# Reactions always must obey the First Law of Thermodynamics

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27 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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SCH4U0

Energy and Rates

Gibb’s Free Energy and Thermodynamics

1

Reaction
Spontaneity and Thermodynamics

Reactions always must obey the First Law of Thermodynamics

The First Law of Thermodynamics:

This is the law of
conservation of energy. The total amount of energy in the
universe is constant.
Therefore, e
nergy
cannot

be
created nor destroyed. Energy can be transferred from
one object/place to another, or transformed from one
form to another.

Remember, molecules will react to form more stable
compounds, thus stable bonds. However, some bonds are
more stable than others

are.

Bond Energy:

The energy required to break a bond.

It is a measure of the stability of a covalent bond.
High
energies correlate to very stable bonds.

Bond Type

Bond Energy

(kJ / mol)

Bond Type

Bond Energy

(kJ / mol)

H

=
e
=
㐳4
=

=
N
=
㌰3
=

=
e
=
4N3
=

=
e
=
㐳4
=

=
e
=
㌹3
=

=
l
=
㌵3
=

=
C
=
㌴3
=
C===l
=
㜴T
=
C===C
=
SN5
=

=
l
=
㈲O
=
i⹥⸠N

N is 945 kJ/mol whereas C
-
H is 413 kJ/mol, this
means that more energy is required to break the triple
SCH4U0

Energy and Rates

Gibb’s Free Energy and Thermodynamics

2

bond than the C
-
H single bond. The higher the E, the
more stable the bond.

Spontaneous Reactions
:

Reactions that occur naturally and do not have to be
forced to occur (by using heat or electricity)

It proceeds without continuous outside assistance.

Products have lower potential energy than reactants

Non
-
spontaneous reactions

R
eactions that do not occur unless energy is

to
force them to occur.

MISCONCEPTION:

All exothermic reactions are spontaneous and endothermic
reactions are not.

Exothermic reactions are
often

spontaneous.

Eg: combustion reactions.

Endothermic reactio
ns are
often non
-
spontaneous

Eg: electrolysis and decomposition

However, many endothermic processes are spontaneous

Eg: dissolving ammonium nitrate in water absorbs heat.

SCH4U0

Energy and Rates

Gibb’s Free Energy and Thermodynamics

3

NH
4
NO
3(s)
+ energy

NH
4
+
(aq)

+ NO
3
-

(aq)

Why do reactions yielding less stable
products occur
spontaneously?

There is a second quantity of energy (other than
enthalpy) that changes i
n

chemical reactions that affects
the spontaneity of a reaction
, this is known as Entropy

Entropy (S):

The measure of disorder or randomness. It can app
ly to a
system, surroundings, or the universe as a whole.

Entropy increases when randomness increases

Eg: A deck of cards laid out with all the suits grouped up and
in order from ace to king is very ordered. This would have a
low entropy. A deck of cards
thrown in the air would land
randomly on the floor with all the cards mixed up. This would
have high entropy.

Whether or not a reaction is spontaneous depends on 2
things:

1.

Enthalpy (ΔH)

ΔH > 0, endothermic, tend NOT to be spontaneous

ΔH < 0 , exothermic,
tend to be spontaneous

SCH4U0

Energy and Rates

Gibb’s Free Energy and Thermodynamics

4

2.

Entropy (ΔS)
-

The measure of disorder or randomness.

ΔS > 0, more disorder, tend to be spontaneous

ΔS < 0 , less disorder, tend NOT to be spontaneous

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

How does entropy or disorder increase?

Change of state reflecting more disorder

(S
solid

<

S
liquid

<S
gas
)

Low S

High

S

SCH4U0

Energy and Rates

Gibb’s Free Energy and Thermodynamics

5

Second Law of Thermodynamics:

All changes either directly
or indirectly increase the entropy of the universe
.

ΔS
universe

= ΔS
system

+ ΔS
surroundings

> 0

all processes tend to be spontaneous if they increase in
entropy

(meaning more disorder)

Processes that decrease entropy are possible only if they
are accompanied by an equal or greater increase in the
entropy of th
e universe.

Eg: People building things must break down glucose and fats
for energy.

ΔS = S
products

S
reactants

> 0

R
eaction Entropy
: ΔS > 0 if;

There are more moles of product made than moles of
reactant used

2NH
3(g)

N
2(g)

+ 3H
2(g)

Complex
molecules are broken into simpler subunits

C
6
H
12
O
6
(s)
+ 6O
2(g)

6CO
2
(g
)

+ 6H
2
O
(
g
)

A substance changes state from solid to liquid/gas or
from
liquid to gas

2H
2
O
(l)

2H
2(g)

+ O
2(g)