Transcript CH 6: Thermochemistry Renee Y. Becker Valencia Community College CHM 1045
CH 6: Thermochemistry
Renee Y. Becker Valencia Community College CHM 1045 1
Energy •
Energy:
is the capacity to do work, or supply heat.
Energy = Work + Heat •
Kinetic Energy:
is the energy of motion.
E
K = 1 / 2
mv
2
(1 Joule = 1 kg
m 2 /s 2 ) (1 calorie = 4.184 J)
•
Potential Energy:
is stored energy.
2
E k & E p 3
Example 1: KE Which of the following has the greatest kinetic energy?
1. A 12 kg toy car moving at 5 mph?
2. A 12 kg toy car standing at the top of a large hill?
4
Energy •
Thermal Energy
is the kinetic energy of molecular motion • Thermal energy is proportional to the temperature in degrees Kelvin.
E
thermal
T
(K) •
Heat
is the amount of thermal energy transferred between two objects at different temperatures.
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•
In an experiment:
Reactants and products are the
system
; everything else is the
surroundings
.
• Energy flow from the
system
to the
surroundings
has a
negative
energy). ( E or H) sign (loss of • Energy flow from the
surroundings
to the
system
has a
positive
(+ E or + H) sign (gain of energy). 6
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•
The law of the conservation of energy:
Energy cannot be created or destroyed.
• The energy of an isolated system must be constant.
• The energy change in a system equals the work done on the system + the heat added.
E
=
E
final –
E
initial =
E
2 –
E
1 =
q
+
w q
= heat,
w
= work 8
• Pressure is the force per unit area.
Pressure = Force Area
F A
(1
N/m 2
= 1
Pa
) (1
atm
= 101,325
Pa
) • Work is a force (
F
) that produces an object’s movement, times the distance moved (
d
): Work = Force x Distance 9
The expansion in volume that occurs during a reaction forces the piston outward against atmospheric pressure, P. Work = -atmospheric pressure * area of piston * distance piston moves 10
Example 2: Work How much work is done (in kilojoules), and in which direction, as a result of the following reaction?
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• The amount of heat exchanged between the system and the surroundings is given the symbol
q
.
q
=
E
+
P
V
At constant volume (
V
= 0):
q v
=
E
At constant pressure:
q p
=
E
+
P
V
=
H
Enthalpy change:
H
=
H
products –
H
reactants 12
Example 3: Work The explosion of 2.00 mol of solid TNT with a volume of approximately 0.274 L produces gases with a volume of 489 L at room temperature. How much
PV
(in kilojoules) work is done during the explosion? Assume 25 °C.
P
= 1 atm,
T
= 2 C 7 H 5 N 3 O 6 (
s
) 12 CO(
g
) + 5 H 2 (
g
) + 3 N 2 (
g
) + 2 C(
s
) 13
•
Enthalpies of Physical Change:
Enthalpy is a state function, the enthalpy change from solid to vapor does not depend on the path taken between the two states. H subl = H fusion + H vap 14
•
Enthalpies of Chemical Change:
Often called heats of reaction (
H
reaction ).
Endothermic :
Heat flows into the system from the surroundings and
H has a positive sign
.
Exothermic :
Heat flows out of the system into the surroundings and
H has a negative sign
.
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Bromination vs. Chlorination
41 16
• Reversing a reaction changes the sign of
H
for a reaction.
C 3 H 8 (
g
) + 5 O 2 (
g
) 3 CO 2 (
g
) + 4 H 2 O(
l
)
H
= –2219 kJ 3 CO 2 (
g
) + 4 H 2 O(
l
) C 3 H 8 (
g
) + 5 O 2 (
g
)
H
= +2219 kJ • Multiplying a reaction increases
H
factor.
by the same 3 [C 3 H 8 (
g
) + 15 O 2 (
g
) 9 CO 2 (
g
) + 12 H 2 O(
l
)]
H
= 3(-2219) kJ
H
= -6657 kJ 17
Example 4: Heat • How much heat (in kilojoules) is evolved or absorbed in each of the following reactions?
a) Burning of 15.5 g of propane: C 3 H 8 (
g
) + 5 O 2 (
g
) 3 CO 2 (
g
) + 4 H 2 O(
l
)
H
= –2219 kJ/mole b) Reaction of 4.88 g of barium hydroxide octahydrate with ammonium chloride: Ba(OH) 2 ·8 H 2 O(
s
) + 2 NH 4 Cl(
s
) BaCl 2 (
aq
) + 2 NH 3 (
aq
) + 10 H 2 O(
l
)
H
= +80.3 kJ/mole 18
•
Thermodynamic Standard State:
Most stable form of a substance at 1 atm pressure and 25 °C; 1 M concentration for all substances in solution.
• These are indicated by a superscript ° to the symbol of the quantity reported. • S
tandard enthalpy change
symbol
H
°.
is indicated by the 19
Example 5: Is an endothermic reaction a favorable process thermodynamically speaking?
