# Thermodynamics

Mechanics

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

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Thermodynamics

Energy and Chemical Change

Energy

The ability to do work or produce heat.

Two basic forms of energy: potential
and kinetic energy.

Potential energy is due to the
composition or position of an object.

Kinetic energy is the energy of motion.

Law of Conservation of Energy

Energy can be converted from one form to another,

but can neither be created nor destroyed.

Chemical potential energy is the energy stored in

a substance due to the arrangement of atoms and

the strength of the chemical bonds.

Heat

q

Energy that flows from a warmer object
to a cooler object.

As a warmer object loses heat, the
temperature decreases.

SI unit of heat and energy is the Joule
(J).

Calories are also often used as heat units. A

calorie is defined as the amount of energy required

to raise the temperature of one gram of pure water

by one degree Celsius.

Food calories (C) are the equivalent of 1000 calories.

1 cal = 4.184 J

Convert 86.5 J to cal; convert 142 Cal to Joules.

Specific Heat

Amount of energy required to raise one
gram of a substance one degree
Celsius.

Water has an extremely high specific
heat (4.18 J) which means that a large
amount of energy must be used to raise
the temperature of a quantity of water.

In order to calculate the heat energy that must be

absorbed or released by a substance, you need to

know the amount of the substance (mass in grams),

the specific heat capacity (C
p
) and the temperature

change that the substance will undergo.

q = m C
p

T

What

is

the

specific

heat

of

iron

if

a

10
.
0

g

sample

changes

from

50
.
4
o
C

to

25
.
0
o
C

with

the

release

of

114

J

of

heat?

If the temperature of 34.4 g of ethanol increases

from 25.0
o
C to 78.8
o
C, how much heat is absorbed

by the ethanol? The specific heat of ethanol is

2.42 J/g
o
C.

What temperature change will 100.0 mL of water

undergo when it absorbs 1360 Joules of heat?

Thermochemistry: the study of heat changes that

accompany chemical reactions and phase changes.

System: the specific part of the universe under study.

Surroundings: everything in the universe other than

the system.

Universe = System + Surroundings

The heat required for one mole of a substance to

change phase is called the molar heat of:

fusion when s

l or

vaporization when l

g

Energy is released when l

s or g

l and

Energy is absorbed when s

l or l

g

Calculate the heat required to melt 25.7 g of solid

methanol at its melting point. The molar heat of

fusion for methanol is 3.22 kJ/mol.

How much heat is evolved when 275 g of ammonia

gas condenses to a liquid at its boiling point? The

molar heat of vaporization for ammonia is

23.3 kJ/mol.

A thermochemical equation is a balanced chemical

equation that includes the physical states of all

reactants and products and the energy change.

C
6
H
12
O
6

(s) + 6O
2
(g)

6CO
2
(g) + 6H
2
O(l)

H =
-
2808 kJ

Enthalpy (H) is the heat content of a system at

constant pressure.

The change in enthalpy for a reaction is called

the Heat of reaction (

H
rxn

= H
final

H
initial

) or

H
rxn

= H
products

H
reactants

The sign for the enthalpy change depends

on whether the products or reactants have

more energy.

Endothermic Reactions:

Reactants + heat

Products

H
rxn

= +

H
products
> H
reactants

Exothermic Reactions:

Reactants

Products + heat

H
rxn

=
-

H
products
< H
reactants

If A

B and B

C what can you conclude?

Hess’s Law:

In going from a particular set of reactants

to a particular set of products, the change in

enthalpy is the same whether the reaction takes

place in one step or a series of steps.

A

C

=

A

B

B

C

A

C

Given:

2CO(g) + O
2

(g)

2CO
2
(g)

H = ?

Use these:

C(s) + O
2
(g)

CO
2
(g)

H =
-
393.5 kJ

2C(s) + O
2
(g)

2CO(g)

H =

+172.5 kJ

Calculate

H
rxn

for the reaction:

HCl(g) + NH
3
(g)

NH
4
Cl(s)

given the following:

H
2
(g) + Cl
2
(g)

2HCl(g)

H=
-
184kJ

N
2
(g) + 3H
2
(g)

2NH
3
(g)

-
92 kJ

N
2
(g) + 4H
2
(g) + Cl
2
(g)

2NH
4
Cl(s)
-
628 kJ

Calculate
Δ
H for the reaction:

2C(s) + H
2
(g)

