid benzene

Production of quicklimeProduction of quicklime

Solid benzene

⇅

CaCO3 (s) CaO + CO⇌ 2

Three Laws of ThermodynamicsThree Laws of Thermodynamics

1st Law1st Law: Energy is conserved in any process: Energy is conserved in any process

22ndnd Law Law: Defines a “spontaneous” process: Defines a “spontaneous” process

33rdrd Law Law: Defines absolute disorder: Defines absolute disorder

Kinetics Kinetics → How fast does a reaction proceed?→ How fast does a reaction proceed?

Thermodynamics → Thermodynamics → DoesDoes a reaction proceed? a reaction proceed?

Fig 19.1 Spontaneous and nonspontaneous processes

spontaneous

nonspontaneous

Spontaneous Physical and Chemical Processes

Reversible ProcessesReversible Processes

System changes in such a way that system and surroundings can be put back in their original states by exactly reversing the process.

Irreversible ProcessesIrreversible Processes Irreversible processes cannot be undone by exactly

reversing the change to the system

Spontaneous processes are irreversible

Fig 19.5

Entropy (S) - measure of randomness or disorder of a system

order SdisorderS

S = Sfinal - Sinitial

For an isothermal process:

S =qrev

T

State functions - properties that are determined by the state of the system, regardless of how that condition was achieved.

at constant T

How does the entropy of a system change for each of the following processes?

(a) Condensing water vapor

Randomness decreases Entropy decreases (S < 0)

(b) Forming sucrose crystals from a supersaturated solution

Randomness decreases Entropy decreases (S < 0)

(c) Heating hydrogen gas from 60°C to 80°C

Randomness increases Entropy increases (S > 0)

(d) Subliming dry ice

Randomness increases Entropy increases (S > 0)

First Law of Thermodynamics

Energy can be converted from one form to another but energy cannot be created or destroyed.

Second Law of Thermodynamics

The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.

Suniv = Ssys + Ssurr > 0Spontaneous process:

Suniv = Ssys + Ssurr = 0Equilibrium process:

Suniv = Ssys + Ssurr ≥ 0For any process:

Entropy on the Molecular Scale Molecules exhibit several types of motion:

Translational: Movement of the entire molecule from one place to another.

Vibrational: Periodic motion of atoms within a molecule.

Rotational: Rotation of the molecule on about an axis or rotation about bonds.

• Boltzmann envisioned the motions of a sample of molecules at a particular instant in time

– e.g., taking a snapshot of all the molecules

• This sampling ≡ a microstate of the thermodynamic system

Entropy on the Molecular Scale

Entropy on the Molecular Scale

Each thermodynamic state has a specific number of microstates, W, associated with it

Entropy ≡ S = k lnW

where k is Boltzmann constant, 1.38 1023 J/K

• Change in entropy for a process:

S = k lnWfinal k lnWinitial

S = k lnWfinal

Winitial

o Entropy increases with the number of microstates in system

Entropy on the Molecular Scale

o Number of microstates and, therefore, the entropy tends to increase with increases in:

o Temperature

o Volume

o Number of independently moving molecules

Entropy and Physical States

• Entropy increases with the freedom of motion of molecules

S(g) > S(l) > S(s)

• Generally, when a solid is dissolved in a solvent, entropy increases.

In general, entropy increases when

◦ Gases are formed from liquids and solids

◦ Liquids or solutions are formed from solids

◦ Number of gas molecules increases

◦ Number of moles increases

Fig 19.11

Third Law of Thermodynamics

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

Fig 19.13 Perfectly ordered crystalline solid at and above 0 K

Fig 19.14 Entropy as a function of temperature