Enthalpy, Entropy,
and Spontaneity
Explained
Review of Enthalpy Change
Review of Enthalpy Change
Enthalpy change (DH) is
amount of heat released or
absorbed in a reaction carried
out at constant pressure.
Review of Enthalpy Change
In an endothermic reaction:
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + :
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
2. Heat term is on the left side:
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
2. Heat term is on the left side:
e.g. A + B + 45 kJ  C
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
2. Heat term is on the left side:
e.g. A + B + 45 kJ  C
3. Potential Energy Diagram looks like:
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
2. Heat term is on the left side:
e.g. A + B + 45 kJ  C
3. Potential Energy Diagram looks like:
PE
Products
Reactants
Reaction Proceeds
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
2. Heat term is on the left side:
e.g. A + B + 45 kJ  C
3. Potential Energy Diagram looks like:
PE
Products
Reactants
Reaction Proceeds
Review of Enthalpy Change
In an endothermic reaction:
1. DH is + : e.g. A + B  C
DH = + 45 kJ
2. Heat term is on the left side:
e.g. A + B + 45 kJ  C
3. Potential Energy Diagram looks like:
PE
Products
DH = + 45 kJ
Reactants
Reaction Proceeds
Review of Enthalpy Change
In an exothermic reaction:
Review of Enthalpy Change
In an exothermic reaction:
1. DH is –
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
2. Heat term is on the right side:
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
2. Heat term is on the right side:
e.g. X + Y  Z + 36 kJ
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
2. Heat term is on the right side:
e.g. X + Y  Z + 36 kJ
3. Potential Energy Diagram looks like:
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
2. Heat term is on the right side:
e.g. X + Y  Z + 36 kJ
3. Potential Energy Diagram looks like:
PE
Reactants
Products
Reaction Proceeds
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
2. Heat term is on the right side:
e.g. X + Y  Z + 36 kJ
3. Potential Energy Diagram looks like:
PE
Reactants
Products
Reaction Proceeds
Review of Enthalpy Change
In an exothermic reaction:
1. DH is – : e.g. X + Y  Z
DH = – 36 kJ
2. Heat term is on the right side:
e.g. X + Y  Z + 36 kJ
3. Potential Energy Diagram looks like:
PE
Reactants
DH = – 36 kJ
Products
Reaction Proceeds
Minimum
gravitational
potential energy
There is a natural tendency for a chemical
system to reach a state of minimum enthalpy.
There is a natural tendency for a chemical
system to reach a state of minimum enthalpy.
There is a natural tendency for the
enthalpy of a chemical system to decrease.
There is a natural tendency for a chemical
system to reach a state of minimum enthalpy.
There is a natural tendency for the
enthalpy of a chemical system to decrease.
Equilibrium tends to favour a state of
minimum enthalpy.
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
PE
Products
DH is +
Reactants
Reaction Proceeds
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
PE
Products
DH is +
Reactants
Reaction Proceeds
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
Enthalpy
Products
DH is +
Reactants
Reaction Proceeds
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
Enthalpy
Products
Reactants have
Minimum Enthalpy
Reactants
Reaction Proceeds
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
Enthalpy
Products
Reactants have
Minimum Enthalpy
Reactants
Reaction Proceeds
In an endothermic reaction, the reactants have
Minimum enthalpy,
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
Enthalpy
Products
Reactants have
Minimum Enthalpy
Reactants
Reaction Proceeds
In an endothermic reaction, the reactants have
Minimum enthalpy,
Equilibrium tends to favour a state of
minimum enthalpy.
An Endothermic Reaction
Enthalpy
Products
Reactants have
Minimum Enthalpy
Reactants
Reaction Proceeds
In an endothermic reaction, the reactants have
Minimum enthalpy, so if no other factors are
considered, equilibrium tends to favour the REACTANTS.
Equilibrium tends to favour a state of
minimum enthalpy.
An Exothermic Reaction
Equilibrium tends to favour a state of
minimum enthalpy.
An Exothermic Reaction
Enthalpy
Reactants
DH = – 36 kJ
Products
Reaction Proceeds
Equilibrium tends to favour a state of
minimum enthalpy.
An Exothermic Reaction
Enthalpy
Reactants
DH = – 36 kJ
Products
Reaction Proceeds
Products have
Minimum Enthalpy
Equilibrium tends to favour a state of
minimum enthalpy.
An Exothermic Reaction
Enthalpy
Reactants
DH = – 36 kJ
Products
Reaction Proceeds
In an exothermic reaction, the products have
Minimum enthalpy,
Products have
Minimum Enthalpy
Equilibrium tends to favour a state of
minimum enthalpy.
An Exothermic Reaction
Enthalpy
Reactants
DH = – 36 kJ
Products
Reaction Proceeds
In an exothermic reaction, the products have
Minimum enthalpy,
Products have
Minimum Enthalpy
Equilibrium tends to favour a state of
minimum enthalpy.
An Exothermic Reaction
Enthalpy
Reactants
DH = – 36 kJ
Products
Reaction Proceeds
Products have
Minimum Enthalpy
In an exothermic reaction, the products have
Minimum enthalpy, so if no other factors are
considered, equilibrium tends to favour the PRODUCTS.
Consider the following reaction:
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Does the tendency toward minimum
enthalpy favour the reactants, or the
products?
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Does the tendency toward minimum enthalpy
