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The effect of Inter-molecular forces

HOW DOES IT WORK?

Molecules are formed when the atoms of non-metal elements share one or more pairs of electrons. Covalent bonds are formed and are classified as intra-molecular or inter-atomic forces. The molecules can be polar or non-polar. The polarity of a molecule determines the type of inter-molecular forces that are possible between molecules of the same substance, molecules of different substances and molecules of one substance and ions of another ionic substance. The strength of inter-molecular forces has an effect of the physical properties of pure substances and mixtures of pure substances.

Experiment 1:
Time allocation: 20 min

The effect of inter-molecular forces on evaporation.

At the same temperature, substances with weak inter-molecular forces will evaporate more easily than ones with stronger inter-molecular forces.

Experiment 2:
Time allocation: 20 min

The Effect of inter-molecular forces on surface tension.

At the same temperature, substances with weak inter-molecular forces will have smaller surface tensions than liquids with stronger inter-molecular forces.

Experiment 3:
Time allocation: 20 min

The effect of inter-molecular forces on solubility.

Substances with inter-molecular force of the same strength will form homogeneous solutions.

Experiment 4:
Time allocation: 20 min

The effect of inter-molecular forces on capillary action

The stronger the inter-molecular forces between the molecules of a liquid, the less the ability to rise in a capillary tube, providing the temperature remains constant.

The stronger the inter-molecular forces between the molecules of a liquid, the higher the boiling point, providing that the atmospheric pressure remains constant.

 

EXPERIMENT 1: THE EFFECT OF INTER-MOLECULAR FORCES ON EVAPORATION.

AIM:      

Time allocation: 20 min

To determine the effect of the strength of inter-molecular forces on evaporation of liquids at the same temperature.

APPARATUS:

  • 350 cm3 tap Water
  • 20 cm3 Ethanol
  • 20 cm3 Acetone (nail polish remover) – Propan-2-one
  • 20 cm3 Methylated spirits (Mixture of 95% ethanol and 5% methanol)
  • 4 x test tubes
  • 5 x Thermometers
  • Gas Burner attached to Fuel cartridge
  • Wire Gauze with ceramic center
  • Test tube stand (6 hole)
  • 5 x Measuring cylinder 25 cm3
  • Test tube Tongs
  • 600 cm3 beaker
  • Beaker tong

METHOD:

  1. Set up the apparatus as indicated in the diagram, you will need 350 cm3 in the beaker for the water bath.
  2. Place a thermometer inside and take a reading of this water temperature.
  3. Then ignite the Bunsen burner and heat the water in the water bath to 60˚C while you prepare your test tubes, remember to keep watching it so it does not exceed this temperature.
  4. Label your test tubes A-D and place them in the stand.
  5. Add 20 cm3 of water with the measuring cylinder to test tube A and mark the level.
  6. Remember to clean the measuring cylinder before adding 20 cm3 of ethanol to test tube B and mark the level.
  7. Add 20 cm3 of Acetone (nail polish remover) – Propan-2-one to test tube C and mark the level.
  8. Add 20 cm3of Methylated spirits (Mixture of 95% ethanol and 5% methanol) to test tube D and mark the level.
  9. Once you have reached 60°C, place the test tubes inside the water bath beaker ensuring that the level of water inside the beaker is above the level of the liquid inside the test tubes adjust if needed.
  10. Note any changes in the levels of liquids in Test Tube A, B, C and D as well as the appearance of the liquids inside the test tube such as effervescence or bubbling.
  11. Ignite the Bunsen burner and heat the water in the water bath to 70˚C (NB: DO NOT EXCEED 70˚C)
  12. Ensure you use tongs to remove and mark the levels of Test Tube A, B, C and D.
  13. Measure and note the differences in the levels of Test Tube A, B, C and D at 60 0C and 70 0

EXPERIMENT 2: THE EFFECT OF INTER-MOLECULAR FORCES ON SURFACE TENSION

AIM:         

    Time allocation: 20 min

To determine the effect of the strength of inter-molecular forces on the surface tension of different liquids at the same temperature.

