This document discusses the topics of entropy and spontaneity in chemistry. It defines entropy as a measure of disorder or randomness in a system. Reactions that increase disorder have a positive change in entropy. The standard entropy change of a reaction can be calculated from standard entropy values. A spontaneous process is one that can occur without outside intervention. The Gibbs free energy change (ΔG) determines if a reaction is spontaneous - a negative ΔG means the reaction proceeds spontaneously. Temperature can affect spontaneity through the entropy term in the ΔG equation.

1. IB Chemistry Power Points
Topic 15
Energetics
www.pedagogics.ca
LESSON
Entropy
Spontaneity

2. ENTROPY
Entropy, S is a measure of the DISORDER or
randomness of a system.
A single coin can have 2 possible configurations.
A system of 4 coins can have 16 (more disorder)

3. The greater the number of configurations (or microstates) in
a particular system, the greater the entropy (disorder) of the
system

4. Entropy on the Molecular Scale
Implications to a chemical system (MUST KNOW):
• more particles
-> more states -> more entropy
• higher T
-> more energy states -> more entropy
• less structure (gas vs solid)
-> more states -> more entropy

5. Entropy changes (dissolving)
ΔS = Sfinal – Sinitial

6. Entropy changes (heating)
ΔS = Sfinal – Sinitial

7. Entropy changes (change of state)
ΔS = Sfinal – Sinitial

8. ѳ
Standard entropy values: S
The standard entropy value for a substance is defined
as the entropy increase of the substance when heated
from 0 K to 298 K (standard conditions)
standard entropy is an absolute value
The standard entropy change for a reaction can be
determined by:
ѳ ѳ ѳ
ΔS = ΣS products – ΣS reactants

9. Consider the following reaction
Describe the change in entropy in this reaction.
Calculate the change in entropy in this reaction.

10. Spontaneous Processes
• Spontaneous processes
are those that can
proceed without any
outside intervention.
• The gas in vessel B will
spontaneously effuse into
vessel A, but once the
gas is in both vessels,
returning to the original
state is not spontaneous

11. Spontaneous Processes
Processes that are spontaneous
in one direction are non
spontaneous in the reverse
direction.

12. Spontaneous Processes
• Processes that are spontaneous at one
temperature may be nonspontaneous at other
temperatures.
• Above 0 C it is spontaneous for ice to melt.
• Below 0 C the reverse process is spontaneous.

13. Spontaneous reactions produce substantial
18.
amounts of products at equilibrium and release free
4
energy.
Free energy is energy that is available to do work
The Gibbs free energy change, G is the maximum
amount of free energy that can be extracted to do work.

14. Gibbs Free Energy Change
(some things to know)
1. If the G (for a particular reaction under a specific set
of conditions) is negative, the forward reaction is
spontaneous. (or vice versa)
2. The G for a reaction can be determined from
standard free energies of formation, Gf values
available on Data Tables. This is analogous to using
standard enthalpies of formation, Hf values.

15. The key equation to predict spontaneity:
G H T S
This equation shows how G changes with
temperature.
(We assume S & H values are not affected by T.)

16. Free Energy and Temperature
• There are two parts to the free energy
equation:
 H the enthalpy term
 T S the entropy term
• The temperature dependence of
spontaneity comes from the entropy
term.

17. By knowing the sign (+ or -) of S and H, we can
get the sign of G and determine if a reaction is
spontaneous.

19. endothermic so ΔH must be positive

20. entropy increasing so ΔS must be positive

21. G H T S
3
176 10 298 284
1
91368 Jmol
1
91.4 kJmol
not spontaneous

22. G H T S
3
176 10 298 284
1
91368 Jmol
1
91.4 kJmol
increase in temperature increases spontaneity

23. G H T S
3
0 176 10 T 284
T 620 K
o
346 C