Download Thermodynamics: Energy Relationships in Chemistry

Thermodynamics: Energy Relationships in Chemistry
The Nature of Energy
•What is force:
•What is work:
Thermodynamics: Energy Relationships in Chemistry
•Mechanical work
w=F
•What is energy:
*
d
Thermodynamics: Energy Relationships in Chemistry
E= joule = 1kg-m2/s2
4.184 J = 1 cal
Thermodynamics: Energy Relationships in Chemistry
Sample problem: A 145g baseball is thrown with a speed
of 25.0 m/s. Calculate the kinetic energy of the ball in joules
and calories
Thermodynamics: Energy Relationships in Chemistry
System and Surroundings
Thermodynamics: Energy Relationships in Chemistry
First law of thermodynamics:
Thermodynamics: Energy Relationships in Chemistry
Thermodynamics: Energy Relationships in Chemistry
+
E = q + w
+
Thermodynamics: Energy Relationships in Chemistry
+
-
Thermodynamics: Energy Relationships in Chemistry
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,
w
q
•A State Function is…
Thermodynamics: Energy Relationships in Chemistry
Thermodynamics: Energy Relationships in Chemistry
P-V work
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
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
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)
-75 kJ
0kJ
-393.5kJ -242kJ
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)
(4.50 g CH4) (1mol CH4) (-802 kJ)
(16.0 g) (1 mole CH4)
= -226 kJ
•The enthalpy change for a reaction is equal in magnitude
but opposite in sign to H for the reverse reaction
•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
2H2O(g)  2H2O(l) H = -88kJ
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -890 kJ
•Here is a second reaction pathway which
produces the same results
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 (molar heat capacity)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
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 .
Thermodynamics: Energy Relationships in Chemistry
Constant Pressure Calorimetry
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).
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.
Thermodynamics: Energy Relationships in Chemistry
Bomb Calorimetry
Thermodynamics: Energy Relationships in Chemistry
Sample exercise:When 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.
HESS’S 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.
CH4 (g) + 2O2(g)  CO(g) + 2H2O + 1/2 O2
H = -607 kJ
CO(g) + 2H2O + 1/2 O2 CO2(g) + 2H2O
H = -283 kJ
CH4 (g) + 2O2(g) CO2(g) + 2H2O
H = -890 kJ
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
Practice Exercise : Calculate the H for the reaction:
NO(g) + O(g) NO2(g)
given the following reactions and their respective enthalpy
changes
NO(g) + O3  NO2(g) + O2(g)
H = -198.9 kJ
O3(g)  3/2O2(g)
H = -142.3 kJ
O2(g) 2O (g)
H = 495.0 kJ
NO(g) + O3  NO2(g) + O2(g)
3/2O2(g) O3(g)
H = -198.9 kJ
H = 142.3 kJ
O (g)  1/2O2(g) )
H = -247.5 kJ
NO(g) + O(g) NO2(g)
H = -304.1 kJ
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

Thermodynamics: Energy Relationships in Chemistry
The standard heat of formation for one mole of ethanol
is the enthalpy change for the following reaction
2C(graphite) + 3H2(g) + ½ O2(g)  C2H5OH

H f = -277.7 kJ
note: the standard heat of formation of the most stable form of any element is 0.
Thermodynamics: Energy Relationships in Chemistry
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) +
15
2
O2  6CO2(g) + 3H2O(l)
H rxn =  n H f (products) -
 m H f (reactants)