favour the reactants, or the products?
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Exothermic
Does the tendency toward minimum enthalpy
favour the reactants, or the products?
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Enthalpy
Exothermic
Reactants
Products
Reaction Proceeds
Does the tendency toward minimum enthalpy
favour the reactants, or the products?
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Exothermic
Enthalpy
Reactants
Products
Products have
Minimum Enthalpy
Reaction Proceeds
Does the tendency toward minimum enthalpy
favour the reactants, or the products?
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Exothermic
Enthalpy
Reactants
Products
Products have
Minimum Enthalpy
Reaction Proceeds
The tendency toward minimum enthalpy
favours the products.
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Equilibrium tends to favour reactions in
which enthalpy is decreasing.
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Equilibrium tends to favour reactions in
which enthalpy is decreasing.
As an exothermic reaction proceeds
in the forward direction, the
enthalpy is decreasing.
Enthalpy
Reactants
Exothermic
Products
Reaction Proceeds
Consider the following reaction:
H 2(g )  F2(g )
2HF(g )
DH  537 kJ
Equilibrium tends to favour reactions in
which enthalpy is decreasing.
Enthalpy
Reactants
Exothermic
Products
Reaction Proceeds
As an exothermic reaction proceeds
in the forward direction, the
enthalpy is decreasing. So this is a
favourable change
Consider the following reaction:
2C(s)  2H 2(g )  52.3 kJ
C2 H 4(g )
Does the tendency toward minimum
enthalpy tend to favour the reactants,
or the products?
Consider the following reaction:
2C(s)  2H 2(g )  52.3 kJ
C2 H 4(g )
The heat term is on the
left side of the equation,
Consider the following reaction:
2C(s)  2H 2(g )  52.3 kJ
C2 H 4(g )
The heat term is on the
left side of the equation,
so the reaction is
endothermic
Consider the following reaction:
2C(s)  2H 2(g )  52.3 kJ
Enthalpy
Products
Reactants
C2 H 4(g )
The heat term is on the
left side of the equation,
so the reaction is
endothermic
Consider the following reaction:
2C(s)  2H 2(g )  52.3 kJ
Enthalpy
Products
Reactants
have
Minimum
Enthalpy
Reactants
C2 H 4(g )
The heat term is on the
left side of the equation,
so the reaction is
endothermic
Consider the following reaction:
2C(s)  2H 2(g )  52.3 kJ
Enthalpy
Products
Reactants
have
Minimum
Enthalpy
Reactants
C2 H 4(g )
The heat term is on the
left side of the equation,
so the reaction is
endothermic
In this reaction, the tendency toward
minimum enthalpy favours the reactants.
The other factor that affects
equilibrium is entropy.
Entropy means disorder,
Entropy means disorder, or
randomness.
Solids are
very ordered,
Solids are
very ordered,
so they have
low entropy
Solids are
Liquids are
very ordered, less ordered,
so they have
low entropy
Solids are
very ordered,
so they have
low entropy
Liquids are
less ordered,
so they have
more entropy
than solids
–
+
Solids are
very ordered,
so they have
low entropy
Aqueous
Liquids are
solutions are
less ordered, mixtures,
so they have
more entropy
than solids
–
+
Solids are
very ordered,
so they have
low entropy
Liquids are
less ordered,
so they have
more entropy
than solids
Aqueous
solutions are
mixtures, so they
have more
disorder (entropy)
than pure solids
or liquids.
–
+
Solids are
very ordered,
so they have
low entropy
Liquids are
less ordered,
so they have
more entropy
than solids
Gases are in
Aqueous
rapid random
solutions are
mixtures, so they motion,
have more
disorder (entropy)
than pure solids
or liquids.
–
+
Solids are
very ordered,
so they have
low entropy
Liquids are
less ordered,
so they have
more entropy
than solids
Gases are in
Aqueous
rapid random
solutions are
mixtures, so they motion, so
They have
have more
disorder (entropy) the most
than pure solids entropy.
or liquids.
–
+
Solids
< Liquids
<
Aqueous Solutions
Increasing Entropy
< Gases
Less gas particles  More gas particles
Less gas particles  More gas particles
Increasing Entropy
Less gas particles in reactants  More gas particles in products
Increasing Entropy
Less gas particles in reactants  More gas particles in products
Increasing Entropy
More gas particles in reactants  Less gas particles in products
Increasing Entropy
PCl5(g)  Cl2(g) + PCl3(g)
PCl5(g)  Cl2(g) + PCl3(g)
1 mol
of gas
PCl5(g)  Cl2(g) + PCl3(g)
1 mol
of gas
2 mol
of gas
PCl5(g)  Cl2(g) + PCl3(g)
1 mol
of gas
2 mol
of gas
Increasing Entropy
CO(g) + 3H2(g)  CH4(g) + H2O(g)
CO(g) + 3H2(g)  CH4(g) + H2O(g)
4 mol
of gas
CO(g) + 3H2(g)  CH4(g) + H2O(g)
4 mol
of gas
2 mol
of gas
CO(g) + 3H2(g)  CH4(g) + H2O(g)
4 mol
of gas
2 mol
of gas
Increasing Entropy
CO(g) + 3H2(g)  CH4(g) + H2O(g)
4 mol
of gas
2 mol
of gas
Decreasing Entropy
There is a natural tendency for a system
to reach a state of minimum enthalpy.
There is a natural tendency for a system
to reach a state of minimum enthalpy.
There is a natural tendency for a
system to reach a state of maximum
entropy.
Both tendencies:
Both tendencies:
• Minimum Enthalpy
Both tendencies:
• Minimum Enthalpy (Minimum H )
Both tendencies:
• Minimum Enthalpy (Minimum H )
• Maximum Entropy
Both tendencies:
• Minimum Enthalpy (Minimum H )
• Maximum Entropy (Maximum S )
Both tendencies:
• Minimum Enthalpy (Minimum H )
• Maximum Entropy (Maximum S )
help determine what will actually happen
when reactants are mixed together.
Here are the possibilities:
Minimum Enthalpy favours Reactants
Reactants

