Energy and Thermochemistry

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Energy and Thermochemistry
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Energy

• The ability to do work
• 4 Types
• Kinetic Energy in motion
• Potential Stored energy
• Chemical potential energy of compounds and
  molecules that can be released as the result of a
  chemical reaction
• Thermal energy of an object due to the random
  motions of its particles

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Thermochemistry

• Changes of heat content and heat transfer
• Follow the Law of Conservation of Energy
• Or, 1st Law of Thermodynamics
• Energy can neither be created nor destroyed only
  transferred
• Heat will always move from HIGH to LOW (never
  reverse)

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Temperature Heat

• Heat not the same as Temperature
• Heat energy transferred to one system by
  another due to temperature difference
• Temperature measurement of heat energy
• Thermometer
• Higher thermal energy, greater motion of
  particles

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System and Surroundings

• System the object in question
• Surrounding(s) everything outside the system
• When both the system and the surroundings equal
  the same temperature it is called Thermal
  equilibrium
• When not equal
• 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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Specific Heat of one object

• Q Cp x m x ?T
• Q heat/energy measured in Joules (J)
• Cp specific heat measured in J/g x 0C
• m mass measured in grams
• ?T change in temperature measured in (0C) (must
  be positive)

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Example

• It takes 487.5 J to heat 25 grams of a substance
  from 25 C to 75 C. What is the specific heat in
  Joules/gC?

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• How many joules of energy must be transferred to
  change the temperature of a piece of iron from 50
  0C to 150C? The sample contains 475 g.

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• To what temperature will a 50.0 g piece of glass
  raise if it absorbs 5275 joules of heat and its
  specific heat capacity is 0.50 J/gC? The
  initial temperature of the glass is 20.0C.

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• If a sample of chloroform is initially at 25C,
  what is its final temperature if 150.0 g of
  chloroform absorbs 1.0 kilojoules of heat, and
  the specific heat of chloroform is 0.96 J/gC?

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Specific heat and Phase Changes
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35 g of water _at_ 180C ? 1500C
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65 g of solid ethanol _at_ -1170C ? 900C
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13 g benzene _at_ -30C ? 250C
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What is the symbol?

• H Enthalpy

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Enthalpy

• Definition amount of heat energy absorbed or
  lost by a system during a chemical reaction

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DH rxn SnH products SnH reactants

• S Sum of. Use Appendix K for the H0f for
  different compounds and elements.
• n number of moles
• If DH0rxn is a negative value the reaction is
  Exothermic
• If DH0rxn is a positive value the reaction is
  Endothermic

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Endothermic vs Exothermic
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Practice

• Na(s) Cl2(g) ? NaCl(s)
• Na(s) H2O(l) ? NaOH(aq) H2(g)

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Entropy and Gibbs free energy
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Entropy

• 2nd Law of Thermodynamics (Entropy)
• Entropy (S) a measurement of the randomness (or
  chaos) of particles in a system
• Systems have an overall tendency to increase the
  entropy

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3rd Law of Thermodynamics

• The entropy (chaos/disorder) of a pure, perfect
  crystalline substance is zero at absolute zero.
• Will entropy ever be negative? Why?
• DSsystem SnSproducts - SnSreactants

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Things that effect Entropy

• Changing from solid to liquid to gas always gives
  you an increase entropy.
• Changing from gas to liquid to solid always gives
  you a decrease entropy.
• As temperature goes up entropy goes up
• As volume of a gas goes up, entropy goes up
• As substances are mixed, entropy increases
• Increasing the number of particles entropy
  increases
• Increasing the number of moles of gas, entropy
  increases

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• 2H2O(g) ? 2H2(g) O2(g)
• CaCO3(s) ? CaO(s) CO2(g)
• NH4Cl(s) ? NH3(g) HCl(g)
• O2(g) ? O3(g)

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Gibbs Free Energy

• Ability to do work
• DGsystem SnGproducts SnGreactants
• If DG is positive nonspontaneous
• If DG is negative spontaneous

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What do the signs mean?
Property Positive Negative
DH
DS
DG
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Quick Review
Enthalpy Entropy Gibbs Free Energy
Define
Symbol
Formula
Unit
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What if it isnt at 250C?

• DG DH TDS
• Temperature must be in Kelvin
• Make sure you convert your S to kilojoules (kJ)

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N2(g) O2(g) ? NO(g)_at_ 1250C
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O2(g) ? O3(g) _at_ 400C
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C2H5OH(l) O2(g) ? CO2(g) H2O(l) _at_ 1250C
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Phase Change and Temperature of Spontaneity
DH DS

• Can be used to determine the boiling point of an
  unknown.

T
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HNO3(l) ? HNO3(g)
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S(s) ? S(g)
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Signs and Result of DG
DG DH DS Spontaneous?




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NH3(g) HCl(g) ? NH4Cl(s)
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CCl4(l) ? C(graphite) Cl2(g)