CSUMB
ESSP 311L
Dr. R. Rinehart

Melting Points

Basic reference: Pavia, Lampman, Kriz, and Engel, Introduction to Organic Laboratory Techniques: A Microscale Approach, 3/e, Saunders, 1999  pp 576-584

Melting Points of Urea, Cinnamic Acid,
and Mixtures of the Two Compounds

Purposes of this experiment:

1)  To familiarize you with the techniques of melting point determination and the use of the melting point apparatus.
2)  To acquaint you with the theory of melting points and to demonstrate that the presence of impurities in a substance lowers the observed melting point of that substance.
3)  To introduce you to the use of the chemical literature in obtaining physical and chemical data on compounds.
4)  To give you some practice in performing chemical calculations and plotting graphs by hand.

Procedure:

1)  Obtain samples of five of the following, including the pure substances, and place into melting point capillaries (or, if using the Fisher-Johns mp apparatus, place between glass cover slips):
NOTE: percent compositions given are on the basis of mass (% w/w)

    urea
    cinnamic acid
    10% urea-90% cinnamic acid
    20% urea-80% cinnamic acid
    40% urea-60% cinnamic acid
    50% urea-50% cinnamic acid
    60% urea-40% cinnamic acid
    80% urea-20% cinnamic acid
    90% urea-10% cinnamic acid

2)  Determine and record the melting ranges of your five samples. The melting range begins at the temperature where the first drop of actual liquid is observed and ends at the first temperature at which the entire sample has liquefied. 

3) Dispose of samples as directed by the instructor or the lab tech. Make sure that the mp apparatus is turned off when you are finished with it.

Calculations:

1)  Convert (% w/w) to mole fraction for each sample. The mole fraction c_i is defined as (mol of i)/(total moles). The mole fraction for a pure substance is 1.000... [duh!].  For a two-component mixture i + j, c_i + c_j = 1.00
    Example:  for simplicity, assume you have 100 g of the mixture, let's say 30% urea-70% cinnamic acid.
            mol urea = (30 g urea)/(m.w. urea)
            mol cinnamic acid = (70 g cinnamic acid)/(m.w. of cinnamic acid)
            total  moles =  mol urea +  mol cinnamic acid
            c_urea = (mol urea)/(total moles)
            c_{cinn. acid}  = (mol cinnamic acid)/(total moles) = 1.00 - c_urea

    Each mixture will have a different value for total moles present, so don't get lazy with the calculations!

Graph:

You are now ready to make a graph along the lines of the one on p. 577 of Pavia et al for inclusion with your report.

Report:

Follow the suggested format. For this particular experiment, the "Overall Reaction" and "Mechanism" sections are clearly irrelevant and hence should not be present. Report due no later than 9/13/02.

© Ronald W. Rinehart, 2002