Determination of Boiling Point of Hydrocarbons:

PURPOSE:

a)  To become acquainted through process in evaluating physical properties such as boiling point and the utilize of boiling point in identifying liquid. 

b)  To find out the boiling points of diverse organic compounds and to utilize such to identify unknowns.

Equipment / materials: 

Discussion:

The boiling point of a compound is the temperature at which it transforms from a liquid to a gas. This is a physical property often utilized to recognize materials or to verify the purity of the compound.

It is hard, although, to find out boiling point. Generally, chemists can only gain a boiling range of 2-3^oC precision. This is generally enough for most utilizes of the boiling point.

The boiling point of a liquid is a significant physical property. A liquid's boiling point is the temperature at which its vapour pressure is equivalent to the atmospheric pressure. Generally, the boiling point is calculated at one atmosphere (101 kPa or 760 mmHg or 760 torr). As melting points, boiling points are trait properties of pure materials. Boiling points are about related to molecular weights; the higher the molecular weight, the higher the boiling point.

Boiling point is the temperature at which the vapour pressure of the liquid precisely equals the pressure exerted on it, causing the liquid to "boil" or transform to the gas phase. For the reasons of this laboratory trial, the boiling point of an organic liquid is the temperature series over that the state of the organic compound transforms from the liquid phase to the gas phase at 760 mm of pressure. Whilst the boiling point is a trait physical property of a compound, many compounds might contain similar boiling point.

The molecules of compounds that survive in the liquid state are moderately close jointly, compared to those of gaseous compounds. The close proximity of molecules in the liquid state permits such molecules to relate by non-covalent interactions (dipole-dipole, H-bonding, van der Waals forces). In common, such interactions are favorable and assist to hold the molecules jointly in an identified volume, but still permit free motion or "flow". On the other hand, molecules of a gaseous compound are much farther away from each other and aren't detained to a precise volume via non-covalent interactions (Figure). If sufficient energy (often in the form of heat) is offered to the liquid, the molecules begin to shift away from each other via "breaking" the non-covalent forces that hold the compound in the liquid state.

Therefore, the boiling point is the temperature range over which sufficient energy is supplied to a liquid compound so that its molecules can divide adequately to change to a gaseous state via breaking non-covalent interactions. No covalent bonds are broken throughout a transform from the liquid phase to the gas phase.

Fig: Phase Change from Liquid to Gas at Boiling Point Temperature Range

The vapour pressure, P_vapour, exerted via a liquid is straight proportional to the temperature. Therefore if the atmospheric pressure, Patm, is lowered, the temperature to which the liquid must be heated in order for Pvapour to Patm is lowered. The boiling point will transform via approximately 0.5^oC for each 10 mm (10 mmHg) transform in pressure.

Therefore, the transform in boiling point =  ΔT_b = ((P_{vapour - Patm})mm / 10 mm) * 0.5

The corrected boiling point= T_b = normal boiling point+?T_b

Example: 

What will be the boiling point of ethanol at 700 mmHg whenever its normal point at 760 mmHg is recognized to be 78.3^oC?

Solution:

?T_b =    (700-760)/10 *0.5 ^oC = -3^oC

 T_b = normal boiling point+ ?T_b = 78.3 ^oC + (-3^oC) = 75.3^oC.

In theory, whenever a liquid is at its boiling point, one should examine bubbles of vapour forming as the liquid transforms to the vapour phase. Though in practice, this is generally not the case. Classically the liquid happens to superheated as its temperature scales above the true boiling point. Then the solution abruptly "bumps" or boils with incredible vigor, bumping the hot liquid out of the bottle. Steps must be taken to guard beside this procedure.

In order to endorse smooth boiling, the solution can be mixed or else boiling stones (boiling chips) can be attached to the liquid. Such glassy kind devices work through providing a sharp surface upon that bubbles as expected form, which promotes smooth creation of bubbles (avoid bumping and formation of huge bubbles).

If the volume of the liquid is little, it is more beneficial to utilize the micro process, as in this test. Though, if the volume of the liquid is huge, its boiling point can be found out via distillation.

Factors influencing boiling point:

Structural traits of a compound influence the boiling point through increasing or decreasing the molecules' ability to begin and keep non-covalent communications that hold the molecules close up jointly in the liquid state. The structural features of a compound that influence boiling point are:

Polarity:  Enhanced H-bonds, polar covalent bonds or formal charges in a molecule tend to enhance the boiling point. More polar elements in a molecule amplify the total number of dipole-dipole, ion-dipole and/or H-bonding interactions. More vigor (higher boiling point temperature) is needed to break such interactions and permit the molecules to shift away from each other into a gaseous state Molecular Weight: amplified molecular weight enhances boiling point. A higher molecular weight compound has additional atoms that can be griped in non-covalent interactions. The greater the number of non-covalent interactions, the more energy (higher boiling point temperature) that is needed to crack the non-covalent interactions to change the compound from the liquid phase into the gas phase.

Branching: This reduces boiling point. Branching chunks molecules from packing jointly as well directly. The closer the molecules are, the stronger the non-covalent interactions. Therefore, molecules that are forced to be beyond away from each other due to branching have weaker non-covalent interactions. Less energy (lower temperatures) is required to induce a phase transform from the liquid phase to the gas for branched compounds comparative to straight chain compounds. (Figure) 

Fig: (a). Straight Chain Compounds (b). Branched Compounds 

EXPERIMENT: Micro Method Determination of Boiling Point of Hydrocarbons

Experimental procedure:

Place about 5 ml of the liquid in a little test tube. A capillary tube, closed at one end, is situated open-end downward into the liquid (Figure). The test tube is resolutely connected to a thermometer via denotes of a rubber band, and this whole assemblage is immersed in a water bath (oil bath for samples through boiling point higher than 100^oC) (Figure). As the temperature is gradually amplified, a quick evolution of bubbles from the end of the tube starts. Carry on heating for about 5-10 seconds to be certain that all of the air has been ejected from the capillary, and the vapour of the liquid continues in the capillary. Take away the heat, but don't obtain the assembly out of water bath (or oil bath), and cautiously watch the capillary. Bubbles persist to be seen until the pressure exerted via the vapour of the liquid turn into like to the full of atmosphere pressure. As the temperature reduces, the bubbles will sluggish down and at several point, the liquid will increase into the capillary. The boiling point of the example is attained whenever the bubbles stop. Read the thermometer and verification the temperature. The temperature examined when this occurs should be the examined boiling point of the liquid. Compare our experimental consequence to the literature value (Table) of the boiling point for the liquid utilized. If our method is good, our experimental value should not fluctuate from the recognized value (literature value) via more than 2-3^oC. Do again the procedure with the known liquids. Every time we execute the process, we must utilize a new capillary. It will as well be needed to permit the hot bath to cool at least 15-20^oC beneath the suspected boiling point previous to repeating our experiment.

Fig: Small Test Tube and Capillary, 

Fig: Small Scale Boiling Point Apparatus sealed at one end   

Table: Literature Values of the Boiling Point for some Liquids Substance Boiling Point Substance Boiling Point

Fig: Boiling Point Apparatus Set Up

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