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Laws of Thermodynamics
1st law:
The universe can be thought of as simply a system we are interested in and its surroundings. For such a system,
E is the internal energy of a system. It is the total of all the possible kinds of energy in the system. E cannot be measured, but DE can.
Q is positive if heat is added to a system from the surroundings and negative if the system gives up heat to its surroundings.
W is positive if work is done on the system by the surroundings (e.g. compressing gas in a container) and negative if a system does work on the surroundings (e.g. if a compressed gas is allowed to expand).
Enthalpy
If DH is positive, heat is absorbed from the surroundings and the reaction is said to be endothermic.
2nd law:
3rd law:
Gibb's Free Energy
DG incorporates both DH and DS, as show below.
where G is Gibb's free energy (so named because it is the energy that is free to do work),
H is enthalpy (heat produced or consumed under conditions of constant pressure),
S is entropy (degree of disorder), and
T is temperature in K.
Why is DS multiplied by T? The higher the temperature, the greater the kinetic energy of the molecules and, thus, the greater the tendency for the system to go to a state of increased disorder (an analogy is the increased disorder an earthquake creates).
If DG <0, the forward reaction is spontaneous.
However, even if a reaction is spontaneous, if the activation energy is too high it will not proceed. Such a reaction is said to be under kinetic control and may require the use of a catalyst or energy input such as heat (which would also alter the equilibrium position).
If a reaction occurs as DG predicts, it is said to be under thermodynamic control.
Keq = e
DG°/-RT
Thus if DG is large and negative,
Keq will be large.
And, Keq = enFE°/RT
If E is large and positive, Keq will be large.
Also,
DG° = -nFE°
Thus, if E is positive, DG will be negative.
(Note: the symbol ° denotes that standard states exist, i.e. the concentration of each reactant and product is 1 M if in solution, and 0.1 MPa if gaseous.)
LeChatelier's principle
When a system at equilibrium is subjected to a stress, it will shift in a direction that minimizes the effect of this stress.