Chemical Thermodynamics
Chapter 19 Notes
We have looked at:
1. How quickly a reaction occurs - rates of reactions
- rate depends on the activation energy - the lower the activation energy, the faster it will proceed
2. How far towards completion does a reaction proceed - equilibrium
- equilibrium depends on the rates of the forward and reverse reactions
Chemical Thermodynamics -- exploring energy relationships
- Reaction that occurs at constant pressure, the enthalpy change equals the heat transferred between the system and its environment (enthalpy is not the only means we have of predicting/determining if a chemical reaction will take place)
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
where,
/\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
- there are some things that happen without any outside intervention
- ice will freeze if put in a freezer and melt if left out, a nail will rust if left outside, you touch something hot, the heat is transferred to you)
These processes are called "spontaneous"
temperature can cause an effect on spontaneity of a process
when, T > OºC --> ice melts spontaneously
when, T < OºC --> ice freezes spontaneously
when, T = OºC --> solid and liquid phases are at equilibrium - not favored in one direction or another
State Functions -- enthalpy, temperature, and internal energy
- defines a state and does not depend on how we get there
- q & w are not state functions --> they depend on the specific path taken from one state to another
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)
- there will be a change in the surroundings that cannot be brought back to the original
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
where,
/\S = change in entropy (depends only on the initial and final states of the system, not the path it took to change)
Positive /\S = final state is more disordered than the initial state
Negatie /\S = final state is more ordered than the initial state
Liquid ---> Gas (process of vaporization = disorder = /\S would be positive
Aqueous ---> Solid (movement in aqueous solution to more ordered) = /\S would be negative
- Liquid ---> more organized
- Solid ---> most organized
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
a change in entropy is related to the heat transferred during the process
For a process that occurs at constant temperature, the entropy change can be expressed as:
/\S = qrev / T
Where,
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
- this law expresses the notion that there is an inherent direction in which processes occur (consider both the entropy of the system and the surroundings)
/\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)
2nd law demands that the increase in entropy of the surroundings must be greater than the decrease of entropy of the system
no process that produces order (decrease in entropy) in a system can proceed without producing an equal or greater disorder (increase in entropy) in its surroundings
Section 19-3
Third Law of Thermodynamics
- entropy of a pure crystalline substance at absolute zero is zero
Section 19-4
Calculation of Entropy Changes
Molar entropy -- of substances in their standard states, Sº (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
/\Sº = ΣnS products - ΣmSºreactants
where,
n & m are the coefficients from the balanced equation
Section 19-5
Gibbs Free Energy
- the spontaneity of a reaction seems to involve 2 concepts - enthalpy and entropy
-
can use /\H & /\S to predict whether a given reaction will be spontaneous ---> Gibbs Free Energy (G) -- state function expressed in kJ
- free (single) elements are zero
G = H - TS
or
/\G = /\H - T/\S
(change in free energy of the system)
- the sign of /\G gives very important information about the spontaneity of processes that occur at constant temperature and pressure
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
if Q < K, then there is an excess of reactants to products and the reaction will proceed spontaneously in the forward direction to reach equilibrium where Q = K
if Q > K, it proceeds in the reverse reaction
/\Gº = Σn/\Gºproducts - Σm/\Gºreactants
This equation tells us whether the reaction will proceed in the forward direction to produce more products (/\Gº < 0), or the reverse direction to produce more reactants (/\Gº > 0)