Transcript Energy and Thermochemistry

Energy and
Thermochemistry
Energy
The ability to do work
 4 Types

Kinetic: Energy in motion
 Potential: Stored energy
 Chemical: potential energy of compounds and
molecules that can be released as the result
of a chemical reaction
 Thermal: energy of an object due to the
random motions of its particles
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Thermochemistry
Changes of heat content and heat transfer
 Follow the Law of Conservation of Energy
Or, 1st Law of Thermodynamics
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Energy can neither be created nor destroyed
only transferred
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Heat will always move from HIGH to LOW
(never reverse)
Temperature & Heat
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Heat not the same as Temperature
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Heat = energy transferred to one system by
another due to temperature difference

Temperature = measurement of heat energy
 Thermometer
Higher thermal energy, greater motion of
particles
System and Surroundings
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System = the object in question
Surrounding(s) = everything outside the system
When both the system and the surroundings
equal the same temperature it is called: Thermal
equilibrium
When not equal
 Heat transfer to surrounding = Exothermic
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You feel the heat  Hot Metal!
Heat transfer to system = Endothermic
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You feel cold  Cold Metal!
Specific Heat of one object
Q = Cp x m x ∆T
Q = heat/energy measured in Joules (J)
 Cp = specific heat measured in J/g x 0C
 m = mass measured in grams
 ∆T = change in temperature measured in
(0C) (must be positive)
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Example
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It takes 487.5 J to heat 25 grams of a
substance from 25 °C to 75 °C. What is
the specific heat in Joules/g·°C?
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How many joules of energy must be
transferred to change the temperature of a
piece of iron from 50 0C to 150C? The
sample contains 475 g.
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To what temperature will a 50.0 g piece of
glass raise if it absorbs 5275 joules of heat
and its specific heat capacity is 0.50
J/g°C? The initial temperature of the glass
is 20.0°C.
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If a sample of chloroform is initially at
25°C, what is its final temperature if 150.0
g of chloroform absorbs 1.0 kilojoules of
heat, and the specific heat of chloroform is
0.96 J/g°C?
Specific heat and Phase
Changes
35 g of water @ 180C  1500C
65 g of solid ethanol @
 900C
0
-117 C
13 g benzene @ -30C  250C
What is the symbol?
H = Enthalpy
Enthalpy
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Definition: amount of heat energy
absorbed or lost by a system during a
chemical reaction
DH rxn = SnH products – SnH reactants
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S = Sum of. Use Appendix K for the H0f for
different compounds and elements.
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n = number of moles
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If DH0rxn is a negative value the reaction is
Exothermic
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If DH0rxn is a positive value the reaction is
Endothermic
Endothermic vs Exothermic
Practice
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Na(s) + Cl2(g)  NaCl(s)
Na(s) + H2O(l)  NaOH(aq) + H2(g)
ENTROPY AND GIBBS FREE
ENERGY
Entropy
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2nd Law of Thermodynamics (Entropy)
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Entropy (S): a measurement of the
randomness (or chaos) of particles in a
system
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Systems have an overall tendency to
increase the entropy
3rd Law of Thermodynamics
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The entropy (chaos/disorder) of a pure,
perfect crystalline substance is zero at
absolute zero.
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Will entropy ever be negative? Why?
DSsystem = SnSproducts - SnSreactants
Things that effect Entropy
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Changing from solid to liquid to gas always
gives you an increase entropy.
Changing from gas to liquid to solid always
gives you a decrease entropy.
As temperature goes up entropy goes up
As volume of a gas goes up, entropy goes up
As substances are mixed, entropy increases
Increasing the number of particles entropy
increases
Increasing the number of moles of gas, entropy
increases
2H2O(g)  2H2(g) + O2(g)
 CaCO3(s)  CaO(s) + CO2(g)
 NH4Cl(s)  NH3(g)+ HCl(g)
 O2(g)  O3(g)
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Gibb’s Free Energy
Ability to do work
 DGsystem = SnGproducts – SnGreactants
 If DG is positive nonspontaneous
 If DG is negative spontaneous
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What do the signs mean?
Property
DH
DS
DG
Positive
Negative
Quick Review
Enthalpy
Define
Symbol
Formula
Unit
Entropy
Gibbs Free
Energy
What if it isn’t at 250C?
 DG
= DH – TDS
 Temperature
 Make
must be in Kelvin
sure you convert your S to
kilojoules (kJ)
N2(g)+ O2(g)  NO(g)@ 1250C
O2(g)  O3(g) @ 400C
C2H5OH(l) + O2(g)  CO2(g) + H2O(l)
@ 1250C
Phase Change and Temperature of
Spontaneity
DH
T=
DS
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Can be used to determine the boiling point
of an unknown.
HNO3(l)  HNO3(g)
S(s)  S(g)
Signs and Result of DG
DG
DH
DS
Spontaneous?
NH3(g) + HCl(g)  NH4Cl(s)
CCl4(l)  C(graphite) + Cl2(g)