Mr. Bouyer
Day 1 | Day 2 - 3 | Day 4 | Lab 1 | Lab 2 | Skills Test

Describe melting and boiling using vapor pressure. Give examples of dynamic equilibrium using the physical states of matter. Apply Le Chatelier's Principle to systems at equilibrium. Describe molecular attraction at the surface of liquids.

Important concepts:

Lab #1
Melting - Commonly used to indicate changing from solid to liquid.

• Normal melting point - The temperature at which the vapor pressure of the solid and the vapor pressure of the liquid are equal.

• Melting and freezing occur at the same temperature.

• Normal boiling point - The temperature at which the vapor pressure of the liquid is equal to standard atmospheric pressure, which is 101.325 kPa.
• Boiling point is a function of pressure.
• At lower pressures, the boiling point is lower.

• Random molecules of a liquid acquire enough energy to escape into the atmosphere because of particle collisions near the surface of the liquid.
• Note that evaporation takes place only at the surface of a liquid while boiling takes place throughout the body of a liquid.

• The best known example is "dry ice", solid CO₂.

Volatile - A liquid that readily evaporates at room temperature.

Equilibrium - When there is no net change in a system.

Dynamic equilibrium -

When a vapor is in equilibrium with its liquid as one molecule leaves the liquid to become a vapor, another molecule leaves the vapor to become a liquid. In other words, an equal number of molecules will be found moving in both directions.

How substances behave when changing phase is determined by intermolecular forces. Here are two rules of thumb: Substances with low vapor pressures have strong intermolecular forces. Those with high vapor pressures have weak intermolecular forces. Ionic compounds do not exert a significant vapor pressure because the interionic forces are too strong to be overcome. Substances with weak intermolecular forces have lower melting points than substances with strong intermolecular forces.

Calculating the energy changes during phase change:  (chem lab 244)

Symbols used in heat calculations:

• - The greek letter "delta" represents change in.
• H - Represents enthalpy, which we will think of as heat.
• q - Represents heat gained or lost.
• m - Represents mass.
• T - Represents temperature.
• C_p - Represents specific heat.

Enthalpy of fusion:

As energy is added to a solid at its melting point, all the energy is used to increase the kinetic energy of the molecules during the phase change. Because of this, the temperature of the melting system remains constant until all the solid has become liquid. If energy is still being added to the system, the temperature will begin to climb when all the solid becomes liquid.

Enthalpy of vaporization:

As energy is added to a liquid at its boiling point, all the energy is used to increase the kinetic energy of the molecules during the phase change. Because of this, the temperature of the boiling system remains constant until all the liquid has become vapor. If energy is still being added to the system, the temperature will begin to climb when all the liquid has been vaporized.

Heat transfer:

The law of conservation of energy tells us that heat lost by one quantity of matter must be gained by another. As a quantity of matter gains heat energy, its temperature will increase based on its individual specific heat. Specific heat is defined as: the heat required to raise the temperature of one gram of a substance by one Celsius degree.

Sample Heat Calculation Problem: How much heat is needed to convert 250g of ice at -30 oC to steam at 150 oC? This problem requires six individual calculations. There are three in which the phase does not change (yellow), two in which the phase changes (green), and a final calculation to solve the problem. (pink) 1. Raise the temperature of ice from -30 oC to 0 oC - no phase change q = m DT Cp = (250g) (30 Co) (2.06 J/g . Co) = 15,450 J 2. Melt the ice - phase change occurs - temperature does not change q = m DHfus = (250g) (334 J/g) = 83,500 J 3. Raise the temperature of liquid water from 0 oC to 100 oC - no phase change q = m DT Cp = (250g) (100 Co) (4.18 J/g . Co) = 104,500 J 4. Vaporize the liquid water - phase change occurs - temperature does not change q = m DHvap = (250g) (2260 J/g) = 565,000 J 5. Raise the temperature of the water vapor from 100 oC to 150 oC - no phase change q = m DT Cp = (250g) (50 Co) (2.02 J/g . Co) = 25,250 J 6. Find the total heat needed for the conversion - Add the q's from steps 1 - 5.       Total q = 793,700 J

Lab #2

Homework Assignment 201:
This assignment must be turned in by the beginning of class tomorrow to receive credit.
Scoring criteria

1. Find the theoretical freezing point and boiling point temperatures of CO₂.
2. Work the following heat calculation practice problems:
   1. How much heat is needed to raise the temperature of a 50g piece of copper metal 10 C^o?
   2. How much heat is needed to change 100g of lead from a solid to a liquid?
   3. How much heat must be removed to lower the temperature of one pound of water from 70 ^oC to 5 ^oC? How many BTU's is that?

Day 2 - 3

• Read about Le Chatelier‘s Principle:
  "If stress is applied to a system at equilibrium,
  the system will tend to readjust so that the stress is reduced"

• Surface Tension - The apparent elasticity of the surface of a liquid.
• Capillary Rise - The rise of a liquid in a tube of small diameter.
• Hydrogen Bonding - When hydrogen forms a covalent bond with a highly electronegative atom, that atom takes almost complete possession of the shared electon pair. This causes hydrogen to have a strong partial positive charge, making the molecule highly "polar". This force is peculiar to hydrogen, since it has no inner levels of electrons shelding the nucleus. Hydrogen bonding helps explain the properties of water, surface tension, and capillary rise.

• Learn why snowflakes are symmetrical.
• Read about Earth's water supply.

In-class Assignment 203:
This assignment must be turned in by the end of class tomorrow to receive credit.

1. Use this planning guide to write an essay about the formation of snowflakes.

Day 4 - Phase diagrams

Research Links:

• Ban Dihydrogen Monoxide! It's a Killer !
• Hydrogen Bonding - University of Idaho
• Ice and its different forms - California Institute of Technology
• Snow Crystals - Cal Tech
• Ice XI and Ice XII - The American Scientist

Chemistry Class X

X

X

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1. q = m T C_p = (50g) (10 C^o) ( .38 J/g . C^o) = 190 J
2. q = m H_fus = (100g) (23 J/g) = 2300 J