Thermodynamics

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Thermodynamics
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Every physical or chemical change is accompanied
by energy change

• Thermodynamics branch of chemistry that studies
  energy changes
• Specifically changes in heat energy

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Thermodynamics

• Tells us if a reaction will occur
• 2 considerations
• Enthalpy or energy
• Entropy or chaos

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Enthalpy, H

• usually do experiments at constant pressure (1
  atm)
• Enthalpy heat content of a system at constant
  pressure
• Symbol H

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Changes in Enthalpy are measurable

• cannot measure enthalpy content of system
  directly
• can measure changes in enthalpy! Symbol ?H
• ?H Hfinal Hinitial Hproducts - Hreactants

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Net gain in energy

• Endothermic Process Energy absorbed
• Hfinal gt Hinitial
• so Hfinal Hinitial is positive
• ?H is positive

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Net loss in energy

• Exothermic Process Energy released
• Hfinal ? Hinitial
• so Hfinal Hinitial is negative
• ?H is negative
• In case you forget the sign of ?H for an
  exothermic process, look at the footnote to table
  I

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Which arrow represents an endothermic change?
An exothermic change?
Energy can move between the system and the
environment
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Change in Energy

• choices in how measure energy change
• depends on how set up experiment
• Monitor the system
• Monitor the environment
• usually easier

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Energy lost Energy gained

• What tells you that energy has moved?
• can measure energy gained or lost by environment
  
• it equals energy lost or gained by system

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The reaction is carried out in the water in the
styrofoam cup The temperature of the water is
monitored
source
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Q mC?T

• Q Energy change
• m mass of water
• C specific heat of water
• ?T temperature change Tf Ti

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Different kinds of ?Hs

• ?H on dissolving heat of solution
• ?H on phase change heat of fusion or heat of
  vaporization
• ?H on reaction heat of reaction
• Categorized by rxn type

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Look at Table I Heats of Reaction

• Rxns 1-6 combustion rxns ?H
  heat of combustion
• Rxns 7-18 formation reactions
• Substance is formed from its elements
  
• ?H heat of formation
• Rxns 19-24 dissolving equations
• ?H heat of solution

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Energy depends on amount

• Remember it takes more energy to heat up water
  in bathtub than to make a cup of tea

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CH4(g) O2(g) ? CO2(g) 2H2O (l)?H -890.4 kJ

• 1 mole of methane 1 mole of oxygen ?
• 1 mole of carbon dioxide gas 2 moles of liquid
  water

reaction is exothermic (negative sign for ?H)
890.4 kJ energy released per mole of CH4(g)
burned
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Energy depends on amount

• Burn 2 moles of CH4(g) with 2 moles of O2(g), get
  2 times as much energy out
• Stoichiometry!
• 2 x 890.4 kJ 1780.8 kJ will be released

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Phase Change Energy depends on direction
Up is endothermic
Down is exothermic
Melting/fusion boiling/ vaporization sublimatio
n
Condensation freezing deposition
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Reactions Energy depends on direction too!

• N2(g) 3H2(g) ? 2NH3(g) ?H -91.8 kJ
• 2NH3(g) ? N2(g) 3H2(g) ?H 91.8 kJ
• If reverse equation, reverse sign of ?H

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Hesss Law

• Can add 2 or more equations by adding the ?Hs
• Enables you to calculate ?H for of rxns
• Say youre interested in
• 2S(s) 3O2(g) ? 2SO3(g)

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2S(s) 3O2(g) ? 2SO3(g)

• Have ?Hs for the following
• a) S(s) O2(g) ? SO2(g) ?H -297 kJ
• b) 2SO3(g) ? 2SO2(g) O2(g) ?H 198 kJ
• Multiply (a) by 2 and reverse (b)

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Adding

• 2S(s) 2O2(g) ? 2SO2(g) ?H -594 kJ
• 2SO2(g) O2(g) ? 2SO3(g) ?H -198 kJ
• 2S(s) 3O2(g) ? 2SO3(g) ?H -792 kJ

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Thermochemical Equations

• balanced chemical equation
• shows physical state of all reactants products
• gives energy change (2 ways)
• energy term can be written as reactant or product
• OR
• ?H is given right after equation

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Exothermic Rxn energy product

• 4Fe(s) 3O2(g) ? 2Fe2O3(s) ?H -1625 kJ
• Or
• 4Fe(s) 3O2(g) ? 2Fe2O3(s) 1625 kJ

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Endothermic Rxn energy reactant

• NH4NO3(s) ? NH4(aq) NO3-(aq) ?H 27 kJ
• 
  
• Or
• NH4NO3(s) 27 kJ ? NH4(aq) NO3-(aq)

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Changes of State

• H2O(s) ? H2O(l) ?Hfusion 333.6 J/g at
  0oC
• H2O(l) ? H2O(s) ?H -333.6 J/g at 0oC
• Energy is released when water freezes!
• H2O(l) ? H2O(g) ?Hvapor 2260 J/g at 100oC
• H2O(g) ? H2O(l) ?H -2260 J/g at 100oC
• Energy is released when water condenses!