Thermochemistry

1
Thermochemistry

• Chapter 6

2
Energy

• The capacity to do work or transfer heat
• Potential Energy Kinetic Energy
• w F x d

3
Other Forms of Energy

• Radiant Energy
• Electromagnetic Radiation
• Thermal Energy
• Associated with random motion of a molecule or
  atom
• Chemical Energy
• Energy stored within the structural limits of a
  molecule or atom

4
Units

• Joule E 1/2 mv2 kg m2/s2
• 1 Joule .239 calories
• 1 calorie 3.965 x 10-3 btu
• How many Joules 1 btu?

5
Dimensional Analysis?
1 cal
1 joule
1 btu
x
x
.239 cal
3.965 x 10-3 btu
6
Dimensional Analysis?
1 cal
1 joule
1 btu
x
x
.239 cal
3.965 x 10-3 btu
1060 Joules (w/ significant figures)
7
First Law of Thermodynamics

• The Energy in the Universe is Constant ?E q
  w
• q heat w work
• But
• WHAT IS THE UNIVERSE?

8
Thermochemistry

• Exothermicity
• out of a system
• ?E lt 0

• Endothermicity
• into a system
• ?E gt 0

Surroundings
Surroundings
Energy
Energy
System
System
9
Thermochemistry

• Exothermicity
• out of a system
• ?H (-)

• Endothermicity
• into a system
• ?H ()

Surroundings
Surroundings
q w
q w
System
System
10
Enthalpy - H

• A chemists description for heat gained () or
  lost (-) by a closed system (constant
  pressure) ?????????final??????initi
  al?
• A state function - depends only on a systems
  present state, not how it got there.

11
Enthalpy is an Extensive Property

• If one mole of isopropanol burns in air with ?H
  - 1987 kJ, what is the change in enthalpy for
  50 g of isopropanol? (Hint MW 60)
• 2 C3H7OH (l) 9 O2 (g) gt 6 CO2 (g) 8 H2O (l)

12
Enthalpy is an Extensive Property

• If one mole of isopropanol burns in air with ?H
  - 1987 kJ, what is the change in enthalpy for
  50 g of isopropanol? (Hint MW 60)

-1987 kJ
1 mole
?H

x
50.0 g
x
1 mole
60.0 g
?H
-1655 -1660 kJ
13
Heat Capacity

• The heat absorbed by the system is a function of
  the mass and the ability to absorb heat.

C Heat Absorbed/ Change in Temperature
Joules/Co
14
Two Flavors

• Specific Heat Capacity
• Joules/ Co g

• Molar Heat Capacity
• Joules/ K mole

15
Calorimetry

• By running a reaction in water, we can use the
  specific heat of water and the observed
  temperature change to measure ?H for the system.

16
Bond Energies/Strengths

• The Bond Dissociation Energy is the ?H for the
  bond breaking reaction in the gas phase.
• CH4 (g) gt C (g) 4 H (g) ?H 1660 kJ

17
Hesss Law

• The change in enthalpy is the same regardless
  of the steps taken to get there.

2 C3H7OH (l) 9 O2 (g) gt 6 CO2 (g) 8 H2O (g)
-1659 kJ/mol
H2O (l) gt H2O (g)
130 kJ/mol
-1529 kJ/mol
2 C3H7OH (l) 9 O2 (g) H2O (l)
gt 6 CO2 (g) 9 H2O (g)
18
Enthalpy Diagrams
130 kJ/mol
to surroundings
-1659 kJ/mol
-1529 kJ/mol
19
The Benefits of Thermochemistry

• Enthalpy
• A means of quantifying chemical energy
• The first law of thermodynamics
• All energy must be accounted for
• Hesss Law
• Known reactions can be used to determine
  energetics of composite, unknown reactions

20
Oxidation of Nitrogen

• What is the enthalpy change for the reaction of
  oxygen and nitrogen?
• N2 (g) 2 O2 (g) gt 2 NO2 (g)
• N2 (g) O2 (g) gt 2 NO (g) ?H 180 kJ
• 2 NO (g) O2 (g) gt 2 NO2 (g) ?H -112 kJ
• N2 (g) 2 O2 (g) gt 2 NO2 (g) ?H 68 kJ

21
Path Independent
NO (g), O2 (g)
NO (g), O2 (g)
-112 kJ
2NO2 (g)
2NO2 (g)
180 kJ
68 kJ
N2 (g), 2O2 (g)
N2 (g), 2O2 (g)
22
Hesss Law

