Kinetic Molecular Theory (KMT)

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Kinetic Molecular Theory (KMT)

• AKA Kinetic Theory of Molecules (KTM)

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Energy is the capacity to do work.

• Energy is measured in Joules
• 1 Joule of energy can raise 1 N of weight exactly
  1 meter
• 1 J1Nm

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Energy is the capacity to do work.

• Forms include
• Kinetic energy
• Gravitational potential energy
• Elastic potential energy
• Electrical energy
• Chemical potential energy
• Heat

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Energy is the capacity to do work.

• Forms include
• Kinetic energy
• Gravitational potential energy
• Elastic potential energy
• Electrical energy
• Chemical potential energy
• Heat

of the greatest interest to a chemist
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Exothermic process
System
Surroundings
Energy
An exothermic process releases energy
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Endothermic process
System
Surroundings
Energy
An endothermic process absorbs energy
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If you add heat to a sample, it may

• a)
• b)
• c)
• d)

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If you add heat to a sample, it may

• a) warm up.
• b) melt
• c) boil
• d) expand (tough to calculate, dont bother)

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Lets try to warm up a cup of cold coffee.
Step 1 Add heat.
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Lets try to warm up a cup of cold coffee.
Step 1 Add heat.
Well, that was easy.
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Lets try to warm up a cup of cold coffee.
How could you add half as much heat?
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Lets try to warm up a cup of cold coffee.
How could you add half as much heat? a) b) c)
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Lets try to warm up a cup of cold coffee.
How could you add half as much heat? a) Raise the
temperature only half as much. b) c)
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Lets try to warm up a cup of cold coffee.
How could you add half as much heat? a) Raise the
temperature only half as much. b) Use half as
much coffee (and cup) c)
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Lets try to warm up a cup of cold coffee.
How could you add half as much heat? a) Raise the
temperature only half as much. b) Use half as
much coffee (and cup) c) Use a different substance
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The effect of heat (q)

• q depends on
• The mass of the sample (m)
• The change in temperature (DT)
• The nature of the sample (C)

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The effect of heat (q)

• q depends on
• The mass of the sample (m)
• The change in temperature (DT)
• The nature of the sample (C)

C is the specific heat capacity for a given
substance. Its units are (J/goC)
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If you add heat to a sample, it may

• a) warm up. qmCDT
• b) melt
• c) boil
• d) expand (tough to calculate, dont bother)

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qmCDT

• q heat, in Joules
• m mass, in grams
• C specific heat capacity, in J/goC
• DTchange in temperature (Tfinal-Tinitial)

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Cwater4.184 J/goC

• Cwater 4.2 J/goC
• Cethanol 2.4 J/goC
• Cice 2.1 J/goC
• CAl .90 J/goC
• CFe .46 J/goC
• Cglass .50 J/goC
• CAg .24 J/goC

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How much heat?

• How much heat does it take to raise 50.g water
  from 15oC to 80.oC?
• qmCDT

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How much heat?

• How much heat does it take to raise 50.g water
  from 15oC to 80.oC?
• qmCDT 50.g x 4.18 J/goC x (80.oC-15oC)

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How much heat?

• How much heat does it take to raise 50.g water
  from 15oC to 80.oC?
• qmCDT 50.g x 4.18 J/goC x (80.oC-15oC)
  50.g x 4.18 J/goC x (65oC)

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How much heat?

• How much heat does it take to raise 50.g water
  from 15oC to 80.oC?
• qmCDT 50.g x 4.18 J/goC x (80.oC-15oC)
  50.g x 4.18 J/goC x (65oC)
• 14000 J (14 kJ)

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What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it
  heat up?
• DT q/mC

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What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it
  heat up?
• DT q/mC 1550 J / (12 g x 4.18 J/goC )

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What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it
  heat up?
• DT q/mC 1550 J / (12 g x 4.18 J/goC )
• 31oC

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What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it
  heat up?
• DT q/mC 1550 J / (12 g x 4.18 J/goC )
• 31oC
• If the temperature starts at 25oC, it will heat
  up to

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What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it
  heat up?
• DT q/mC 1550 J / (12 g x 4.18 J/goC )
• 31oC
• If the temperature starts at 25oC, it will heat
  up to 56oC

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Calorimetry

• --the measurement of heat.

