LAB #9:  INTERMOLECULAR FORCES AND EVAPORATION




SAFETY PRECAUTIONS:
Handle concentrated HCI in the fume hood only, wear gloves and safety goggles (as always!). Rinse any spills immediately.


Introduction and Background Information


In this experiment, we will determine what type of molecule has stronger intermolecular force between molecules by measuring evaporation. Evaporation is an endothermic process that results in a temperature decrease. For a molecule to break free from the attractive forces of a liquid state (and evaporate) a molecule must absorb a certain amount of energy from its surroundings.

One major attractive force which “holds” a molecule in the liquid state is a hydrogen bond. In general, hydrogen bonds form between molecules that contain N, O, F, and/or H. The mass of a molecule can act as a dispersal force. Lighter molecules are not subjected to attractive forces as heavier molecules are. The amount of a temperature decrease is a sign of the strength of intermolecular forces of attraction in a molecule. The greater the temperature decrease the more energy which is required to allow a molecule to break free from the liquid state.


You will encounter two types of organic compounds in this experiment—alkanes and alcohols. Alkanes are a chain of carbons with hydrogen atoms bonded to the carbon backbone. Alcohols, in addition to carbon and hydrogen atoms, contain the -OH functional group. Methanol, CH3OH, and ethanol, C2H5OH, are two of the alcohols that we will use in this experiment.

You will examine the molecular structure of alkanes and alcohols for the presence and relative strength of two intermolecular forces—hydrogen bonding and dispersion forces due to mass of a molecule. You will study temperature changes caused by the evaporation of several liquids and relate the temperature changes to the strength of intermolecular forces of attraction. You will use the results to predict, and then measure, the temperature change for several other liquids.



Substances to be tested

hexane  C6H14


heptane  C7H16


methanol   CH3OH


ethanol   C2H5OH


propanol    C3H7OH


butanol      C4H9OH


pentanol     C5H11OH


acetone   C3H6O


other substances

pentane


ethylene glycol



Experimental Set-up


Molecular Workbench - Java-based applications produced by the Concord Consortium  - to access click on the link, then scroll down at the bottom of the "Selected Curriculum Modules" section click "More" and then in the chemistry column select "Intermolecular attractions"




Procedure 

View the procedure handout attached at the bottom of this page.  Please note that we will not look at the kinetics of crystal violet, but of another, very similar molecule. The principles operate similarly to the discussion above. Apply what you learned about crystal violet to the molecules and reactions under investigation.