Spontaneity and Thermodynamics - My Class Sites

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27 Οκτ 2013 (πριν από 3 χρόνια και 5 μήνες)

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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