Download Ch 9 lecture notes, part 1

4/22/2014
Chapter 9
Why Study Thermochemistry?
Thermochemistry: Energy Changes in
Chemical Reactions
What are bond energies? What heat changes occur in chemical
reactions? Will a reaction occur? If we can show that the
reaction gives off heat, then the answer is frequently yes.
C6H12O6(s) + 4KClO3(l) → 6CO2(g) + 6H2O(g) + 4KCl(s)
Chemical and physical changes frequently
involve the gain or loss of HEAT physical changes (homogeneous mixtures):
NaCl(aq) + KBr(aq) → COLD
H2SO4(aq) + H2O(l) → HOT
Physical mixing;
no reaction
chemical changes:
H2SO4(aq) + C6H12O6(s) → products + HEAT
HEAT is a form of ENERGY
Chapter Outline
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9.1
9.2
9.3
9.4
9.5
9.6
9.7
Energy as a Reactant or a Product
Transferring Heat and Doing Work
Enthalpy and Enthalpy Changes
Heating Curves and Heat Capacity
Heats of Reaction and Calorimetry
Hess’s Law and Standard Heats of Reaction
Heats of Reaction from Heats of Formation and
Bond Energies
 9.8 More Applications of Thermochemistry
Energy Units
1 calorie (cal) = heat required to raise 1.0 gram
of H2O 1.0 oC
1 Joule = the KE of 2.0 kg mass moving 1.0 m/s
= ½mv2
= ½ (2.0 kg) (1.0 m/s)2
= 1.0 kg m2/s2
= 1.0 J
1.0 cal = 4.184 J
Definitions
Thermochemistry - the study of heat changes in chemical
reactions
Heat - the transfer of thermal energy between two bodies at
different temperatures
System - specific part of the Universe under study
Surroundings - the rest of the Universe outside the system
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Energy and Work
ENERGY is the capacity to perform WORK
WORK is a force over a distance (F x d)
Kinetic energy (KE) = energy of motion
KE = ½ mv2
m = mass
v = velocity
An object can possess energy in only two
ways, (1) kinetic and (2) potential energy
from Paul A. Tipler and Gene Mosca, "Physics for Scientists and Engineers", Fifth
Edition, 2003, W. H. Freeman & Company
Kinetic and Potential Energy at the Molecular Level
Energy of position or stored energy
Kinetic Energy - in any substance, whether it’s a solid,
liquid, or gas, the individual particles are in constant,
random, motion
PE = mgh
m = mass
g = gravitational constant
h = height
Gas =
“bouncing”
around
Liquids =
“sliding” around
Solids =
vibrations
The average Kinetic Energy is directly proportional to the
absolute Kelvin temperature
Heat = Kinetic Energy
= High KE
= Low KE
Potential Energy in Atoms and Molecules
electron-proton attraction
Weaker attraction
+
-
nucleus
HOT
COLD
When brought into
contact...
Collisions transfer
KE from hot to
cold until
equalized
Heat “flows” from the hot object to the cold
Higher potential energy
(further apart)
Stronger attraction
+
electron
Coulomb’s Law:
-
F = k q1q2/d2
lower potential energy
(closer together)
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Chemical Potential Energy
Coal is a complicated collection of organic
molecules which when burned, releases a great
amount of energy
The energy comes from
the chemical bonds
between atoms, which
are broken during
combustion
Energy Changes in Chemical Reactions
In virtually every chemical reaction, heat is either lost
or gained. How can we think of this in terms of kinetic
and potential energy?
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O + heat
high PE, low KE
Low PE, high KE
Internal Energy, E
The First Law of Thermodynamics
The sum of all the kinetic and potential energies (including
chemical) of an object, or a collection of many objects.
(Conservation of Energy)
Different types of molecular motion contribute to overall internal
energy: (a) translational, (b) rotational, and (c) vibrational.
E = q + w
“internal
energy”
Energy Flow Diagrams
Heat flow
Work
(F x d)
= 1 atm
E = q + w
w
It can be shown that
the q that flows at
constant pressure is
known as Enthalpy:
H = qp
Boiling water
performs work on
the surroundings
q
Heat flows into
the system
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State functions are properties that are determined by the state
of the system, regardless of how that condition was achieved.
State functions are PATH-INDEPENDENT
State functions are PATH-INDEPENDENT
energy, pressure, volume, temperature
E = Efinal - Einitial
P = Pfinal - Pinitial
V = Vfinal - Vinitial
T = Tfinal - Tinitial
Potential energy of the skier and hiker
is the same even though they took
different paths.
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