Intermolecular Forces

1
Water and the Changes of State
The energy required to heat (or cool) a solid (or
heat/cool a liquid or a gas) can be calculated
using q msDT. It requires additional energy
to change states. The energy required to convert
a specific amount of the solid to a liquid is
known as the heat of fusion (q DHfus) and the
energy required to convert a specific amount of a
liquid to a gas is the heat of vaporization (q
DHvap). The total amount of energy can be
calculated from qT q1 q2 q3...
Heating curve for water
Temperature oC
2
Water and the Changes of State
Q. How many kilojoules of energy are needed to
change 15.0 g of ice at -5.00oC to steam at 125.0
oC? The first step is to design a pathway q1
msDT for ice from -5.0 to 0.0 oC, the specific
heat of ice is 4.213 J/g oC q2 DHfus for ice to
liquid at 0.0oC q3 msDT for liquid 0.0oC to
100.0 oC q4 DHvap for liquid to steam at
100.0oC q5 msDT for steam 100.0 to 125.0 oC
the specific heat of steam is 1.900 J/g oC so qT
q1 q2 q3 q4 q5 The next step is to
calculate each q q1 (15.0 g) (4.213 J/g oC)
(0.0 - (-5.0) oC) 316 J q2 (335 J / g) (15.0
g) 5025 J q3 (15.0 g) (4.184 J/g oC) (100.0 -
(0.0) oC) 6276 J q4 (2260 J / g) (15.0 g)
33900 J q5 (15.0 g) (1.900 J/g oC) (110 - 100
oC) 285 J qT 316 J 5025 J 6276 J 33900
J 285 J 45.8 kJ
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Energy Change of State

• SI unit of energy Joule (J)
• The energy required to vaporize a substance, q,
  is proportional to the amount of substance. This
  energy is called the heat of vaporization, D
  Hvap. The process is endothermic, therefore D
  Hvap is positive.
• D Hvap q/m ? q D Hvap x m
• When vapor condenses into a liquid, the reverse
  energy change, heat energy is removed from the
  substance. Known as the heat of condensation.
  This process is exothermic, therefore D Hcon is
  negative.

4
Energy Change of State

• To melt a solid, energy must be added to overcome
  the forces that hold the crystal structure
  together. This is known as the heat of fusion, D
  Hfus. The process is endothermic, therefore D
  Hfus is positive.
• D Hfus q/m ? q D Hfus x m
• D Hfus of a substance is the energy required to
  melt one gram of that substance.
• The heat of solidification is the reverse of heat
  of fusion and is exothermic and D Hsol is
  negative.

5
Energy Change of Temperature Specific Heat

• The specific heat is the heat flow required to
  change the temperature of one gram of a substance
  by one degree Celsius, c.
• q m x c x DT
• Specific heat of water 4.18 J/g C
• 1 cal/g
  C
• 1 cal 4.184 J

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Change in Temperature plus Change of State

• If you were to take some ice from a freezer,
  place it in a flask and apply steady heat to it,
  five things would happen
• The ice would warm to its melting point.
• The ice would melt at the melting point.
• The water would warm to its boiling point.
• The water would boil at the boiling point.
• The steam would become hotter.

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How to calculate the total heat flow for a change
in temperature plus a change of state

• Step 1 Sketch a graph having the shape shown on
  the next slide. Be sure to mark all relevant
  starting and ending points.
• Step 2 Calculate the heat flow, q, for each
  parts of the graph between all of the starting
  and ending points.
• Step 3 Add the heat flows. Be sure the units are
  the same, either kilojoules or joules, for all
  numbers being added.

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Sample problem for a change in temperature plus a
change of state

• Problem Calculate the total heat flow when 19.6
  grams of ice, initially at -12C, is heated to
  steam at 115C.

Required information
Given m 19.6 g Ti -12C Tf 115C
Look up the following Tm 0C Tb 100ºC DHfus
335 J/g DHvap 2.26 kJ/g
10
5
4
3
2
1
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Set up for answers

• q1 m x c x DT
• q2 m x DHfus
• q3 m x c x DT
• q4 m x DHvap
• q5 m x c x DT

Answer q1 0.48 kJ q2 6.53 kJ q3 8.19
kJ q4 44.3 kJ q5 0.59 kJ Sq q1 q2
q3 q4 q5 60.1 kJ
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PRACTICE PROBLEMS 21a

• 1. Which contains less heat, ice at 0oC or
  water at 0oC? Explain your answer.
• 2. On the basis of KMT, explain why vapor
  pressure increases with temperature.
• 3. Write equations to show how the following
  metals react with water.
• a) aluminum b) calcium c)
  potassium d) iron

