Chemical Thermodynamics

Chapter 19 Notes

We have looked at:

1. How quickly a reaction occurs - rates of reactions
2. How far towards completion does a reaction proceed - equilibrium

Chemical Thermodynamics -- exploring energy relationships

Section 19-1
Spontaneous Processes

1st Law of Thermodynamics (Law of Conservation of Energy) -- energy is conserved -- energy is neither created nor destroyed, but changed from one form to another
/\E = q + w

/\E = change in energy of the system
q = heat absorbed by the system from its surroundings
w = work done on the system by its surroundings
These processes are called "spontaneous"

when, T > OC --> ice melts spontaneously
when, T < OC --> ice freezes spontaneously
when, T = OC --> solid and liquid phases are at equilibrium - not favored in one direction or another

State Functions -- enthalpy, temperature, and internal energy
Reversible Process --> a way in which a system can change its state -- the system can be restored to its original state by exactly reversing the process (there is only one reversible path between any 2 states of a system)

Irreversible Process --> cannot be reversed to restore the system to its original state (must take a different path to get back to its original state) So,
1. When a system is at chemical equilibrium, we can go reversibly between the products and reactants.
2. In any spontaneous process, the path between the products and reactants is irreversible.

Section 19-2
Entropy & the 2nd Law of Thermodynamics

The change in disorder along with the change in energy affects the spontaneity of chemical processess --> disorder is called Entropy (the more disorder or randomness, the larger the entropy value)

/\S = Sfinal - Sinitial

/\S = change in entropy (depends only on the initial and final states of the system, not the path it took to change)

Liquid ---> Gas (process of vaporization = disorder = /\S would be positive

Aqueous ---> Solid (movement in aqueous solution to more ordered) = /\S would be negative

From a gas to a solid, /\S becomes more negative (more organized)

From a solid to a gas, /\S becomes more positive (more unorganized)

Entropy is a state function - heat is not

For a process that occurs at constant temperature, the entropy change can be expressed as:

/\S = qrev / T


T = absolute temperature
qrev = heat transferred in the reverse reaction

Remember, /\Hfusion is defined for melting (freezing would be the reverse, therefore, you reverse the sign)

2nd Law of Thermodynamics

/\Suniv = /\Ssystem + /\Ssurr

so, in any reversible process, /\Suniv = 0 and in any irreversible process, /\Suniv > 0 (spontaneous process)

Reversible = /\Suniv = /\Ssys + /\Ssurr = 0 (at equilibrium)

Irreversible = /\Suniv = /\Ssys + /\Ssurr > 0 (spontaneous)

and if,

/\Suniv = /\Ssys + /\Ssurr < 0 (non-spontaneous)

Section 19-3
Third Law of Thermodynamics

Section 19-4
Calculation of Entropy Changes

Molar entropy -- of substances in their standard states, S (standard state -- pure substance at 1 atm)

1. Stardard molar entropies are not zero (single elements will have a value -- they will not be zero)
2. Standard molar entropies of gases are greater than those of liquids and solids
3. Standard molar entropies generally increase with increasing molar mass of the substance.
4. Standard molar entropies generally increase with the number of atoms in the formula of the substance

/\S = ΣnS products - ΣmSreactants

n & m are the coefficients from the balanced equation

Section 19-5
Gibbs Free Energy

G = H - TS
/\G = /\H - T/\S
(change in free energy of the system)

If temperature and pressure are constant, then:

1. If G = negative value --> reaction is spontaneous in the forward direction
2. If G = 0 --> reaction is at equilibrium
3. If G = positive value --> reaction in the forward direction in nonspontaneous (work must be supplied from the surroundings to make it occur, however, the reverse reaction will be spontaneous)

In any spontaneous process at constant temperature and pressure, the free energy always decreases

/\G = Σn/\Gproducts - Σm/\Greactants

This equation tells us whether the reaction will proceed in the forward direction to produce more products (/\G < 0), or the reverse direction to produce more reactants (/\G > 0)