Transcript CHM 111 CHAPTER 5 Thermochemistry © 2012 by W. W. Norton & Company.

CHM 111
CHAPTER 5
Thermochemistry
© 2012 by W. W. Norton & Company
Thermodynamics
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Energy: is the capacity to do work, or supply heat.
Energy = Work + Heat
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Kinetic Energy: is the energy of motion.
EK = 1/2 mv2
(1 Joule = 1 kgm2/s2)
(1 calorie = 4.184 J)
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Potential Energy: is stored energy.
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Thermodynamics
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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 sign (loss of energy).
•
Energy flow from the surroundings to the system
has a positive sign (gain of energy).
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Thermodynamics
Closed System: Only energy can be lost or gained.
• Isolated System: No matter or energy is exchanged.
•
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First Law of Thermodynamics
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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.
DE = Efinal – Einitial = E2 – E1 = q + w
q = heat, w = work
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Enthalpy Changes
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Enthalpies of Physical Change:
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Change of State
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Enthalpy Changes
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Enthalpies of Chemical Change: Often called
heats of reaction (DHreaction).
Endothermic: Heat flows into the system from the
surroundings and DH has a positive sign.
Exothermic: Heat flows out of the system into the
surroundings and DH has a negative sign.
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Enthalpy Changes
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Reversing a reaction changes the sign of DH for a
reaction.
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) DH = –2219 kJ
3 CO2(g) + 4 H2O(l)  C3H8(g) + 5 O2(g) DH = +2219 kJ
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Multiplying a reaction increases DH by the same factor.
3 C3H8(g) + 15 O2(g)  9 CO2(g) + 12 H2O(l) DH = –6657 kJ
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Enthalpy Changes
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How much heat (in kilojoules) is evolved or absorbed in
each of the following reactions?
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Burning of 15.5 g of propane:
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
DH = –2219 kJ
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Reaction of 4.88 g of barium hydroxide octahydrate with
ammonium chloride:
Ba(OH)2·8 H2O(s) + 2 NH4Cl(s)  BaCl2(aq) + 2 NH3(aq) + 10 H2O(l)
DH = +80.3 kJ
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Enthalpy Changes
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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.
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Standard enthalpy change is indicated by the
symbol DH°.
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Hess’s Law
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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.
3 H2(g) + N2(g)  2 NH3(g) DH° = –92.2 kJ
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Hess’s Law
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The industrial degreasing solvent methylene
chloride (CH2Cl2, dichloromethane) is prepared
from methane by reaction with chlorine:
CH4(g) + 2 Cl2(g)
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CH2Cl2(g) + 2 HCl(g)
Use the following data to calculate DH° (in kilojoules)
for the above reaction:
CH4(g) + Cl2(g)
DH° = –98.3 kJ
CH3Cl(g) + Cl2(g)
DH° = –104 kJ
CH3Cl(g) + HCl(g)
CH2Cl2(g) + HCl(g)
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Standard Heats of Formation
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Standard Heats of Formation (DH°f): The
enthalpy change for the formation of 1 mole of
substance in its standard state from its constituent
elements in their standard states.
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The standard heat of formation for any element in
its standard state is defined as being ZERO.
•
DH°f = 0 for an element in its standard state
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Standard Heats of Formation
H2(g) + 1/2 O2(g)  H2O(l)
3/
2 H2(g)
+ 1/2 N2(g)  NH3(g)
2 C(s) + H2(g)  C2H2(g)
DH°f = –286 kJ/mol
DH°f = –46 kJ/mol
DH°f = +227 kJ/mol
2 C(s) + 3 H2(g) + 1/2 O2(g)  C2H5OH(g) DH°f = –235 kJ/mol
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Standard Heats of Formation
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Calculating DH° for a reaction:
DH° = DH°f (Products) – DH°f (Reactants)
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For a balanced equation, each heat of formation must
be multiplied by the stoichiometric coefficient.
aA + bB
cC + dD
DH° = [cDH°f (C) + dDH°f (D)] – [aDH°f (A) + bDH°f (B)]
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Standard Heats of Formation
Some Heats of Formation, DHf° (kJ/mol)
CO(g)
-111
C2H2(g)
227
Ag+(aq)
106
CO2(g)
-394
C2H4(g)
52
Na+(aq)
-240
H2O(l)
-286
C2H6(g)
-85
NO3-(aq)
-207
NH3(g)
-46
CH3OH(g)
-201
Cl-(aq)
-167
N2H4(g)
95.4
C2H5OH(g)
-235
AgCl(s)
-127
HCl(g)
-92
C6H6(l)
49
Na2CO3(s)
-1131
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Standard Heats of Formation
•
Calculate DH° (in kilojoules) for the reaction of
ammonia with O2 to yield nitric oxide (NO) and
H2O(g), a step in the Ostwald process for the
commercial production of nitric acid.
•
Calculate DH° (in kilojoules) for the photosynthesis
of glucose from CO2 and liquid water, a reaction
carried out by all green plants.
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Calorimetry and Heat Capacity
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Calorimetry is the science of measuring heat
changes (q) for chemical reactions. There are two
types of calorimeters:
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Bomb Calorimetry: A bomb calorimeter measures the
heat change at constant volume such that q = DE.
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Constant Pressure Calorimetry: A constant pressure
calorimeter measures the heat change at constant
pressure such that q = DH.
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Calorimetry and Heat Capacity
Constant Pressure
Bomb
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Calorimetry and Heat Capacity
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Heat capacity (C) is the amount of heat required to
raise the temperature of an object or substance a
given amount.
q
C =
DT
Specific Heat: The amount of heat required to raise the
temperature of 1.00 g of substance by 1.00°C.
Molar Heat: The amount of heat required to raise the
temperature of 1.00 mole of substance by 1.00°C.
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Calorimetry and Heat Capacity
Specific Heat: The amount of heat required to raise the
temperature of 1.00 g of substance by 1.00°C.
S.H. = q/ (g °C)
q = S.H. x mass x ΔT
q = S.H. x mass x (Tf – Ti)
ΔH = q/(moles in reaction)
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Calorimetry and Heat Capacity
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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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When 25.0 mL of 1.0 M H2SO4 is added to 50.0 mL
of 1.0 M NaOH at 25.0°C in a calorimeter, the
temperature of the solution increases to 33.9°C.
Assume specific heat of solution is 4.184 J/(g–1·°C–1),
and the density is 1.00 g/mL–1, calculate DH for the
reaction.
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Calorimetry and Heat Capacity
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