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

• We 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.

• We cannot measure the enthalpy content of a
  system directly.
• We 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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Energy of universe is conserved.
Universe
Which arrow represents an endothermic change? An
exothermic change?
Environment
Energy can move between the system and the
environment.
System
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Change in Energy

• You have choices in how you measure the energy
  change. Depends on how you set up the
  experiment.
• Monitor the system.
• Monitor the environment.
• This is usually easier.

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Energy lost Energy gained

• What signal tells you that energy has moved?
• You can measure the energy gained or lost by the
  environment.
• It equals the energy lost or gained by the
  system.

Changes in temperature!
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The reaction is carried out in the water in the
styrofoam cup. The temperature of the water is
monitored.
source
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 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
• Categorize these by rxn type.

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Look at Table I Heats of Reaction

• Rxns 1-7 combustion rxns or rxn with O2
• ?H heat of combustion
• Rxns 8-18 formation reactions
• 1 mole of 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 the
  water in a 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 yields 1
  mole of carbon dioxide gas 2 moles of liquid
  water.
• The reaction is exothermic. (negative sign)
• 890.4 kJ of energy is 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
  2X 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
Gas
Melting or fusion, boiling or vaporization,
sublimation
Condensation, freezing, deposition
P.E.
Liquid
Solid
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Rxns 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 you reverse the equation, you reverse the sign
  of ?H.

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Hesss Law

• If you add 2 or more equations, then you add
  the ?Hs.
• Enables you to calculate ?H for a huge 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.
• Gives physical state of all reactants products.
• Gives the energy change. (2 ways)
• Energy term can be written as a reactant or
  product.
• ?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

Exothermic
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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!