MELTING POINT

DETERMINATIONEXPERIMENT#8

DIAZ, HOVELYN LOU MARTINEZ, MA. CZARINA

ESTEBAN, REYNAN JAMES SANOTA, MARIELA

GREGORIO, KRYSTAL JANE

INTRODUCTION

A melting point can be used to identify a substance and to

get an indication of its purity. The melting point of a solid is

the temperature at which the solid exists in equilibrium with

its liquid state under an external pressure of one

atmosphere. Both the melting point range (the interval

between the beginning of liquefaction and complete

liquefaction) and the temperature of complete

liquefaction are valuable indicators of the purity of the

solid compound. A pure crystalline organic compound

usually possesses a sharp melting point and it melts

completely over a narrow temperature range.

PROCEDUREPrepare 3 melting point capillary tubes by

heating the middle portion of the capillary

tube (1mm diameter) and when soft

enough, pull in opposite directions to

completely seal and separate the two. Test

the tube if completely sealed by blowing air

on the open end, and feeling with fingers if

air comes out from the sealed end, it should

be heated further to completely seal it.

PROCEDURE

Introduce powdered samples of acetanilide,

benzoic acid and salicylic acid into 3 separate

prepared capillary by putting a small amount

of sample (about 3-4 mm diameter) on a piece

of clean dry paper and pushing the open end

of the capillary tube into the sample to scoop it

up. The column of solid should not be more

than 1cm, in the tube length and it should be

tightly packed.

PROCEDURE

Prepare the set-up for melting point

determination

PROCEDURE

Fasten the capillary tube containing

acetanilide to the thermometer with string

or a rubber bond so that the sample is close

to and on a level with the center bulb of the

thermometer bulb. Attach the thermometer

in its place in the set-up where its bulb and

the capillary tube (with sample) is centered

in the beaker and submerged in the oil.

PROCEDURE

Heat the oil bath with a moderate

flame. Then gradually increase the

size of the flame so as to cause a

rise in temperature of 1 or 2 per

minute. Stir the bath continuously

to have a uniform heating.

PROCEDURE

RECORD:

a. The temperature at which melting begins and

b. The temperature when the entire 1cm

powdered sample in the capillary tube is

completely melted

Repeat the same procedure by using benzoic and

salicylic acid

DATA AND RESULTSCOMPOUND INITIAL TEMPERATURE FINAL TEMPERATURE

ACETANILIDE 105C 114C

BENZOIC ACID 110C 122C

SALICYLIC ACID 140C 150C

COMPOUND MELTING POINT

ACETANILIDE 114.3 °C

BENZOIC ACID 121-123°C

SALICYLIC ACID 159°C

QUESTIONS1. What are the bonding forces that hold together the molecules in a crystal? Give examples of compounds with such types of bonding forces.

Intermolecular forces hold its molecules together such as Van der Waals forces, Ionic bonding, Covalent bonding, Hydrogen bonding and Metallic forces.

Examples:

I2, P8 and solid SO2 =Van der Waals forces. Weakly held and easily broken with low melting points

NaCl, MgO, CaCo= Ionic forces composed of the electrostatic attractions of anions and cations. Hard, brittle poor electrical conductors.

SiO = Covalent bonded crystal forming quartz and diamond crystals. Hard, poor conductors of heat and electricity and have high melting points

All of the metallic elements: Ag, Mg, Fe Cu, etc= Metallic bonding positive ions surrounded by delocalized valance electrons. Conduct heat and electricity well, have low to high melting points and can be soft or hard.

QUESTIONS2. What is the effect of an impurity on the melting

point of a substance?

They cause the melting point to turn from the standard.

It actually makes the magnitude of the melting point

lower because impurities decreases the quality of a

material. The melting point is not anymore sharp, but a

range of values. The more impurities present, the

greater the range of values.

QUESTIONS3. How can melting point be used in identifying

and/or characterizing unknown compounds?

Compounds are identified because each

compound have different melting points. The melting

point value can then be matched to a table of

melting points to identify the unknown.

APPLICATION

Melting points of organic solids are useful to organic chemists because:

They (and other physical constants) aid in the recognition of such

compounds. Extensive tables have been compiled, giving the melting points of different compounds and their derivatives.

They aid in obtaining information about the purity of an organic

compound. A pure organic compound has a "sharp" melting point.

They are useful in establishing the identity of a solid compound:

Mixed Melting Point Determination. This technique is based on the

fact, that one substance will lower the melting point of another

substance if mixed with it. A "mixture" of two specimens of the identical material, by contrast, shows no change in melting point.

GENERALIZATIONBy performing this experiment, we learned that the more carbon a

chemical or compound have, the higher the melting point.

Salicylic acid got the highest melting point among the three

because electrostatic bonds between the constituents of

molecules of salicylic acid are increasing over time through a series

of additions of heat than those between acetanilide and benzoic

acid. It needs more heat or energy to melt. Melting Point tells us

that it is something about the forces that hold the molecules

together, and the fact that some substances are not molecules at

all, but networks of atoms held in place by either ionic or covalent

bonds. It is the formation of a network of interconnected atoms

that makes for the high melting point, not the type of bonding

between the atoms.

GENERALIZATION

It is those compounds which do not form networks that

have lower melting points, because weaker Van der

Waals forces are the only forces keep the molecules

together.