APPARATUS:

  • 50 cm3 Water
  • 50 cm3 Cooking / Vegetable oil
  • 50 cm3 Glycerine
  • 50 cm3 Acetone
  • 50 cm3 Methylated spirits
  • Five small 5mℓ syringes

Supplied by Educator

  • Five dry coins (5 cent coins)
  • White sheet of paper

METHOD:

  1. Set down a white piece of paper on a flat surface, place the 5 coins down with enough distance apart on the sheet of paper.
  2. Label the coins as A, B, C, D and E.
  3. Using the syringe pull up 5 cm3 of the water in the syringe.
  4. Using coin A, Slowly press the plunger down, allowing one drop of water at a time to fall onto coin.
  5. Watch the coin very carefully. The drop of water forming on top of it will gradually get larger. Stop pushing on the plunger as soon as the drop spills over the edge of the coin.
  6. Calculate the volume of liquid needed, before the droplet spills over the edge.
  7. Using Coin B, follow steps 3 to 5 using Cooking / Vegetable Oil.
  8. Calculate the volume of liquid needed, before the droplet spills over the edge.
  9. Using Coin C, follow steps 3 to 5 using Glycerine.
  10. Calculate the volume of liquid needed, before the droplet spills over the edge.
  11. Using Coin D, follow steps 3 to 5 using Acetone.
  12. Calculate the volume of liquid needed, before the droplet spills over the edge.
  13. Using Coin E, follow steps 3 to 5 using Methylated spirits
  14. Calculate the volume of liquid needed, before the droplet spills over the edge.

EXPERIMENT 3: THE EFFECT OF INTER-MOLECULAR FORCES ON SOLUBILITY

AIM:                                                                                                                                     

Time allocation: 20 min

To determine the effect of the strength of inter-molecular forces on the solubility of substances at the same temperature.

APPARATUS:

  • 50 cm3 Water
  • 50 cm3 Ethanol (CH3CH2OH)
  • 50 cm3 Chloroform (CHCℓ3)
  • 2g Iodine (I2(s))
  • 2g Potassium permanganate (KMnO4(s))
  • Four test tubes
  • 5 Pipettes

METHOD:

  1. Mark the test tubes A, B, C and D
  2. Fill test tube A, B and C with 10 cm3 of water.
  3. Use a clean pipette for each substance as below instructed.
  4. Add 5 cm3 of ethanol (CH3CH2OH) to test Tube A and test tube D
  5. Add 5 cm3 chloroform (CHCℓ3) to test Tube B, C and D.
  6. Note your observations.
  7. Now add 5 cm3 Chloroform (CHCℓ3 (ℓ)) to test tube D.
  8. Note your observations.
  9. Add 2g Potassium permanganate (KMnO4 (s)) to test tube B and note your observation.
  10. Add 2g Iodine (I2 (s)) to test tube C and note your observation 

PRECAUTION: 

DO NOT INHALE THE CHLOROFORM FUMES

 

EXPERIMENT 4: THE EFFECT OF INTERMOLECULAR FORCES ON CAPILLARY ACTION

AIM:                                                                                                                        

        Time allocation: 20 min

To determine the effect of the strength of intermolecular forces on the capillary action of liquids at the same temperature.

APPARATUS:

  • 50 cm3 Coloured water
  • 50 cm3 Coloured acetone (nail polish remover)
  • 50 cm3 Cooking / Vegetable Oil
  • 50 cm3 Glycerine
  • 50 cm3 Methylated spirits
  • Five test tubes
  • 25 cm3 Measuring cylinder
  • 5 Pieces of Perspex capillary tubing: 5 – 7 mm in diameter and 15 cm long
  • 5 x 1 Hole Rubber Stoppers

METHOD:

  1. Label the test tubes as A, B, C, D and E.
  2. Place test tubes in an upright position in a test tubes stand.
  3. Use the measuring cylinder to transfer the following:
    • 20 cm3 of Coloured water in test tube A.
    • 20 cm3 of Coloured Acetone in test tube B.
    • 20 cm3of Cooking Oil in test tube C.
    • 20 cm3 of Glycerine in test tube D.
    • 20 cm3 of Methylated Spirits in test tube E.
  4. Set the capillary tubing into the liquid.
  5. Ensure that it is upright and secure by using the 1 x hole Rubber stopper in the test tubes.
  6. Record the height of the meniscus of each liquid after 10 minutes.

Note:

Use test tube holder to handle test tubes & beaker holder to handle the beaker for All Experiments!

PRECAUTIONS:

  • Regard all chemicals as harmful – do not eat and drink in the laboratory during practical sessions.
  • Organic compounds are flammable and must be heated in a water bath.
  • Wear safety glasses and clothing and handle the burner with care.