# Spontaneity and Thermodynamics - My Class Sites

Mechanics

Oct 27, 2013 (4 years and 8 months ago)

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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