We are most familiar with
H, the change in enthalpy.
As studied previously,
corresponds to q, the heat transferred to the system from the surroundings, when measured at
constant pressure conditions.
If q is positive,
H is positive. Heat enters the system from the surroundings; the reaction
If q is negative,
H is negative. Heat leaves the system into the surroundings; the reaction
Thermodynamics investigates reaction
If we have well-defined initial and
final states (states A and B) , will the reaction tend to from A to
B or from B to A.
not at all?
We’ve already analyzed this question with the Q test.
If we know the equilibrium
constant for the reaction:
Q < K
Reaction is spontaneous to the right
Q > K
Reaction is spontaneous to
Q = K
Reaction is at equilibrium
As we will be studying in this chapter, we will relate reaction spontaneity, (and indeed,
even the value of K), to thermodynamic quantities,
It is important to understand that “spontaneous” does not necessarily mean “fast.”
spontaneous reaction is one that will go, sooner or later.
The thermodynamic functions,
G, are all
This means that their values depend only on final and initial states,
not on the path or process of the change.
Thermodynamics does not examine the path or process
—this is studied in kinetics, where we have already examined, activation energies and
mechanisms, which both relate to process and speed of reaction.
Thermodynamics only looks at
initial and final states, examines differences between these states (
G), and then
calculates reaction spontaneity (will it go from A to B or from B to A) and also calculates the
extent of the reaction (also measured by the equilibrium constant K).