Thermochemistry Notes

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ThermochemistryNotes 3
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Energy and Change of State

• OBJECTIVES
• Describe Melting and Boiling
• Solve Latent Heat of Fusion and Latent Heat of
  Vaporization problems
• Solve change of state problems

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Heat and Change of State

• During all changes in state energy will either be
  gained or lost
• Two Changes of State -
• Melting/Freezing the phase change between solid
  and liquid states.
• Vaporization/Condensation the phase change
  between liquid and gas states.

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Heat and Change of State
Melting - As energy is gained by a solid the
particles gain kinetic energy. - The bonds within
the solid will break allowing the particles to
move around more making a liquid Freezing -
Energy is released by the liquid which slows down
the particles to a vibration
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Heat and Change of State

• - The energy involved in melting/freezing is
  called the Latent Heat of Fusion (?Hfus)
• - Water has its own value
• ?Hfus 334 J/g
• During melting/freezing there is no change in
  temp. All the energy is spent to break bonds or
  slow down particles.

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Heat and Change of State

• To calculate heat required for melting/freezing
  use the formula -
• q m ?Hfus
• m mass
• ?Hfus Latent Heat of fusion
• Changes for each substance

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Energy and Change of State
Example 1 Find how much heat is required to melt
65g of water? q m ?Hfus q (65g)(334
J/g) q 21,710 J 21,700 J 21.7 kJ
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Energy and Change of State

• Vaporization
• As energy is gained by a liquid the particles
  gain kinetic energy.
• The particles gain so much energy they break the
  water surface and change to a gas
• Condensation
• - Energy is released and the gas particles slow
  down and group together creating a liquid

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Energy and Change of State

• The energy involved in vaporization/condensation
  is called the Latent Heat of Vaporization (?Hvap)
• - Water has its own value
• ?Hvap 2260 J/g
• Again during this process all the energy is spent
  to break bonds or slow down particles.

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Heat and Change of State

• To calculate heat required for boiling, use the
  formula -
• q m ?Hvap
• m mass
• ?Hvap Latent Heat of vaporization

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Energy and Change of State
Example 2 Find how much heat is required to
boil 65g of water? q m ?Hvap q
(65g)(2260 J/g) q 146,900 J
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• Classwork
• How much Heat is required to vaporize 343 g of
  Ethanol?
• ?Hvap 38.6 kJ/g
• How much heat is evolved when 1255 g of water
  condenses to a liquid at 100C?
• A sample of ammonia liberates 5.66kJ of heat as
  it solidifies at its melting point. What is the
  mass? ?Hfus 5.66 kJ/g