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Calorimetry

• --the measurement of heat.
• If one thing gains heat

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Calorimetry

• --the measurement of heat.
• If one thing gains heat
• something else lost it.

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• If 75 g of a metal at 96oC is placed in 58 g of
  water at 21oC and the final temperature reaches
  35oC, what is the specific heat capacity of the
  metal?

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Step 1

• How much heat did the water gain?

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Step 1

• How much heat did the water gain?
• qmCDT

Mass of water, in grams
Specific heat of water, 4.18 J/goC
Change in the temperature of water, in oC
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Step 2

• How much heat did the metal lose?

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Step 2

• How much heat did the metal lose?
• Heat lost - heat gained
• qlost-qgained

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Step 3

• What is the specific heat capacity of the metal?

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Step 3

• What is the specific heat capacity of the metal?
• Cq/mDT

Heat lost by metal
Mass of metal, in grams
Change in the temperature of metal, in oC
Specific heat of metal, in J/goC
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• If 75 g of a metal at 96oC is placed in 58 g of
  water at 21oC and the final temperature reaches
  35oC, what is the specific heat capacity of the
  metal?

.74 J/goC
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Thermochemistry

• 2H2(g)O2(g) ?2H2O(g) 443,000 J
• Two moles of hydrogen gas reacts with one mole of
  oxygen gas to form two moles of water vapor,
  releasing 443 kJ of heat.

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Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.

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Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.
• Forming bonds releases energy
• Breaking bonds requires energy

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Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.
• Forming bonds is exothermic
• Breaking bonds is endothermic

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Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.
• Exothermic reactions have a negative change in
  enthalpy
• Endothermic reactions have a positive change in
  enthalpy

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Thermochemistry

• 2H2 O2?2H2O

Breaking these bonds requires energy
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Thermochemistry

• 2H2 O2?2H2O

Breaking these bonds requires energy
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Thermochemistry

• 2H2 O2?2H2O

Breaking these bonds requires energy
Forming these bonds releases a lot more energy
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The mass to heat problem
g
1 mol
kJ
kJ
g
mol
The heat of reaction, DHrxn
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How about some stoichiometry?

• The oxidation of carbon releases 394 kJ/mol.
• How much heat is produced from the oxidation of
  15 g C?

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Special reactions

• Formation
• Combustion
• Fusion
• Vaporization
• Dissolution

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Special reactions

• Formation formating of 1 mole of a compound from
  its elements in their normal state
• Combustion burning 1 mole of a substance in
  oxygen
• Fusion freezing 1 mole of a substance at its
  melting point
• Vaporization boiling 1 mole of a substance at
  its boiling point
• Dissolution dissolving 1 mole of a substance in
  water

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Special Heats

• The Heat of Formation
• DHformation of Mg(OH)2 (s) -925 kJ/mol
• says
• Mg(s)O2(g)H2(g)?Mg(OH)2(s) 925 kJ

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Special Heats

• DHformation of CO2 (g) -393.5 kJ/mol
• ?
• 2Na(s)C(s)1½O2(g)?Na2CO3(s)1131 kJ
• ?

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Special Heats

• The Heat of Combustion
• DHcombustion of N2 (g) 90.4 kJ/mol
• says
• N2(g)O2(g) 90.4 kJ ?2NO(g)

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Special Heats

• DHcombustion of H2 (g) -286 kJ/mol
• ?
• CH3OH1½O2(g)?CO2(g)H2O(l) 726 kJ
• ?

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Special Heats

• The Heat of Fusion
• DHfusion of Fe 13.8 kJ/mol
• says
• Fe(s) 13.8 kJ ? Fe(l) at 1536oC

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Special Heats

• DHfusion of C6H6 9.87 kJ/mol
• ?
• CH3OH(s) 3160 J ? CH3OH(l) at -98oC
• ?

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Special Heats

• The Heat of Vaporization
• DHvaporization of CS2 28 kJ/mol
• says
• CS2(l) 28 kJ ? CS2(g) at 46.3oC

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Special Heats

• DHvaporization of CH4 8.2 kJ/mol
• ?
• O2(l) 6820 J ? O2(g) at -183oC
• ?

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Special Heats

• The Heat of Solution
• DHsolution of (NH2)2CO -14.0 kJ/mol
• says
• (NH2)2CO(s) ? (NH2)2CO (aq) 14.0 kJ

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Special Heats

• Formation
• Combustion
• Fusion
• Vaporization
• Dissolution