Water and Changes of State

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Water and Changes of State

• Chapter 14.1

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Water

• Water is the most common liquid.
• Approximately 97 of the earths water is found
  in oceans.
• Uses of water
• Sustains reactions within our bodies.
• Cools auto engines and nuclear power plants.
• Helps in the growth of many creatures.

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Pure Water

• Colorless and Tasteless
• At 1 atm
• Forms a solid at O?C
• Forms a gas at IOO?C
• The liquid range of water is O?C to IOO?C

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Figure 14.7 The heating/cooling curve for water
heated or cooled at a constant rate.
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What does the heating/cooling curve show?

• The motions of the water molecules increase as it
  is heated.
• When the temperature reaches IOO?C, bubbles
  develop and float to the surface.
• When liquid water is cooled, the liquid begins to
  freeze.
• During boiling and freezing, temperature stays
  constant.

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Water is Unique

• When water freezes, it expands.
• Accounts for bursting of pipes and engine blocks
  that are left unprotected in freezing weather.
• The expansion of water is why ice cubes float.
  (Also why ice floats on the surface of lakes
  allowing life under water during winter.)
• Density of water 1.00g/mL
• Density of ice 0.917 g/mL

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Energy Requirements for Changes in State
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Physical Changes

• A change from solid to liquid or liquid to gas
  are physical changes.
• No chemical bonds are broken when going from ice
  to water to steam.

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It takes Energy!

• It takes energy to melt ice and vaporize water.
• Even though the temperature doesnt change,
  energy is being used to move the molecules apart.
• It would take much more energy to break apart the
  covalent bonds.

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Molar Heats

• Molar heat of fusion energy required to melt
  one mole of a substance
• For ice 6.02 kJ/mol
• Molar heat of vaporization energy required to
  change one mole of a substance to its vapor
• For water 40.6 kJ/mol

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Calculating Energy Changes Solid to Liquid

• Calculate the energy required to melt 8.5 g of
  ice at O?C. The molar heat of fusion for ice is
  6.02 kJ/mol.
• Change grams into moles.
• Change moles into kJ.

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Calculating Energy Changes Liquid to Gas

• Calculate the energy (in kJ) required to heat 25
  g of liquid water from 25?C to IOO?C and change
  it to steam at IOO?C. The specific heat capacity
  of liquid water is 4.184 J/g?C, and the molar
  heat of vaporization is 40.6 kJ/mol.
• First use Q s x m x ?T during temperature
  change.
• 7.8 kJ
• Use molar heat of vaporizaton.
• 56.4kJ
• Add.
• 64.2kJ

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Transition Page
14
Elements Page