Energy and Thermochemistry

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Energy and Thermochemistry
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Energy

• The ability to do work
• 2 types
• Potential stored energy
• Kinetic energy in motion

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Thermochemistry

• Changes of heat content and heat transfer
• Follow Law of Conservation of Energy
• Or, 1st Law of Thermodynamics
• Energy can neither be created nor destroyed
• Thus, total energy of universe constant

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Temperature Heat

• Heat not same as temperature
• Heat energy transferred to one system by
  another due to temperature difference
• Temperature measure of heat energy content
  ability to transfer heat
• Thermometer
• Higher thermal energy, greater motion of
  constituents
• Sum of individual energies of constituents
  total thermal energy

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Systems and Surroundings

• System the object in question
• Surrounding(s) everything outside the system
• When both system and surrounding at same
  temperature ? thermal equilibrium
• When not
• Heat transfer to surrounding exothermic
• (you feel the heat) ? hot metal!
• Heat transfer to system endothermic
• (you feel cold) ? cold metal!

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Math!

• Joules (J) used for energy quantities
• But usually kJ (1000 J) used
• Ye Royal Olde School used calorie (cal)
• cal amt of heat required to raise the
  temperature of 1.00 g of water by 1?C
• 1 cal 4.184 J (SI-unit)
• ButCalorie (Cal) 1000 cal
• Used in nutrition science and on food labels

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Heat Capacity

• Specific heat capacity
• Quantity of heat required to raise the temp of 1
  gram of any substance by 1 K

• Molar heat capacity
• Quantity of heat required to raise the temp of 1
  mole of any substance by 1 K

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Calculating heat transfer

• FYI
• Specific heat capacity of metals is very low
• ? lt 1.000 J/(g?K)
• What does this tell us about heat transfer in
  metals?

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Lets do an example

• In your backyard, you have a swimming pool that
  contains 5.19 x 103 kg of water. How many kJ are
  required to raise the temperature of this water
  from 7.2 C to 25.0 C?

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Example solved

• Trick ? T in K ?T in C

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Practice

• How many kJ are required to raise the temperature
  of 25.8 g of quicksilver from 22.5 C to 28.0 C?
  CHg 0.1395 J/(g?K)

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But what if theres a change of state?

• Temperature constant throughout change of state
• Added energy overcomes inter-molecular forces

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Change of state

• What do the flat areas represent?

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• qtot qs qs?l ql ql?g qg
• qs?l heat of fusion
• Heat required to convert solid at melting pt. to
  liq
• Ice 333 J/g
• ql?g heat of vaporization
• Heat required to convert liq. at boiling pt. to
  gas
• Water 2256 J/g

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Practice

• How much heat is required to vaporize 250.0 g of
  ice at -25.0 C to 110.0 C?
• Given
• Specific heat capacity of ice 2.06 J/g?K
• Specific heat capacity of water 4.184 J/g?K
• Specific heat capacity of steam 1.92 J/g?K
• Lets do this

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Calorimetry

• The process of measuring heat transfer in
  chemical/physical process
• qrxn qsoln 0
• qrxn -qsoln
• Rxn system
• Soln surrounding
• What youll do in lab
• Heat given off by rxn
• Measured by thermometer
• Figure out qrxn indirectly

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Enthalpy

• heat content at constant pressure
• If ?H , process endothermic
• If ?H -, process exothermic
• Enthalpy change dependent on states of matter and
  molar quantities
• For example
• Is vaporizing ice an exothermic or endothermic
  process?
• Thus, will ?H be or -?

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Hesss Law

• If a rxn is the sum of 2 or more other reactions,
  ?H sum of ?Hs for those rxns
• So, ?Htot ?H1 ?H2 ?H3 ?Hn

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Lets solve a problem

• C(s) 2S(s) ? CS2(l) ?H ?
• Given
• C(s) O2(g) ? CO2(g) ?H -393.5 kJ/mol
• S(s) O2(g) ? SO2(g) ?H -296.8 kJ/mol
• CS2(l) 3O2(g) ? CO2(g) 2SO2(g) ?H -1103.9
  kJ/mol
• How do we manipulate the 3 rxns to achieve the
  necessary net rxn?
• Does ?H change if the rxns are reversed and/or
  their mole ratios are changed?
• Lets talk about this on the next slide

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Lets work it out

• 1. Switch this rxn CS2(l) 3O2(g) ? CO2(g)
  2SO2(g) ?H -1103.9 kJ
• Thus, CO2(g) 2SO2(g) ? CS2(l) 3O2(g) ?H
  1103.9 kJ
• Thus, -?Hfwd ?Hrev
• 2. Double this rxn S(s) O2(g) ? SO2(g) ?H
  -296.8 kJ
• Thus, 2S(s) 2O2(g) ? 2SO2(g) ?H (-296.8 kJ)
  x 2 -593.6 kJ
• Since ?H is per mole, changing the stoichiometric
  ratios entails an equivalent change in ?H
• 3. Keep this rxn C(s) O2(g) ? CO2(g) ?H
  -393.5 kJ
• 4. Add those on same side of rxns/eliminate those
  on opposite sides of rxn
• CO2, 2SO2, 3O2
• 5. Net rxn C(s) 2S(s) ? CS2(l)
• ?H 1103.9 kJ - 593.6 kJ 393.5 kJ 116.8 kJ
• Is it an exo- or endothermic rxn?

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Practice

• Given
• CH4(g) ? C(s) 2H2(g) ?H 74.6 kJ/mol
• C(s) O2(g) ? CO2(g) ?H -393.5 kJ/mol
• H2(g) O2(g) ? H2O(g) ? -241.8 kJ/mol
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g) ?Hrxn ?

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Standard Energies of Formation

• Standard molar enthalpies of formation ?Hf?
  enthalpy change for formation of 1 mol of cmpd
  directly from component elements in standard
  states
• Standard state most stable form of substance in
  physical state that exists _at_ 1 bar pressure a
  specific temp., usually 25?C (298K)
• 1 bar 100kPa
• 101.325 kPa 1 atm
• So 1 bar ? 1 atm (an SI unit)
• Example
• C(s) O2(g) ? CO2(g) ?H ?Hf? -393.5 kJ
• ?Hf? 0 for elements in standard state

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Enthalpy Change for a Rxn

• Must know all std molar enthalpies
• ?H?rxn ?Hf?prods - ?Hf?reactants
• Given to you in a table in the back of the book
• Again, keep in mind the mole ratios for each
  species involved!

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Example

• ?H?rxn for 10.0 g of nitroglycerin?
• 2C3H5(NO3)3(l) ? 3N2(g) ½O2(g) 6CO2(g)
  5H2O(g)
• C3H5(NO3)3(l) -364 kJ/mol
• CO2(g) -393.5 kJ/mol
• H2O(g) -241.8 kJ/mol

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Solution
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Practice

• Determine ?Hrxn for
• 4NH3(g) 5O2(g) ? 4NO(g) 6H2O(g)
• Given
• NH3(g) -45.9 kJ/mol
• NO(g) 91.3
• H2O(g) -241.8