Thermodynamics: Energy Relationships in Chemistry

1
Thermodynamics Energy Relationships in Chemistry
The Nature of Energy

• What is force

A push or pull exerted on an object

• What is work

An act or series of acts which overcome a force
2
Thermodynamics Energy Relationships in Chemistry

• Mechanical work

The amount of energy required to move an object
over a certain distance
w F d

• What is energy

The capacity to do work
3
Thermodynamics Energy Relationships in Chemistry
potential energy
Kinetic energy
Ek 1/2 mv 2
E joule 1kg-m2/s2
4.184 J 1 cal
4
Thermodynamics Energy Relationships in Chemistry
Sample problem A 252 g baseball is thrown with
a speed of 39.3 m/s. Calculate the kinetic
energy of the ball in joules and calories
Ek 1/2 mv2
1/2 (0.145 kg)(25m/s)2 45 kg m2/s2 45J
(45 J)
11 cal
5
Thermodynamics Energy Relationships in Chemistry
System and Surroundings
surroundings
system
6
Thermodynamics Energy Relationships in Chemistry
First law of thermodynamics

• Energy can neither be created nor destroyed

• The energy lost by a system equals the energy
  gained
• by its surroundings

• Everything wants to go to a lower energy state

?E E final - E initial
7
Thermodynamics Energy Relationships in Chemistry
endothermic
exothermic
E final lt E initial
E final gt E initial
8
Thermodynamics Energy Relationships in Chemistry

?E q w

9
Thermodynamics Energy Relationships in Chemistry

-
10
Thermodynamics Energy Relationships in Chemistry

-

-
11
Thermodynamics Energy Relationships in Chemistry
Sample problem During the course of a
reaction a system loses 550 J of heat to its
surroundings. As the gases in the system expand,
the piston moves up. The work on the piston by
the gas is determined to be 240 J. What is the
change in the internal energy of the system,
?E q w
?E (-550 J) (-240 J
?E -790 J
12

• A State Function is independent of
• pathway and is capitalized.
• ?E, energy is an extensive property
• and is a State function.
• heat (q) and work (w) are not state
• functions.

13
Thermodynamics Energy Relationships in Chemistry
14
Thermodynamics Energy Relationships in Chemistry
P-V work
15
Thermodynamics Energy Relationships in Chemistry
Let work w -P ? V
If ?E q w, then ?E q -P ? V
When a reaction is carried out in a
constant-volume container (? V 0) then, ?E
q v
When a reaction is carried out at constant
pressure container then, ?E q p - P ? V, or
q p ?E P ? V
16
Thermodynamics Energy Relationships in Chemistry

• Chemical reactions usually occur under conditions
  where
• the pressure is held constant, therefore

change in enthalpy ?H ?E P ? V
?H q p

• Since chemical reactions usually occur under
  conditions
• where the volume of the system undergoes little
  change

?H ?E

• Since ?H H final H initial, then for any type
  of chemical reaction, ?H H products - H
  reactants

17
Thermodynamics Energy Relationships in Chemistry
Some things you may never have wished to know
about enthalpy

• Enthalpy is an extensive property

CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
?H -802 kJ
-75 kJ 0kJ -393.5kJ -242kJ
(-393.5) 2(-242) (-75) 2(0) -802.5kJ
18
Sample problem How much heat is produced when
4.50 g of methane gas (CH4) is burned in a
constant pressure environment
CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
(1mol CH4)
(-802 kJ)
(4.50 g CH4)
-226 kJ
(1 mole CH4)
(16.0 g)
19

• The enthalpy change for a reaction is equal in
  magnitude
• but opposite in sign to ?H for the reverse
  reaction

20

• The enthalpy change for a reaction depends
• on the state of the reactants and products

Assume
CH4(g) 2O2(g) ? CO2(g) 2H2O(l)
?H -890 kJ

• The following process would also produce the same
  
• result

CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
?H -802 kJ
?H -88kJ
2H2O(g) ? 2H2O(l)
CH4(g) 2O2(g) ? CO2(g) 2H2O(l)
?H -890 kJ
21

