Ch. 10.4 and 11 Thermodynamics

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Ch. 10.4 and 11Thermodynamics

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Energy

• Definition the capacity to do work, or to
  produce heat
• SI (metric) unit is Joule
• Non-SI unit is calorie
• 1 cal 4.18 J
• 1 calorie the amount of energy needed to raise
  1 gram of water 1 C
• Heat - energy that is transferred from a warm
  object to a cooler object represented by q
• Temperature - a measure of the average kinetic
  energy of an object

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Specific heat/Change of Temp

• Quantity of heat needed to raise 1 gram
• of a substance 1 C unit is J/gC
• FORMULA q c x m x ?T
• q heat (J)
• c specific heat (J/gC)
• m mass (g)
• T change in temp. (C)

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Specific Heat Practice Problem 1

• Calculate the amount of heat in joules needed to
  warm 250. g of water from 25.0C to 95.0C.
  (c4.184 J/C ?g)

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Practice Problem 2

• How much heat is lost when 50.0 grams of Al is
  cooled from 130.0 C to 62.0 C? The specific
  heat of Al is 0.897 J/gC

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Change of state and heat

• heat of fusion- amount of heat needed to melt 1
  gram of a substance at its melting point
• Hf copper 205 J/g
• Hf water 80 cal/g 334 J/g
• q mHf
• heat of vaporization- amount of heat needed to
  boil 1 gram of a substance at its boiling point
• Hv water 540 cal/g 2260 J/g
• q mHv

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Heating/Cooling Curves
Change of Temperature
Change of Phase
Know which formula to use when!
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Change of state Practice Problems

1. Calculate the amount of heat, in Joules, needed
   to melt 70.0g of copper at its melting point.
2. Calculate the heat required, in calories, to
   change 250g of water at 100C to steam at 100C.
3. Calculate the amount of heat needed to change 20g
   of ice at -10.0C to water at 80.0C.

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Enthalpy (H)

• a measure of heat content of a system
• ?H change in heat content that accompanies a
  process
• ?Hrxn Hfinal - Hinitial
• ?Hrxn Hproducts - Hreactants
• ?Hrxn can also be written as ?Hf, for heat of
  formation
• Chemical systems in the world tend to achieve
  the lowest possible energy. Would this occur in
  an exothermic or an endothermic reaction?

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Exothermic reactions

• chemicals react and give off heat (feel hot) ?H
  is negative products are more stable
• 4Fe 3O2 ? 2Fe2O3 1625 kJ

reactants
products
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Endothermic reactions

• chemicals need to absorb energy in order for the
  reaction to take place (feel cool) ?H is
  positive reactants are more stable
• 27 kJ NH4NO3 ? NH4 NO3-

products
reactants
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Practice problems 1

• CO (g) NO (g) ? CO2 (g) N2 (g)

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Practice 2

• CH4 (g) O2 (g) ? CO2 (g) H2O (g)

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Practice 3

• N2 (g) O2 (g) ? NO2 (g)

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Entropy (S)

• Measure of disorder or chaos in a system
• Law of disorder states that things move
  spontaneously in the direction of maximum chaos
  or disorder
• ?Ssystem SP - SR
• If ?S is , there is an increase in entropy.
• If ?S is -, there is a decrease in entropy

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Rules for disorder

• 1. Entropy increases as particles move apart.
• Gas gt Liquid gt Solid
• I2 (s) ? I2 (g)
• 2. Entropy increases when you divide a substance
  into parts (when the total number of products gt
  the total number of reactants)
• 2H2O ? 2H2 O2
• 3. Entropy increases as temp increases b/c
  particles move faster. (unit for entropy is J/K
  mole)
• 4. Entropy increases when you dissolve a solid
  into a liquid. Entropy decreases when you
  dissolve a gas into a liquid.

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Entropy Examples

• Water (liquid) -? water (solid)
• KCl(s) ? KCl (l)
• C(s) O2 (g) ? CO2 (g)

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Gibbs Free Energy (G)

• Energy available to do work
• Relates enthalpy (H) and entropy (S), using the
  equation
• ?G ?H T?S
• ?G GP - GR

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Still confused?

• Try these online notes
• http//www.sciencegeek.net/Chemistry/Powerpoint/Un
  it7/Unit7_files/frame.htm

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Spontaneous Reactions

• If ?G is negative, the reaction will occur
  spontaneously.
• If ?G is positive, the reaction will NOT occur
  spontaneously.

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Quick Spontaneous Chart
?H - -
?S - -
?G - Depends on temp Depends on temp
spontaneous Non- spontaneous