15.2  ENTROPY & SPONTANEITY
*SPONTANEOUS CHANGE defined  one that, given the necessary ______________________ energy, proceeds
without continuous outside assistance
*WHAT CONDITIONS FAVOUR A SPONTANEOUS CHANGE?
1. Enthalpy and Spontaneous Changes:
 When a change lowers the energy of a system, events tend to occur _______________. In chemical reactions,
________________ reactions tend to be spontaneous.
*However, not every exothermic reaction is spontaneous and not every endothermic reaction is non-spontaneous. The
important thing to consider is that an energy decrease works as one factor in favour of spontaneity.
2. Entropy and Spontaneous Changes:
*Entropy, S,  a measure of the randomness or _________________ of a system
 An ordered arrangement of particles (atoms, ions or molecules) has lower entropy (less disorder) than the same number
of particles in random arrangements. The greater the _______________ of movement, the larger the entropy.
 Unlike enthalpy, absolute entropy values can actually be measured, but usually it is the change in entropy that is
important.
The Formula: [ΔS˚ =
*Units of entropy = ______________
*Any event that is accompanied by an _____________in the entropy of the system tend to occur spontaneously.
(*Second Law of Thermodynamics = The entropy of the universe is constantly increasing / Third Law of
Thermodynamics = As the temperature of a system approaches zero, the entropy of a system approaches zero.)
In general, a system will experience an increase in entropy (i.e.  disorder /  freedom of movement / S > 0) if:






the _______________of a gaseous system increases,
the ________________ of a system increases (e.g. heating a liquid, solid, or gas)
there is a change of physical _____________ where the distance between the particles increases (e.g. solid to liquid
or gas, or liquid to gas (Sgas > Sliquid > Ssolid)
different types of particles mix (e.g. a solute dissolving in a solvent. (There are some exceptions.)
the number of particles increases (e.g. N2O4(g)  2NO2(g))
the number of moles of gas increases
*Activity: Determine whether the following reactions/changes show an increase or decrease in entropy
(Hint:  disorder = +ve S ;  disorder = -ve S)
1. 2KClO3(s)  2KCl(s) + 3O2(g)
2. H2O(l)  H2O(s)
3. N2(g) + 3H2(g)  2NH3(g)
4. NaCl(s)  Na+(aq) + Cl-(aq)
5. KCl(s)  KCl(l)
6. Ag+(aq) + Cl-(aq)  AgCl(s)
7. 2OH-(aq) + CO2(g)  H2O(l) +
_________
_________
_________
_________
_________
_________
CO32-(aq) ___________
1
GIBBS FREE ENERGY (G)  the amount of energy available to do work
*For a reaction to be spontaneous, the sign of G has to be ______________________.
To calculate Gibbs Free Energy use: G˚ =
Where:
G˚ = change in free energy;
H˚ = enthalpy change;
OR ΔG˚ = ΣnG˚products - ΣnG˚reactants
S˚ = entropy change
T = temperature in _________________ ;
***Note that H˚ values are given terms of ________ and S˚ values are given in terms of ________. (Therefore, divide S˚ value by
1000 before calculating G˚.)
 When G is ______________, the forward reaction is SPONTANEOUS.
 When G is ______________, the forward reaction is NON-SPONTANEOUS.
 When G is ______________, the reverse reaction is SPONTANEOUS.
 When G is __________________, the reaction has established equilibrium….more on this in grade 12.
*PREDICTING SPONTANEITY  SUMMARY:
H
S
G
SPONTANEITY
-
+
Always ________________________
__________________ spontaneous
+
-
Always _______________________
___________________ spontaneous
-
-
Depends on ____________________
Spontaneous ____________ a specific, calculated,
threshold temperature; spontaneous when TS<H
+
+
Depends on ____________________
Spontaneous ____________ a specific, calculated
threshold temperatures; when TS>H
*MEMORIZE  [negative, negative, below] and [positive, positive, above]
*Other scenario: (i) MIXING GASES  ∆H =_____ ; ∆S = _____ therefore ∆G =____ and process occurs___________________
*Useful rearrangement of G˚ =H˚ - TS˚ :
*By setting G˚ = 0, the temperature at which a reaction becomes spontaneous or non-spontaneous can
be determined. When G˚ = 0,
T=
*Remember: the T you attain will be in _____________.
*PRACTICE Questions: 6.8 & 6.9 Questions (IB Text) and the following:
1. a) Write the equation for the combustion of one mole of hydrogen to form water under standard conditions, and state whether you
would expect the entropy change to have a positive or negative value.
b) The standard enthalpy of combustion of hydrogen is -286 kJ mol-1. The standard entropy values for hydrogen, oxygen, and water
are 131, 205, and 69.9 J K-1 mol-1, respectively. Calculate the value for the Gibbs free energy at 298 K to show that this reaction is
spontaneous at this temperature.
**Be sure to be consistent with units!
c) Above what temperature will this reaction become non-spontaneous?
2. For the melting of ice, ΔH = +6.00 kJ mol-1 and ΔS = 22.0 J K-1 mol-1.
a) Calculate ΔG for the process at -10˚C. (2 marks)
b) Use this to explain which form of water is stable at this temperature. (1 mark)
c) Repeat this for water at +10˚C. (1 mark)
d) Calculate the temperature at which ΔG = 0. What is the significance of this
temperature? (2 marks)
e) Suppose that the entropy of liquid water was increased by the presence of a solute such
as sodium chloride. How would this affect ΔS and the temperature at which ΔG is
zero? Does this correspond to any common phenomenon? (2 marks)
2