Thermodynamics

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Thermodynamics
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Every physical/chemical change is accompanied by
change in energy

• 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 (heat energy)
• entropy (chaos/randomness)

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Enthalpy, H

• enthalpy heat content of system at constant
  pressure
• use 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
• result ?H is positive

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net loss in energy

• Exothermic Process energy released
• Hfinal ? Hinitial
• so Hfinal Hinitial
• result ?H is negative
• see footnote to table I

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

• choose how measure energy change
• depends on how set up experiment
• monitor the system
• monitor the environment (this is easier)

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Energy lost Energy gained

• how do you know energy has moved?
• can measure energy gained/lost by environment
• equals energy lost/gained by system

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• reaction is carried out in water in styrofoam
  cup
• - cup is the universe!
• temperature of water is monitored
• - water is the environment!

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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 types of ?Hs

• ?H of dissolving heat of solution
• ?H of phase change
• heat of fusion/heat of vaporization
• ?H of reaction heat of reaction
• categorized by rxn type

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Table I Heats of Reaction

• rxns 1-6 combustion rxns
  ?H heat of combustion
• rxns 7-18 formation reactions (synthesis)
  
• ?H heat of formation
• rxns 19-24 dissolving equations
• ?H heat of solution

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energy depends on amount

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

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

• 1 mole of methane 2 moles 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

• CH4(g) 2O2(g) ? CO2(g) 2H2O (l) ?H
  -890.4 kJ
• burn 2 moles of CH4(g) with 4 moles of O2(g), get
  2 times as much energy out
• remember stoichiometry!
• (2)(890.4 kJ) 1780.8 kJ is released

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Phase Change Energy depends on direction
endothermic
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
• If look at reverse reaction, then need to reverse
  sign of ?H
• 2NH3(g) ? N2(g) 3H2(g) ?H 91.8 kJ

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Thermochemical Equations

• balanced chemical equations
• show physical state of all reactants products
• energy change can be given in 2 ways
• 1. energy term written as reactant or product
• OR
• 2. ?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) ? NH41(aq) NO3-1(aq) ?H 27 kJ
• 
  
• OR
• NH4NO3(s) 27 kJ ? NH41(aq) NO3-1(aq)

endothermic if ()
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Changes of State

• H2O(s) ? H2O(l) ?Hf 334 J/g at 0oC
• H2O(l) ? H2O(s) ?Hf -334 J/g at 0oC
• energy is absorbed when water melts
• energy is released when water freezes!
• H2O(l) ? H2O(g) ?Hv 2260 J/g at 100oC
• H2O(g) ? H2O(l) ?Hv -2260 J/g at 100oC
• energy is absorbed when water evaporates
• energy is released when water condenses!