CHEMISTRY 161

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CHEMISTRY 161 Energy and Chemical
Change Chapter 7
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Energy an Chemical Change

• Forms of Energy
• SI Unit of Energy
• Energy in Atoms and Molecules
• Thermodynamics
• Calculation of Heat and Energy Changes
• Measuring Heat and Energy Changes

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1. Forms of Energy
1. Kinetic energy
energy of a moving microscopic or macroscopic
object E ½ m v2
2. Radiant energy
energy in form of photons (light) (solar
energy) E h ? (h Plancks constant) (Chapter
8)
3. Potential energy
energy by changing objects position in height E
m g h (h height)
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4. Thermal Energy
energy associated with random motion of atoms and
molecules Ekin ½ M v2 3/2 R T (Chapter 7) M
m ? Na
5. Chemical Energy
EXP1
energy stored in chemical bonds of substances
(Chapter 7)
LAW OF CONSERVATION OF ENERGY
THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT
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2. SI Unit of Energy
1 Joule 1 J
1 cal 4.184 J
1 J 1 Nm 1 kg m2 s-2
Ekin ½ ? m ? v2
macroscopic versus microscopic
1 J vs. 1 kJ mol-1
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3. Energy in Atoms and Molecules
Atoms Kinetic and Thermal Energy
gases are constantly in motion and hold a
kinetic energy
Ekin ½ M v2 3/2 R T
EXP2
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Molecules Kinetic, Thermal, Potential Energy
(N2)
molecules have different internal (vibrational)
energy when bond distances are changed
EXP3/4
different bonds have different bond strength
(stabilities) (H2 vs. N2)
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4. Thermodynamics
reactants ? products
(different energies)
THERMODYNAMICS
HEAT
CHANGE
study of the energy associated with change
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THERMOCHEMISTRY
study of the energy associated with chemical
change
2 H2(g) O2(g) ? 2 H2O(l)
energy
Hindenburg 1937
Challenger 1986
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ENDOTHERMIC
EXOTHERMIC
EXP
2 H2(g) O2(g) ? 2 H2O(l)
2 HgO(s) ? O2(g) 2 Hg(l)
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Energy
NH4NO3 (aq)
2 H2(g) O2(g)
Exothermic (heat given off by system)
Endothermic (heat absorbed by system)
NH4NO3(s) H2O (l)
2 H2O(l)
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QUANTIFICATION
Enthalpy of Reaction
Enthalpy is the heat release at a constant
pressure (mostly atmospheric pressure)
DH Hfinal - Hinitial
DH Hproducts - Hreactants
board
Hfinal gt Hinitial DH gt 0 ENDOTHERMIC
Hfinal lt Hinitial DH lt 0 EXOTHERMIC
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Energy
NH4NO3(aq)
2 H2(g) O2(g)
Hfinal gt Hinitial
Hfinal lt Hinitial
Exothermic
Endothermic
NH4NO3(s) H2O(l)
2 H2O(l)
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Energy
H2O(l)
Hfinal gt Hinitial
DH Hfinal Hinitial
Endothermic
H2O(s)
H2O(s) ? H2O(l) ?H 6.01 kJ mol-1
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Energy
H2O(l)
Hfinal lt Hinitial
DH Hfinal Hinitial
Exothermic
H2O(s)
H2O(l) ? H2O(s) ?H - 6.01 kJ mol-1
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THERMOCHEMICAL EQUATIONS
H2O(l) ? H2O(s) ?H - 6.01 kJ mol-1
CH4(g) 2 O2(g) ? 2 H2O(l) CO2(g) ?H-890.4
kJ mol-1
Calculate the heat evolved when combusting 24.0
g of methane gas.
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5. Calculation of Heat and Enthalpy Changes
DHm Hm,products Hm,reactants
molar
REFERENCE SYSTEM
e.g. oxidation numbers of elements are zero
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Standard Enthalpy of Formation
DHfO
heat change when 1 mole of a compound is formed
from its elements at a pressure of 1 atm
(T 298 K)
DHfO (element) 0 kJ/mol
DHfO (graphite) 0 kJ/mol
DHfO (diamond) 1.9 kJ/mol
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C(s, graphite) O2(g)
Hreactants
0
ENTHALPY, H
DHf0 - 393.51 kJ mol-1
Hproducts
-393.51
CO2(g)
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Standard Enthalpy of Formation
C(s, graphite) O2(g)
CO2(g)
DHf0 - 393.51 kJ mol-1
CH4(g)
DHf0 - 74.81 kJ mol-1
C(s, graphite) 2H2(g)
DHf0 - 46.11 kJ mol-1
½ N2(g) 3/2 H2(g)
NH3(g)
NO(g)
DHf0 33.18 kJ mol-1
(1/2) N2(g) (1/2) O2(g)
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Standard Enthalpy of Reaction
a A b B ? c C d D
a A b B
a DHfO (A) b ?HfO(B)
Hreactants
ENTHALPY, H
DHOrxn S?Hf0(prod) S?Hf0(react)
Hproducts
c DHfO(C) d ?HfO(D)
c C d D
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Standard Enthalpy of Reaction
DHOrxn Sn?Hf0(prod) Sm?Hf0(react)
CaO(s) CO2(g) ? CaCO3(s)
kJ/mol
-393.5
-1206.9
-635.6
DHOrxn -177.8 kJ/mol
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Standard Enthalpy of Reaction
CH4(g) 2 O2(g) ? CO2(g) 2 H2O(l)
DHOrxn Sn?Hf0(prod) Sm?Hf0(react)
CH4(g) 2 O2(g) ? CO2(g) 2 H2O(g)
2 H2O(g) ? 2 H2O(l)
CH4(g) 2 O2(g) ? CO2(g) 2 H2O(l)
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Hesss Law
The overall reaction enthalpy is the sum of the
reaction enthalpies of the steps in which the
reaction can be divided
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CH4(g) 2O2(g)
Reactants
- 802 kJ
- 890 kJ
ENTHALPY, H
CO2(g) 2H2O(g)
- 88 kJ
CO2(g) 2H2O(l)
Products
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DHrxn for S(s) 3/2 O2(g) ?SO3(g)
S(s) O2(g) ? SO2(g) DH1 -320.5
kJ SO2(g) 1/2 O2(g) ?SO3(g) DH2 -75.2 kJ
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6. State Functions
THERMODYNAMICS
quantitative study of heat and energy changes of
a system
CH4(g) 2 O2(g) ? CO2(g) 2 H2O(l)
the state (condition) of a system is defined by
T, p, n, V, E
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the state (condition) of a system is defined by
T, p, n, V, E
STATE FUNCTIONS
properties which depend only on the initial and
final state, but not on the way how this
condition was achieved
Hess Law
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?V Vfinal Vinitial
?p pfinal pinitial
?T Tfinal Tinitial
?E Efinal Einitial
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Energy is a STATE FUNCTION
?E m g ?h
IT DOES NOT MATTER WHICH PATH YOU TAKE
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Hess Law
CH4(g) 2O2(g)
Reactants
- 802 kJ
- 890 kJ
ENTHALPY, H
CO2(g) 2H2O(g)
- 88 kJ
CO2(g) 2H2O(l)
Products
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Applications
Zeroth Law of Thermodynamics
a system at thermodynamical equilibrium has a
constant temperature
heat is spontaneous transfer of thermal energy
two bodies at different temperatures T1 gt T2
spontaneous
T2
T1
EXP LN2/Metal/H2O
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First Law of Thermodynamics
energy can be converted from one form to another,
but cannot be created or destroyed
CONSERVATION OF ENERGY
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SURROUNDINGS