C
2
H
2
(g)

Given:

Δ
H=

C
2
H
2
(g) + 5/2 O
2
(g)

2CO
2
(g) + H
2
O(l)
-
1299.6

C(s) + O
2
(g)

CO
2
(g)
-
393.5

H
2
(g) + ½ O
2
(g)

H
2
O(l)
-
285.8

Given the following reaction:

Ca(s) + H
2
O(l) + CO
2
(g)

H
2
(g) + CaCO
3
(s)

Use the following reactions:

CaCO
3
(s)

CaO(s) + CO
2
(g)
Δ
H = +177.8 kJ

Ca(s) + 2 H
2
O(l)

Ca(OH)
2
(s) + H
2
(g)
-
414.3 kJ

Ca(OH)
2
(s)

CaO(s) + H
2
O(l) +65.3 kJ

Find
Δ
H
o
rxn

for

C
2
H
2
(g) + 2 H
2
(g)

C
2
H
6
(g)

Using the standard enthalpies of reaction below:

2 H
2
(g) + O
2
(g)

2 H
2
O(l)

-
572 kJ

2 C
2
H
2
(g) + 5 O
2
(g)

4 CO
2
(g) + 2 H
2
O(l)

-
2598 kJ

2 C
2
H
6
(g) + 7 O
2
(g)

4 CO
2
(g) + 6 H
2
O(l)

-
3122 kJ

Hess’s Law Quiz Tomorrow

Enthalpies of Formation

The standard enthalpy of formation of
a compound is called
Δ
H
o
f

Δ
H
o
f

is the change in enthalpy for the
reaction that forms 1 mole of the
compound from its elements.

All elements must be in their standard
states at 298 K.

The enthalpy of formation for ethanol is

-
277.7 kJ. Write the equation for the formation

of ethanol.

Standard enthalpies of formation can be used to

determine the enthalpy of reaction using:

H
rxn

=

H
products

H
reactants

H
2
S(g) + 4F
2
(g)

2HF(g) + SF
6
(g)

Write the thermochemical equation for the

formation of zinc nitrate.

Determine
Δ
H for the following reaction:

NH
4
NO
3
(s)

N
2
O(g) + 2H
2
O(l)

l
1st Law
-

the energy of the universe
is constant.

l
Keeps track of thermodynamics
doesn’t correctly predict spontaneity.

l
Entropy

(S) is disorder or
randomness

l
2nd Law: the entropy of the universe
increases.

Laws of Thermodynamics

Entropy

S

Measure of the randomness or disorder
of the particles in a system.

Gas > Liquid > Solid

Substances at higher temperatures
have a higher entropy than substances
at lower temperatures.

Determine whether the following results in an

increase or decrease in the entropy:

CO
2
(s)

CO
2
(l)

CaO(s) + CO
2
(g)

CaCO
3
(s)

Ag
+
(aq) + Cl
-
(aq)

AgCl(s)

For each of the following pairs, choose the

substance with the higher entropy:

Ar(l) or Ar(g)

1 mol As at 400K or 1 mol As at 298K

100g Na
2
SO
4
(s) at 30
o
C or 100g Na
2
SO
4
(l)

at 30
o
C or 100g

l
A reaction that will occur without
outside intervention.

l
We can’t determine how fast.

l
We need both thermodynamics and
kinetics to describe a reaction
completely.

l
Thermodynamics compares initial
and final states.

l
Kinetics describes pathway
between.

Spontaneous

Spontaneous Reactions

Most spontaneous reactions are
exothermic.

However, some endothermic reactions
occur spontaneously due to an increase
in entropy of the system.

Two factors need to be considered,
enthalpy and entropy in deciding
spontaneity

Gibbs Free Energy (G) is the energy available

to do work.

G =

H
-

T

S

G =
-

reaction is spontaneous

G = + reaction is not spontaneous

H

S

G

Charac
-
teristics

-

+

-

Spon.

+

-

+

Non
-

Spon.

-

-

+

Spon @

Low T

+

+

-

Spon @

High T

The enthalpy of formation of hydrogen chloride is

-
92.31 kJ/mol.

1. Write the thermochemical equation for the

formation of hydrogen chloride.

2. Determine the heat of reaction when hydrogen

chloride reacts with ammonia to form

ammonium chloride with an enthalpy of
formation of
-
314.4 kJ/mol.