• Here is a second reaction pathway which
• produces the same results

CH4 (g) 2O2(g) ? CO(g) 2H2O 1/2 O2
CO(g) 2H2O 1/2 O2 ?CO2(g) 2H2O
CH4 (g) 2O2(g) ?CO2(g) 2H2O
22
Thermodynamics Energy Relationships in Chemistry
Calorimetry Things are heating up
Calorimetry Measurement of heat flow
Molar Heat capacity The energy required to
raise the temperature of 1 mole of a substance
by 1?C (C q/?T, J/mol-?C )
q n C?T
Specific Heat The energy required to raise the
temperature of 1 gram of a substance by 1?C (C
q/?T, J/g-?C )
q m S?T
23
Thermodynamics Energy Relationships in Chemistry
Sample exercise The specific heat of Fe2O3 is
0.75 J/g-?C. A.) What is the heat capacity of a
2.00 kg brick of Fe2O3. B.) What quantity of
heat is required to increase the temperature of
1.75 g of Fe2O3 from 25 ?C to 380 ?C .
A.) (2.00 kg)
(1000g)
(0.75 J)
1.50 x 103 J/ ?C
(1 g - ?C)
(1kg)
B.) q m S?T 1.75 g (0.75 J/g-?C ) (355 ?C)
465 J
24
Thermodynamics Energy Relationships in Chemistry
Constant Pressure Calorimetry
25
Sample exercise 50 ml of 1.0 M HCl and 50 ml of
1.0 M NaOH are reacted together in a coffee cup
calorimeter. The temperature of the resulting
solution increased from 21.0 ?C to 27.5 ?C .
Calculate the enthalpy change of the reaction
(the specific heat of water 4.18 J/g-?C).
q mS ?T
q (100g)(4.18 J/ g-?C )(6.5 ?C )
q 2717 J 2.7 kJ
54kJ/Mol
1mol .050L L
assume the calorimeter absorbs negligible heat
and that the density of the solution is 1.0 g/ml.
26
Heat capacity of a metal
What is the heat capacity of a 5.05g chunk of an
unknown metal. The metal was heated in boiling
water and then placed in 50 mL of water in a
coffee cup calorimeter at a temperature of
24.5ºC. The highest temperature achieved was
28.9ºC. What is the heat capacity of the metal.
50g x 4.184j/gºC x 4.5ºC 5.05g x X x 71.1ºC
X 2.62J/gºC
27
Thermodynamics Energy Relationships in Chemistry
Bomb Calorimetry
28
Thermodynamics Energy Relationships in Chemistry
Sample exerciseWhen 1.00 g of the rocket fuel,
hydrazine (N2H2) is burned in a bomb calorimeter,
the temperature of the system increases by 3.51
?C. If the calorimeter has a heat capacity of
5.510 kJ/ ?C what is the quantity of heat
evolved. What is the heat evolved upon
combustion of one mole of N2H4.
qevolved -Ccalorimeter x ?T
-(5.510 kJ)
19.3 kJ
(3.51 ?C)
(?C)
(32 g)
(19.3 kJ)
(1.00mol)
618 kJ
(1.00 mol)
(1.00 g)
29
HESSS Law If a reaction is carried out in a
series of steps, ?H for the reaction will be
equal to the sum of the enthalpy changes for the
individual steps.
30
(No Transcript)
31
Sample exercise Calculate the ?H for the
reaction 2C(s) H2(g)? C2H2(g) given the
following reactions and their respective enthalpy
changes C2H2(g) 5/2O2 ? 2CO2(g) H2O(l) ?H
-1299.6 kJ C(s) O2(g) ? CO2(g) ?H
-393.5 kJ H2(g) 1/2O2 ? H2O(l) ?H -285.9
kJ
2CO2(g) H2O(l) ? C2H2(g) 5/2O2 ?H
1299.6 kJ
2C(s) 2O2(g) ? 2CO2(g) ?H
-787.0 kJ
H2(g) 1/2O2 ? H2O(l) ?H
-285.9 kJ
2C(s) H2(g) ? C2H2(g) ?H
226.7 kJ
32
(No Transcript)
33
Heats of formation, ?Hºf

• A thermodynamic description of the formation of
• compounds from their constituent elements.

Heat of vaporization ?H for converting liquids
to gases
Heat of fusion ?H for melting solids
Heat of combustion ?H for combusting a substance
in oxygen

• A thermodynamic description of the formation of
• compounds under standard conditions (1 atm, 298 K
  
• (25 ?C)) is called the standard heat of
  formation, ?Hºf

?
34
Thermodynamics Energy Relationships in Chemistry
The standard heat of formation for one mole of
ethanol is the enthalpy change for the following
reaction

? C2H5OH ?H f -277.7 kJ
?
2C(graphite) 3H2(g) ½ O2(g)
note the standard heat of formation of the most
stable form of any element is 0.
35
Thermodynamics Energy Relationships in Chemistry
36
Thermodynamics Energy Relationships in Chemistry
Sample exercise The quantity of heat produced
from one gram of propane (C3H8) is -50.5 kJ/gram.
How does this compare with the heat produced from
one gram of benzene (C6H6)? C6H6(l) O2 ?
6CO2(g) 3H2O(l)
(-285.8 kJ)
(-393.5 kJ)
?H rxn
(0)
(-49.04 kJ)
-
-
?H rxn
6(-393.5 kJ)
3(-285.8 kJ)
(- 49.04 kJ) -3267 kJ
note the standard heat of formation of the most
stable form of any element is 0.