SCH4U0
First Law of Thermodynamics
This is the law of conservation of energy.
The total amount of energy in the universe is constant
Energy is transferred between forms and cannot be
created nor destroyed
This relates to enthalpy because the enthalpy
released/absorbed in a reaction is the exact energy
difference between the reactants and products
But what is this energy exactly?
Bond Energy
The enthalpy difference arises from the difference in
potential energy between the reactants and products
This difference comes from the different potential
energies of different chemical bonds
Bond Energy
The energy required to break a chemical bond
The larger the bond energy, the more stable a bond is
Stable bonds have low potential energy
Bond Energy
Bond Type
Bond Energy
(kJ / mol)
H
–
H
436
C
–
H
413
N
–
H
391
C
–
C
346
C = C
615
C
–
N
305
O
–
H
436
C
–
O
358
C = O
749
N
–
O
222
O = O
498
N
–
N
170
N
–
N
(triple)
945
PE
CH
4
, 2 O
2
CO
2
, 2 H
2
O
Δ
H
•
Energy is released because the bonds
being formed release more energy
than is required to break apart the
reactants.
Ie: the products are more stable than
the reactants
Spontaneity
Since the products have lower potential energy than
the reactants, this reaction occurs spontaneously
Meaning that it happens naturally without any human
interference
This suggests that;
Exothermic reactions are always spontaneous
Endothermic reactions are always non
-
spontaneous
But this is not always the case
Why not?
Entropy
There is a second quantity of energy (other than
enthalpy) that changes is chemical reactions that
affects the spontaneity of a reaction
Entropy (S)
The measure of randomness or disorder
Large entropy implies a large amount of disorder
This means that the atoms are not ordered, and are free to
move in any way
Low entropy implies a small amount of disorder
This means that the atoms are highly ordered, and have
restricted motion
Every compound has an associated entropy
Entropy
Large molecules have low entropy while small
molecules have high entropy
Large molecules have several atoms placed in a specific
order
Small molecules only have a few atoms placed in a
specific order
Glucose (C
6
H
12
O
6
) has low entropy
Carbon dioxide has high entropy
2
nd
Law of Thermodynamics
The entropy of the universe is always increasing, or
remaining constant
This means that processes in the universe tend to
increase in entropy (get more disordered)
The spontaneous direction for a reaction to proceed is
the way that increases the entropy of the system
This in turn increases the entropy of the universe
The 2
nd
Law
The 2
nd
law
does not
imply that reactions cannot
decrease in entropy (get more ordered)
If that was the case, biological molecules (and thus life)
could never have formed
Processes that decrease in entropy are only possible if
they are accompanied by equal or greater increases in
the entropy of the universe
Eg: Humans building a house decreases entropy, but
they must break down glucose to do this (increase
entropy)
Change in Entropy
Δ
S
Δ
S > 0 if;
There are more moles of the products than reactants
Complex molecules are broken into smaller molecules
A substance changes state from a more ordered state to
a less ordered state
solid to liquid, solid to gas, or liquid to gas
States
Low S High S
Gibbs Free Energy
There are two types of energy exchanged in a
chemical reaction
Enthalpy (H) and entropy (S)
Gibbs Free Energy (G) is the combination of both
enthalpy and entropy
Definition: The amount of energy available to do work
in chemical system
This means that G is the energy that can be released
from a chemical system and used to do work on the
surroundings
Gibbs Free Energy
The change in Gibbs free energy for a reaction
depends on the enthalpy change, entropy change,
and the temperature
Spontaneity
This means that to determine whether or not a
reaction is spontaneous we must think about all three
of these variable
Spontaneity
Exergonic reactions:
Release energy and are
therefore spontaneous
ΔG < 0
Endergonic reactions:
Absorb energy and are
therefore non
-
spontaneous
ΔG > 0
G
Reactants
Products
Δ
G
G
Reactants
Products
Δ
G
Spontaneity
When is this reaction spontaneous?
Enthalpy is negative
–
spontaneous
Entropy is positive
–
spontaneous
At all temperatures this reaction is spontaneous
Spontaneity
When is this reaction spontaneous?
Enthalpy is positive
–
non
-
spontaneous
Entropy is negative
–
non
-
spontaneous
At all temperatures this reaction is non
-
spontaneous
Spontaneity
When is this process spontaneous?
Enthalpy is positive
–
non
-
spontaneous
Entropy is positive
–
spontaneous
If temp. is high, this reaction is spontaneous
If temp. is low, this reaction is non
-
spontaneous
Spontaneity
When is this reaction spontaneous?
Enthalpy is negative
–
spontaneous
Entropy is negative
–
non
-
spontaneous
If temp. is low, this reaction is spontaneous
If temp. is high, this reaction is non
-
spontaneous
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