MELTING AND BOILING POINTS

Agenda

1. Intramolecular and Intermolecular Bonding Review

2. Teacher-Led Discussion Melting Points Boiling Points

3. The Beginning of Nomenclature

Learning Goal

By the end of this class, the students will be able to: Predict the relative melting and boiling

points of different substances based on the type of intermolecular bonds this molecules form

Intramolecular Bonds – Review Questions From Yesterday

What do we remember?? 1. Compare and contrast the following:

Covalent and Ionic Bonds Ionization Energy and Electron Affinity

2. Place the following elements in order of increasing atomic size Nb, Pd, Rh, Sr, Te, Sb Be, Ba, Ra, Ca, Mg

3. List three qualities of the following groups Metals Non-Metals Metalloids

Intramolecular Forces - Review Questions Continued

4. Determine the intramolecular forces present in the following compounds (Hint: You will need to determine the ∆EN) CH4

N2Br5

CaCl2 HF SF4

5. Describe Hydrogen Bonding. What must be present for a Hydrogen Bond to occur?

6. Order the following bonds in order from strongest to weakest: Covalent Bonds, London

Dispersion Forces, Hydrogen Bonds, Ionic Bonds, Dipole-Dipole Interactions

Penny Challenge

The Challenge: In Groups of 3 to 4, you will attempt to place as many drops of water as you can on a penny, before any water spills over it. Every member of the group will make an attempt to place as many drops of water as they can to see who is the winner.

Procedure

Before anything be sure to make your predictions

1. Place a paper towel on the desk, and place a penny on top of the paper towel

2. Without touching the penny, use a water dropper to place the water on the penny

3. Count each drop of water that successfully lands and stays on the penny until the water spills over.

Discussion Questions

1. Did your actual performance exceed or not meet your prediction in your hypothesis?

2. What property of water allows it to retain such a number of water drops on a penny?

3. Knowing this property of water exists, explain why belly flops hurt when you jump into a pool of water.

What do intermolecular forces determine?

Bonds that are between 2 or more different molecules These forces help determine

the different properties of compounds

For example, the temperatures at which a compound boils or melts can be determined by the way the molecules bonds with other molecules of itself

Melting Point

Is the temperature at which a pure sample of a compound changes from a solid to a liquid at standard atmospheric pressure 101.325 kPa

The temperature at which the compound melts is determined by the strength of the attractions holding the particles of the compound together

How do Compounds Melt?

You may remember from previous science courses that compounds, even at extremely low temperatures, have at least some kinetic energy That is they are vibrating at

least a small amount They do not pull away from

surrounding particles but are moving at least a small amount

How do Compounds Melt?

As the compound is heated up and kinetic energy increases, the vibrating becomes more vigorous This continues until the

vibration gets strong enough to allow the particles to pull away from each other

Causing melting

Boiling Point

Is the temperature at which a pure sample of a compound changes from a liquid to a gas at atmospheric pressure Similar to above, but the

particles go from sliding past one another to breaking free completely from one another

So how is this related to bonding?

High Melting and Boiling Points

All ionic Due to the high electrostatic

forces between oppositely charged particles Very strong bond!

Also the way ionic compounds are structured adds to the strength of the compound Crystalline

Intermediate Melting and Boiling Points

All molecular All have 1 or more polar

bond Whole molecule or part of

molecule may be polar Held together by Dipole-

Dipole attractive forces Also a special kind of

dipole-dipole called a hydrogen bond

Low Melting and Boiling Points All molecular All non-polar Symmetrical charge Thus only held together by weak Van

Der Walls forces