Ice at 0oC contains less heat than liquid water
at the same temperature. Heat must be added to
convert ice to water, so the water will contain
that much more additional heat energy. Also the
liquid state is in motion much more than the
solid state. An increase in motion can only be
accomplished by an increase in energy.
According to the kinetic molecular theory, the
vapor pressure of a liquid should increase with
temperature because of the increase in collisions
and kinetic energy that always accompanies an
increase in heat energy (temperature). KEm
3/2 RT. The increase in energy thus motion
allows the liquid molecules to escape (overcome
the surface tension and other cohesive forces
maintaining the liquid state) from the surface of
the liquid into the gas phase.
a) Al (s) 3H2O (g) ? 3H2 (g) Al2O3
requires steam b) Ca (s) 2H2O ? H2
(g) Ca(OH)2 slowly at ambient
temperature c) 2K (s) 2H2O ? H2 (g)
2KOH heat vigorous at ambient
temperature d) 3Fe (s) 4H2O (g) ? 4H2 (g)
Fe3O4 requires steam
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PRACTICE PROBLEMS 21b

• 1. Explain the physical process of boiling.
• 2. Why does ice float in water?
• 3. Why does water have a relatively high boiling
  point?
• 4. Explain if ice will float in ethyl alcohol (d
  0.789 g/L)?
• 5. How much energy is needed to change 62.74 g of
  water at 15.00oC to steam at 103.0 oC?
• 6. Magnesium carbonate, MgCO3, forms a hydrate
  containing 39.1 water of hydration. Calculate
  the formula of this hydrate.

See next slide for essay/answer
1.645 x 105 J or 3.931 x 104 cal
MgCO3 . 3 H2O
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1. Explain the physical process of boiling.

• At room temperature the water molecules have
  enough energy to allow the particles to move past
  each other but not enough to escape the surface
  tension. As the temperature of water increases,
  the heat energy (from the burner) is transferred
  to kinetic energy (for the molecules) leading to
  an increase in the molecular motion of the
  molecules. This action results in an increase in
  the vapor pressure above the surface of the
  liquid. When the vapor pressure of the water
  equals the external pressure, boiling begins.
  Now a sufficient amount of the molecules have
  enough energy to resist the attractive forces.
  Bubbles of vapor are formed throughout the liquid
  and these bubbles rise to the surface to escape.

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2. Why does ice float in water?

• Ice floats in its own liquid due to the
  intermolecular force, hydrogen bonding. As water
  freezes, the molecular motion of the molecules
  slow down and the partial positive end (hydrogen)
  of one water molecule is attracted to the partial
  negative end (oxygen) of another water molecule.
  Combine this event with the bent shape of water
  and the molecules become arranged in a 3-D
  hexagonal array. This array creates pockets of
  vacuum (empty space) in the lattice structure as
  well as a decrease in the number of molecules per
  unit volume. The mass is directly related to the
  number of molecules therefore, in the solid
  state, since there are less particles then there
  must be less mass per unit volume therefore the
  solid is less dense than the liquid.

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3. Why does water have a relatively high boiling
point?

• Water has a relatively high boiling point
  because of the amount of intermolecular forces
  present. Water experiences LDF (London
  Dispersion Forces) and d-d (dipole-dipole)
  forces, along with the additional attractive
  force, Hydrogen bonding. A large amount of heat
  energy is required to break all of these forces
  in order for a phase transition to occur, thus
  the high boiling point.

4. Explain if ice will float in ethyl alcohol (d
0.789 g/L)?
Ice would not float in pure ethyl alcohol because
the density of water is 1.000 g/mL which is
greater than 0.789 g/mL for ethyl alcohol. Yet
since ethyl alcohol also undergoes a small degree
of hydrogen bonding, the sinking effect is not as
dramatic as it would be with a nonpolar
substance.
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GROUP STUDY PROBLEMS 21

• Short Essay
• 1. Can ice be colder than 0.0oC? Justify your
  answer.
• 2. Why does a boiling liquid maintain a constant
  temperature when heat is continually being added?
• 3. Why does a lake freeze from the top down?
• Math
• 1. Suppose 50.0 g of ice at 0.0oC are added to
  285g of water at 22.0oC. Is there sufficient ice
  to lower the temperature of the system to 0.0oc
  and still have ice remaining? Show all work.
• 2. A mixture of 70.0 mL of hydrogen and 50.0 mL
  of oxygen is ignited to form water. Does any gas
  remain unreacted?
• 3. A 25.0 g sample of a hydrate of FePO4 was
  heated until all the water was driven off. The
  mass of anhydrous sample is 16.9 g. What is the
  formula of the hydrate?