Products
Minimum Enthalpy favours Reactants
Maximum Entropy favours Reactants
Reactants

Products
Minimum Enthalpy favours Reactants
Maximum Entropy favours Reactants
Reactants

Products
No reaction will occur
when reactants are mixed.
Minimum Enthalpy favours Products
Reactants

Products
Minimum Enthalpy favours Products
Maximum Entropy favours Products
Reactants

Products
Minimum Enthalpy favours Products
Maximum Entropy favours Products
Reactants

Products
The reaction will go to
completion when
reactants are mixed.
Minimum Enthalpy favours Products
Reactants
Products
Minimum Enthalpy favours Products
Maximum Entropy favours Reactants
Reactants
Products
Minimum Enthalpy favours Products
Maximum Entropy favours Reactants
Reactants
Products
The reaction will reach a
state of equilibrium when
reactants are mixed.
Minimum Enthalpy favours Reactants
Reactants
Products
Minimum Enthalpy favours Reactants
Maximum Entropy favours Products
Reactants
Products
Minimum Enthalpy favours Reactants
Maximum Entropy favours Products
Reactants
Products
The reaction will reach a
state of equilibrium when
reactants are mixed.
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Reactants
Reactants
No Reaction
Spontaneity
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Reactants
Products
Equilibrium
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Reactants
Products
Equilibrium
Spontaneous
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Reactants
Products
Equilibrium
Spontaneous
Products
Reactants
Equilibrium
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Reactants
Products
Equilibrium
Spontaneous
Products
Reactants
Equilibrium
Spontaneous
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Reactants
Products
Equilibrium
Spontaneous
Products
Reactants
Equilibrium
Spontaneous
Products
Products
Completion
Minimum
Enthalpy
Favours
Maximum
Entropy
Favours
Result
Spontaneity
Reactants
Reactants
No Reaction
Non-spontaneous
Reactants
Products
Equilibrium
Spontaneous
Products
Reactants
Equilibrium
Spontaneous
Products
Products
Completion
Spontaneous