experiment 2 ( benzoic acid) by skysoda

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									                   UNIVERSITI KUALA LUMPUR

                         Organic Chemistry
                            CLD 10302

                            EXPERIMENT 2


Group Member      : Muhamad Syafiq Bin Ali   (55101210311)
                    Muhamad Faris Bin Hamir (55101210280)
                    Nor Azizul Akili Adnan  (55101210137)

Section           : 2DipA
                                     EXPERIMENT 2



To purify benzoic acid by recrystallization
To determine the melting point of pure benzoic acid


The products of chemical reactions can be impure. Purification of your products must be
performed to remove by-products and impurities. Liquids are customarily purified by
distillation, while solids are purified by recrystallization (sometimes called simply
Recrystallization is a method of purifying a solid. There are two types of impurities:
those more soluble in a given solvent than the main component and those less soluble. (If
there are any impurities that have the same solubility as the main component, then a
different solvent needs to be chosen.)
When organic substances are synthesized in the laboratory or isolated from plants, they
will obviously contain impurities. Several techniques for purifying these compounds have
been developed. The most basic of these techniques for the purification of organic solids
is recrystallization, which relies on the different solubilities of solutes in a solvent.
Compounds, which are less soluble, will crystallize first. The crystallization process itself
helps in the purification because as the crystals form, they select the correct molecules,
which fit into the crystal lattice and ignore the wrong molecules. This is of course not a
perfect process, but it does increase the purity of the final product.
Organic compounds that are solids at room temperature are usually purified by
recrystallization. The general technique involves dissolving the materials to be
recrystallized in hot solvent (or solvent mixture) and cooling the solution slowly. The
solid that crystallized out from the solution is very pure material.
During the recrystallization process, solid impurities (Such as dust, filter paper etc) that
do not dissolve in hot solution are normally eliminated through filtration. The dissolved
impurities remain in cold solution while the pure compound recystallize out of the
solution . The general procedure for recrystallization is as shown in the flow chart below:

                             MATERIALS AND METHODS

     Chemicals:                                       Apparatus:
                                                      Conical Flasks
     Benzoic acid
                                                      Filter funnel
     Distilled water
                                                      Hirsch / Buchner funnel
                                                      Watch glass


1.        1.0 g benzoic acid weighed and putted into a 100 ml conical flask.15 ml water and
          anti – bumping granules ( 3-5 pieces) was added.

2.        The mixture heated on a hot plate until the solvent boils. successive small volumes
          of water (2-3 ml) added and continue boiling until all benzoic acid has dissolved
          (apart from insoluble impurities), then 5 ml hot water added into the solution.

3.        If the solution is coloured, the solution removed from the hot plate. the solution was
          cooled to room temperature and decolouring charcoal (0.2 – 0.3 g) added. The
          mixture mixed and boiled for several minutes.

4.        While waiting for the solution to boil, the fluted filter paper prepared and putted
          into the funnel. The funnel fitted with fluted filter paper and putted in a conical
          flask. A little water and anti bumping granules added into the conical flask and
          heated on a hot plate.

5.        The hot mixture filtered of benzoic acid through a fluted filter into conical flask. If
          the filtration is done in batches, keep the remaining solution hot throughout the
          filtration process.

6.        If crystallization occurs on the filter paper, a minimum volume of boiling water
          was added to redissolve the crystals, and allowed the solution to pass through the
          funnel. Hot solvent in small volumes until all crystals are dissolved. After filtration,
          boil the filtrate to produce a more concentrated solution.

7.        The conical flask was covered with a watch glass and the solution to cooled to room
          temperature, then putted in an ice-bath after the crystallization has occurred. If no
          recrystallisation occurs at this stage, it may be due to the fact that too much solvent
          was used. The solution by heating on the hot plate concerntrated and cooled.

8.        When all the benzoic filter the crystals through a Hirsch/ buchner funnel by rinsing
          the flask with some of the filtrate. The crystal washed with a little cold water and
          dried. The crystals placed in a watch glass to air dry or dry the crystal rubbing
          between two filter papers.

     9.   The crystal dried completely before taking the melting point. The pure benzoic
          acid recovered, the % recovered was calculated.

   1. Data on the impure Benzoic Acid

    a)    Mass of benzoic acid + Weighing paper          :    1.23 g

    b)    Mass of weighing paper                         :    0.46g

    c)    Mass of impure benzoic acid                    :    0.77g

2. Data for recrystallized benzoic acid

          Mass of recrystallized benzoic acid +
    a)                                                   :     0.74 g
          Weighing paper
    b)    Mass of weighing paper                         :   0.5985 g

    c)    Mass of recrystallized benzoic acid            :   0.1415 g

          Calculation of percentage recovery
    d)                                                   :
          (Show calculation )

    e)    Melting point of recrystallized benzoic acid

2. Melting point of recrystallized benzoic acid (Theory) :

                                                                        [70 marks]

(Hints: Discuss on your findings and relate to your theory and objective of

                                                                        [5 marks]


(Hints: Conclusion should contain summary of the results, sum up what you have
learned from the lab. The conclusion should be one paragraph of 5 – 7 sentences).

                                                                        [5 marks]

1. What is the purpose of the recrystallisation process?

       -   To purify a solid by allowing it to recrystallize out of solution. The impurities
           stay in solution while the desired solid precipitates out.

                                                                                    [5 marks]

2. Explain why anti – bumping are added before any solution is heated ?

       -   Because a heated solution is likely to boil, probably sooner than later.
       -   For a liquid to boil smoothly, small bubbles have to nucleate on the bottom
           and rise to the top. The anti-bumping allows the bubbles to form.
       -   Most liquids, especially small quantities in a glass test tube, don't boil
           smoothly. A big bubble forms at the bottom, and abruptly ejects the liquid on
           top all over the lab bench.

                                                                                    [5 marks]
3. Why is suction filtration favoured over gravitational filtration when separating pure
   crystals from its supernatant liquid after the recrystallisation

       -   Vacuum filtration is often faster than gravity filtration and is most useful if the
           solid must be kept or if a filtration must be done rapidly.

                                                                                    [5 marks]

4. In general, water is not a good solvent for the recrystallisation. Explain this statement.

       -   non volatile solvent
       -   A good solvent for recrystallisation is one where the substance is considerably
           more soluble in hot solvent than cold solvent. Also after recrystallising and
           filtering, the residual solvent should evaporate quickly. Water is not that good
           for this point either.

                                                                                    [5 marks]

                                      Total Marks: 100

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