Spontaneity and Thermodynamics - My Class Sites

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