Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

AS Intermolecular Forces

Intermolecular Forces

word document

 Pre-Lab Assignment

  1. Read the entire lab handout.
  2. For all the compounds used in this lab, make a table showing:

a)       Names
b)       chemical formulas
c)       Lewis structure
d)       Displayed formula
e)       3D structure
f)        How many polar/non-polar bonds are present in the compound?
g)       Is the molecule polar or non-polar

Experimental Questions

  1. Which types of compounds have the strongest intermolecular forces?
  2. How can certain physical properties give an indication of the strength of IMFs of compounds?

Learning goals

  1. Relate molecular polarity to the intermolecular forces of the molecule.
  2. Relate intermolecular forces of the molecule to physical properties such as evaporation rate.

Background

The forces between molecules that hold molecules together are called Intermolecular Forces (IMF)

These are:

  1. London Dispersion forces (Van der Waals’ forces) the weakest ones
  2. Dipole – Dipole forces – stronger than Van der Waals’ forces but weaker than Hydrogen Bonds.
  3. Hydrogen bonds (H-bonding). The strongest ones.

Cohesive forces: They are attractive forces between particles that hold them together. They are weak in gases and get progressively stronger as we go from liquid to solid. There are only three ways that elements are held together:

  1. Sharing of electrons as in covalent bonding. (strong bond)
  2. Positivenegative charge interaction as in ionic compounds. (strong bond)
  3. Partial charge interactions as in intermolecular forces. (weak bond)

Disruptive forces: They are forces that tend to separate molecules. There are two main types of disruptive forces:

  1. The motion of atoms or molecules (translation, rotation or vibration)
  2. The repulsion of like charges. Positive çèPositive; Negative çè Negative

Recall that the force of attraction decreases with increasing distance of separation or decreasing charge.

The temperature of an object is proportional to the average of all its kinetic energies. T ~ K.E. = ½ m v2 

This means that at the same temperature, objects with a smaller mass must be moving faster, and the faster they move, the easier is to break away from attractive forces. 


Part 1: Predicting polarity based on models

Draw the Lewis structure for the following compounds, you will make the model using the molecular kit, sketch its 3-D shape using the wedge and dash notation, look at the intramolecular bonds to see if any of them are polar and then look at the shape of the molecule to see if the molecule is polar.

You should now be able to predict the type of intermolecular forces and how the properties will vary between the molecules that you will be testing.

Compounds to be studied:

COMPOUND FORMULA LEWIS STRUCTURE 3D   STRUCTURE   POLAR / NON POLAR

BONDS

POLAR / NON POLAR

MOLECULE

WATER  

 

 

ACETONE  

 

 

ETHANOL  

 

 

HEXANE  

 

 


Analysis questions Part 1:

1)       Which substance(s) have Van der Waals’ intermolecular forces?

2)       Which substance(s) have dipole-dipole forces

3)       Which substance(s) present hydrogen bonds between their molecules?

4)       Which should be the most polar molecule present?

5)       Which substance will not mix with water?


Part 2: Comparison of Evaporation Rates 

  1. Obtain 4 pieces of filter paper or paper towel and five rubber bands or tape to attach the paper to the bulb of your thermometers.
  2. Wrap one piece of paper around each thermometer bulb and fix it with the rubber bands (or with tape)
  3. Prepare 4 test tubes in a rack and place around 2 mL of water in the first one, 2 mL of ethanol in another, 2 mL of acetone in the 3rd one and 2 mL of hexane in the last one.
  4. Fix the thermometers to a paper with tape so you can remove them together.
  5. Wait until the temperature stabilizes.
    This will be your measurement for time = 0.0 min.
  6.  Prepare your recording device. Be sure you see the temperature values at all times.
  7. As soon as you press recording and without stopping the recording process, remove the thermometers and place them on the table so the papers do not touch the table. (see the picture on the right) 
  8. Continue recording for at least 3 minutes (180 sec).
  9. After stopping the recording build up a suitable table to show your results. Use the correspondent decimal places for your readings and use units in the data table according to the measurements you will do. Watch your video and record the values of the time and temperature every 5 seconds.
  10. Make a line graph showing how the temperature of each liquid changes through time.
    • Use a correct scale.
    • Label the axis
    • Use a suitable title for your table

Analysis questions Part 2:

6)       Which substance evaporated at the highest rate?

7)       Which substance evaporated at the lowest rate?

8)       Which is the substance with stronger intermolecular forces?

9)       Why you need to wait until the temperature in all thermometers stabilize before the measurements?

10)       Order the substances by increasing order of intermolecular forces.


Part 3: Evaporation rate of different alcohols

In the same way you tested the water, alcohol, acetone and Hexane, now you will compare the rate of evaporation for different alcohols: (see your teacher for the list of alcohols available.

COMPOUND FORMULA LEWIS STRUCTURE 3D   STRUCTURE   POLAR / NON POLAR

BONDS

POLAR / NON POLAR

MOLECULE

METHANOL  

 

 

ETHANOL  

 

 

   

 

 

   

 

 

The procedures are the same as in PART 2 – but this time you will build a data table for the alcohols and graph a data table for the rate of evaporation of each one of them.

Analysis questions Part 3:

11)       In Part 1, how is the polarity of the molecule related to the intermolecular forces?

12)       Place the molecules used in both experiments in order of rate of evaporation

13)       Which intermolecular forces are present in each one?

14)       How can be the intermolecular forces related to the rate of evaporation?

15)       How can you identify which is the alcohol with the biggest IMF present?


 

You should show in your lab report:

  • PURPOSE OF THE LAB
  • HYPOTHESIS (IN AN IF THEN STATEMENT)
  • MATERIALS
  • PROCEDURES
  • LAB SETUP
  • DATA TABLES AND GRAPHS in GRAPH PAPER USING RULER!!!
  • ANALYSIS QUESTIONS