The Driving Forces of
Reactions
AP Chemistry

In chemistry we are concerned with
whether a reaction will occur
spontaneously, and under what conditions
will it occur spontaneously.

Spontaneous reactions are reactions
that naturally favor the products at
specific conditions. They also release free
energy (the energy available to do work)

Non-Spontaneous reactions are ones
that do not favor the products and do not
release free energy, they use it!
Free Energy

Free Energy is given the symbol “G” and we look at
the change in free energy (ΔG) in a reaction to
determine if it is spontaneous or not.

A positive ΔG means the reaction is not
spontaneous .
A negative ΔG means the reaction is spontaneous.
ΔG = 0 if the system is at equilibrium. (such as at
melting or boiling temperature)




The change in free energy, ΔG, is determine by the
Gibbs Free Energy Equation
ΔG = ΔH - TΔS
T = Temperature in Kelvin
ENTHALPY
ΔH represents the change in enthalpy for the
reaction.
 (remember: +ΔH is endothermic and
-ΔH is exothermic.)
 Most reactions are exothermic because there is a
natural tendency in nature to move to a lower
energy state, and that is what happens in an
exothermic reaction (think about the potential
energy diagrams!)

ENTROPY

ΔS represents the change in entropy
(S)

Entropy is the measure of the degree of
randomness in a system. Gases have
higher entropy than a liquid or solid,
because the particles have less order.

There is a tendency in nature to proceed
in the direction that increases entropy (to
get more random).
Entropy

+ΔS means there is an increase in
entropy for a reaction
( liquid gas) gets more random.

–ΔS means there is a decrease in
entropy for a reaction.
( a liquid  solid) gets more ordered
Gibbs free energy equation
We use the Gibbs free energy equation to
determine that ΔG for a reaction if we know
the enthalpy change, the entropy change
and the temperature (in Kelvin).
 This will tell us if a reaction will be
spontaneous at that temperature.(you look
at the sign (+ or -) on the answer for ΔG)
 - ΔG = spontaneous
 + ΔG = non-spontaneous

You can calculate ∆Go from ∆Ho and ∆So
and Temperature or you can look up the
standard free energies of formation ∆G of
and use the products minus reactants
procedure we used for ∆H and ∆S.
 (see appendix handout for values or look
at appendix in book!)

ΔG = ΔH - TΔS
ΔH
ΔS
+
ΔG
-
+
-
-
+
+
+
-
Depends on
Temperature
Depends on
Temperature
Always Spontaneous
Never Spontaneous
Spontaneous at
Low Temps
Spontaneous at
High Temps
Example:

Determine the values of H, S and G at
standard conditions (25oC and 1 atm)
for the following reaction using the
data given.

2SO2(g) + O2(g)  2SO3(g)
Substance
S (J/K mol)
H kJ/ mol
SO2 (g)
248
-297
O2 (g)
205
0
SO3(g)
257
-396
∆H = 2∆HSO3 - [2∆HSO2 + ∆HO2]

= 2(-396) – [ 2(-297) + 0]

= -198 kJ
∆S = 2 ∆SSO3 - [2(∆SSO2 + ∆SO2]
= 2(257) – [2(248) + 205]
= -187 J/K
Now plug into the Gibbs Equation using
T=298K (convert ∆S to kJ from J first!)
∆G = ∆H - T ∆S

= (-198kJ - (298K)(-0.187kJ/K)

= -142 kJ

H2O(s) = H2O(l)

Is Enthalpy (+) or (-)?
 (+) because it is an endothermic change

Is Entropy Increasing or Decreasing?
 Increasing
 ∆S = (+)

Is the reaction Spontaneous?




Depends on Temperature.
Spontaneous, ∆G = (-), at High Temps.
Heat is required to melt Ice.
If you are at the right temperature, it will
spontaneously melt.
H2SO4 =

+
2H
+
2SO4
+ Heat
Is ∆H (+) or (-)?
 (-) because it is exothermic

Is ∆S (+) or (-)?
 (+) because entropy is increasing, more particles are
more random!

Is the reaction spontaneous?
 Yes, it is always spontaneous.

The universe favors lower energy and higher
entropy!