Worksheet 5.6
Thermodynamics Continued
We have been discussing enthalpy (H), entropy (S) and Gibbs energy (G). The second law of
thermodynamics states that the entropy of the universe must always increase. ΔS (for the
universe) being positive is equivalent to ΔG being negative, in which case means that the
reaction/process will “go”.
ΔG = ΔH - TΔS
since Tcannotbe negative
1. For the following conditions, predict whether or not the process will occur:
ΔH
ΔS
Negative
(exothermic)
Positive (system
entropy
increases)
negative
NO
Positive
Negative (system
(endothermic) entropy
decreases)
positive
No
Negative (system
entropy
decreases)
positive
negative
Yes
Positive
Positive (system
(endothermic) entropy
increases)
positive
negative
Yes
Negative
(exothermic)
G
705
What would Would the
you predict value of T
ΔG to be?
impact the
sign of ΔG?
2. Given the following process, what would you predict is the sign of each of the following.
Be sure to explain the reasoning for your selection.
a. ΔH
0
H
b. ΔS
0
S
c. ΔG
U
Dispersing the particles requires breaking
the intermolecular forces and ΔH
becomes positive.
The particles change from the aggregated
state to the dispersed state, and ΔS
increases.
oit T.IT always be positive
G T
3. For the reaction 2NO2(g) à N2O4(g) ΔS° = –175.8 J/K.mol ΔH° = –57.2 kJ/mol
a. What is ΔG°? (at 298K)
G
298K
ΔHTΔS
57.2KJ mol 298K 175.871km01
G 52331.2J1m01
b. Is the reaction favorable at 298 K? Explain why or why not.
NO
0 unfavorable
10G
G O favorable
Because ΔG is positive, this chemical reaction cannot
proceed spontaneously. Which means that at 298K,
the reaction needs to be pushed by external energy.