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. Thermodynamics: Energy Relationships in Chemistry. Mechanical work.
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The Nature of Energy
A push or pull exerted on an object
An act or series of acts which overcome a force
The amount of energy required to move an
object over a certain distance
w = F * d
The capacity to do work
potential energy
Kinetic energy
Ek= 1/2 mv 2
E= joule = 1kgm2/s2
4.184 J = 1 cal
(4.184 J)
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
First law of thermodynamics:
E = E final  E initial
endothermic
exothermic
E final < E initial
E final > E initial
q
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
Let work w = P V
If E = q + w, then E = q + P V
When a reaction is carried out in a constantvolume
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
change in enthalpy: H = E+ P V
H = q p
H =E
Some things you may never have wished to know about enthalpy
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
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)
Assume:
CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
H = 890 kJ
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
CH4 (g) + 2O2(g) CO(g) + 2H2O + 1/2 O2
CO(g) + 2H2O + 1/2 O2CO2(g) + 2H2O
CH4 (g) + 2O2(g) CO2(g) + 2H2O
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 1C (C = q/T, J/molC )
q = n (molar heat capacity)T
Specific Heat: The energy required to raise the temperature
of 1 gram of a substance by 1C (C = q/T, J/gC )
q = m sT
Sample exercise: The specific heat of Fe2O3 is 0.75 J/gC. 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 = mST = 1.75 g (0.75 J/gC ) (355 C) =465 J
Constant Pressure Calorimetry
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/gC).
q = mST
q = (100g)(4.18 J/ gC )(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.
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
Bomb Calorimetry
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.
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)
in a series of steps,H for the reaction
will be equal to the sum of the enthalpy
changes for the individual steps.
CO(g) + 2H2O + 1/2 O2CO2(g) + 2H2O H = 283 kJ
CH4 (g) + 2O2(g) CO2(g) + 2H2O H = 890 kJ
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
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) H = 198.9 kJ
3/2O2(g) O3(g) H = 142.3 kJ
O (g) 1/2O2(g) ) H = 247.5 kJ
NO(g) + O(g) NO2(g) H = 304.1 kJ
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
m
(reactants)
n
H rxn =
H f
H f
Thermodynamics: Energy Relationships in Chemistry
The standard heat of formation for one mole of ethanol
is the enthalpy change for the following reaction
C2H5OHH 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.
15
2
2
(products) 
m
(reactants)
H rxn =
n
H f
[1H f
3H f
H f
H f
[6H f
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.