1) Yes 2) No 20
Hess’s Law •
Hess’s Law:
The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.(not a physical change, chemical change) 3 H 2 (
g
) + N 2 (
g
) 2 NH 3 (
g
)
H
° = –92.2 kJ 21
• Reactants and products in individual steps can be added and subtracted to determine the overall equation.
(1) 2 H 2 (
g
) + N 2 (
g
) (2) N 2 H 4 (
g
) + H 2 (
g
) (3) 3 H 2 (
g
) + N 2 (
g
) N 2 H 4 (
g
)
H
° 1 = ?
2 NH 3 (
g
)
H
° 2 = –187.6 kJ 2 NH 3 (
g
)
H
° 3 = –92.2 kJ
H
° 1 +
H
° 2 =
H
° reaction Then
H
° 1 =
H
° reaction
H
° 2
H
° 1 =
H
° 3 –
H
° 2 = ( –92.2 kJ) – (–187.6 kJ) = +95.4 kJ 22
Example 6: Hess’s Law • The industrial degreasing solvent methylene chloride (CH 2 Cl 2 , dichloromethane) is prepared from methane by reaction with chlorine: CH 4 (
g
) + 2 Cl 2 (
g
) CH 2 Cl 2 (
g
) + 2 HCl(
g
) Use the following data to calculate
H
° (in kilojoules) for the above reaction: CH 4 (
g
) + Cl 2 (
g
)
H
° = –98.3 kJ CH 3 Cl(
g
) + HCl(
g
) CH 3 Cl(
g
) + Cl 2 (
g
)
H
° = –104 kJ CH 2 Cl 2 (
g
) + HCl(
g
) 23
•
Standard Heats of Formation (
H
° f ):
The enthalpy change for the formation of 1 mole of substance in its standard state from its constituent elements in their standard states.
• The standard heat of formation for any element in its standard state is defined as being ZERO.
H
° f
= 0 for an element in its standard state
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Standard Heats of Formation •
Calculating
H
° for a reaction:
H
° =
H
°
f
(Products) –
H
°
f
(Reactants)
• For a balanced equation, each heat of formation must be multiplied by the stoichiometric coefficient.
aA + bB cC + dD
H
° = [c
H
° f (C) + d
H
° f (D)] – [a
H
° f (A) + b
H
° f (B)] 25
Standard Heats of Formation
Some Heats of Formation,
H
f
°
(kJ/mol)
CO(
g
) CO 2 (
g
) H 2 O(
l
) NH 3 (
g
) N 2 H 4 (
g
) HCl(
g
) -111 -394 -286 -46 95.4
-92 C 2 H 2 (
g
) C 2 H 4 (
g
) C 2 H 6 (
g
) CH 3 OH(
g
) C 2 H 5 OH(
g
) C 6 H 6 (
l
) 227 52 -85 -201 -235 49 Ag + (
aq
) Na + (
aq
) NO 3 (
aq
) Cl (
aq
) AgCl(
s
) Na 2 CO 3 (
s
) 106 -240 -207 -167 -127 -1131 26
Example 7: Standard heat of formation Calculate
H
° (in kilojoules) for the reaction of ammonia with O 2 to yield nitric oxide (NO) and H 2 O(
g
), a step in the Ostwald process for the commercial production of nitric acid.
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Example 8: Standard heat of formation Calculate
H
° (in kilojoules) for the photosynthesis of glucose and O 2 from CO 2 and liquid water, a reaction carried out by all green plants.
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Example 9: Which of the following would indicate an endothermic reaction? Why?
1. H 2. + H 29
Heat of Phase Transitions from H f Calculate the heat of vaporization, H vap water, using standard enthalpies of of formation H H 2 2 O O (g) (l) H f -241.8 kJ/mol -285.8 kJ/mol 30
Calorimetry and Heat Capacity •
Calorimetry
is the science of measuring heat changes (
q
) for chemical reactions. There are two types of calorimeters: • Bomb Calorimetry: A bomb calorimeter measures the heat change at constant volume such that
q
=
E
.
• Constant Pressure Calorimetry: A constant pressure calorimeter measures the heat change at constant pressure such that
q
=
H
.
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Constant Pressure Bomb
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Calorimetry and Heat Capacity •
Heat capacity (C)
is the amount of heat required to raise the temperature of an object or substance a given amount.
Specific Heat:
The amount of heat required to raise the temperature of 1.00 g of substance by 1.00
°C.
q = s x m x t q = heat required (energy) s = specific heat m = mass in grams t = T f - T i 33
Calorimetry and Heat Capacity •
Molar Heat:
The amount of heat required to raise the temperature of 1.00 mole of substance by 1.00
°C.
q = MH x n x t q = heat required (energy) MH = molar heat n = moles t = T f - T i 34
Example 10: Specific Heat What is the specific heat of lead if it takes 96 J to raise the temperature of a 75 g block by 10.0
°C?
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Example 11: Specific Heat How much energy (in J) does it take to increase the temperature of 12.8 g of Gold from 56 C to 85 C?
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Example 12: Molar Heat • How much energy (in J) does it take to increase the temperature of 1.45 x10 4 moles of water from 69 C to 94 C?
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