1. Thermodynamics Lecture 3
Gibb’s Free Energy
Thermodynamics Lecture 3

2. Gibb’s Free Energy (ΔG)
Energy associated with a chemical reaction that can be used to do work
Value tells whether a reaction is spontaneous or not
Spontaneous: reaction that occurs without being driven by some outside force
Two driving forces for chemical reactions:
ΔS= entropy (disorder)
ΔH= enthalpy (heat)

3. ΔG= ΔH- TΔS If ΔG is negative If ΔG is positive
Reaction is spontaneous
If ΔG is positive
Reaction is non spontaneous (work must be done to make the reaction occur)

4. Question 1
Is ice melting at a temperature less than 0°C a spontaneous reaction?
H2O (s)  H2O (l)
ΔG= ΔH- TΔS
Melting is endothermic reaction so ΔH is positive
T is low
Entropy increases as you go from solid to liquid

5. Question 2
Is condensation at a temperature less than 100 °C a spontaneous reaction?
H2O (g)  H2O (l)
ΔG= ΔH- TΔS
Condensation is exothermic reaction so ΔH is negative
T is low
Entropy decreases as you go from gas to liquid so it is a negative value

6. ΔH ΔS T
-TΔS
ΔG= ΔH- TΔS
Reaction Characteristics - +
Low
spontaneous
High
Nonspontaneous
Spontaneous