• Hydrogen peroxide, H2O2, is a colorless liquid.
  Solutions of H2O2 are used as bleach and
  antiseptics. It can be prepared in a process
  whose overall change is

H2(g) O2(g) H2O2(l)
Calculate the enthalpy change using the following
data
1
H2O2(l)
H2O(l)
O2 (g) H -98.0 kJ
2
2H2(g) O2(g)
2H2O(l) H -571.6 kJ
23
Calculate the enthalpy change using the following
data
1
kJ
H2O2(l)
H2O(l)
O2 (g) ? H -98.0
2
2H2(g) O2(g)
2H2O(l) ? H -571.6
kJ
1
H2O O2
H2O2
? H (-98.0 kJ x (-1)
2
(-571.6 kJ)
1
H2 O2
H2O
? H
2
(2)
H2 O2
H2O2
? H -187.8 kJ
24
Enthalpies of Formation-Heat of Formation

• In order to standardize a set of data, we have
  established a set of conditions under which ?Hfo
  are reported.
• Standard State - most stable form of an element
  at STP
• STP - standard temperature and pressure
• P 105 Pascals 1 atm
• T 298 K (25 oC)

25
Elemental Heats of Formation

• For an element, the heat of formation is
  defined as 0.0 kJ/mol under STP conditions.

Note the standard state of an element is its
form at STP
26
To Calculate ?Ho Reaction

• ?Horxn ? n?Hfo(prod) - ? m?Hfo(react)
• Remember to change the sign of ?Hfo if the
  reaction is reversed.
• If the formation reaction must be multiplied by
  an integer, ?Hfo must be as well.
• Elements in their standard states dont
  contribute to ?Ho.

27
An example - Alternative Automobile Fuels

• Methanol is sometimes used as a fuel alternative
  to octane (gasoline) in high performance race
  cars. Determine the enthalpy of combustion per
  gram of each.
• ?Hfo CH3OH (l) -239 kJ/mole
• ?Hfo C8H18 (l) -269 kJ/mole
• ?Hfo CO2 (g) -394 kJ/mole
• ?Hfo H2O (l) -286 kJ/mole

28
Methanol Combustion

• CH3OH (l) O2 (g) gt CO2 (g) H2O (l)

29
Methanol Combustion

• 2CH3OH (l) 3O2 (g) gt 2CO2 (g) 4H2O (l)
• ?Horeaction ? n?Hfo(prod) - ? m?Hfo(react)
• 2 x ?Hfo (CO2) 4 x ?Hfo (H2O) - 2 x ?Hfo
  (CH3OH) - 3 x ?Hfo (H2O)

30
Methanol Combustion

• 2CH3OH (l) 3O2 (g) gt 2CO2 (g) 4H2O (l)
• ?Horeaction ? n?Hfo(prod) - ? m?Hfo(react)
• 2 x ?Hfo (CO2) 4 x ?Hfo (H2O) - 2 x
  ?Hfo (CH3OH)
• 2 x (-394 kJ) 4 x (-286 kJ) - 2 x
  (-239 kJ)
• -1454 kJ/ 2 moles CH3OH

31
Conversion to ?Hfo/g

• ?Horeaction/g -1454 kJ/ 2 moles CH3OH
  -1454 kJ/2 moles x (32.0 g/mol)
  -22.7 kJ/g

32
Combustion of Octane

• C8H18 (l) O2 (g) gt CO2 (g) H2O (l)
• Complete for next time!
• HINT 2 moles of octane are consumed.

33
Standard Enthalpies of Formation

• Hydrogen sulfide gas is a poisonous gas with the
  odor of rotten eggs. The gas burns in oxygen as
  follows Calculate the
  standard enthalpy change for this reaction using
  standard enthalpies of formation.

2H2S(g) 3O2(g)
2H2O(l) 2SO2(g)
34
Standard Enthalpies of Formation
2H2S(g) 3O2(g)
2H2O(l) 2SO2(g)
2(-20)
3(0)
2(-285.8)
2(-296.8)
(kJ)
35
Standard Enthalpies of Formation
2H2S(g) 3O2(g)
2H2O(l) 2SO2(g)
2(-20)
3(0)
2(-285.8)
2(-296.8)
(kJ)
?Ho ?n ?Ho (products) - ?m ?Ho (reactants)
2(-285.8) 2(-296.8)-2(-20) 3(0)
kJ -1125.2 -1125 kJ