-
SYSTEM
THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT
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First Law of Thermodynamics
?Esystem ?Q ?W
?Q heat change
?W work done
DQ gt 0 ENDOTHERMIC
?
DQ lt 0 EXOTHERMIC
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mechanical work
?W - p ?V
?V lt 0
?V gt 0
the energy of gas goes up
the energy of gas goes down
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6. Measurement of Heat Changes
temperature increase
DH ?Q 8 ?T
(pressure is constant)
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Where does the heat go?
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DH ?Q 8 ?T
DH ?Q const ?T
DH ?Q C ? ?T
temperature change
enthalpy change
heat capacity
C m ? s s specific heat capacity
DH ?Q m s ?T
EXP
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specific heat capacity
capability of substances to store heat and energy
s J g-1 K-1
the J necessary to increase the temperature of 1
g of a compound by 1 K
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DH ?Q m s ?T

• prepare two styrofoam cups

2. carry out chemical reaction in a compound with
known s
s (H2O) 4.184 J g-1 K-1
3. measure temperature change
4. determine ?H
calorimeter
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100 ml of 0.5 M HCl is mixed with 100 ml 0.5 M
NaOH in a constant pressure calorimeter (scup
335 J K-1). The initial temperature of the HCl
and NaOH solutions are 22.5C, and the final
temperature of the solution is 24.9C. Calculate
the molar heat of neutralization assuming the
specific heat of the solution is the same as for
water.
DH ?Q C ?T
DH ?Q (c1 c2) ?T

• Neutralization reactions
• Redox reactions
• 3. Precipitation reactions

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Constant Volume Calorimeter
?Q (m s(H2O) cbomb) ?T
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Energy an Chemical Change

• Forms of Energy
• SI Unit of Energy
• Energy in Atoms and Molecules
• Thermodynamics
• Calculation of Heat and Energy Changes
• Measuring Heat and Energy Changes