Chemistry

Ebulioscopy (continued)


Phases diagram

Phases diagram is the name of the graph which simultaneously contains the curves of the boiling, solidification and sublimation temperatures of a given chemical as a function of vapor pressure.

Every substance has its phase diagram, with its triple point (T) and its curves.

In a phase diagram we can verify that:

- in the triple point the three phases of equilibrium coexist;
- In the solidification curve, the solid phase and the liquid phase coexist;
- in the sublimation curve the solid phase and the vapor phase coexist;
- In the boiling curve the liquid phase and the vapor phase coexist.


source: profpc.com.br/coligativas_properties.htm

Still regarding the phase diagram:

- any region to the left of the solidification and sublimation curves exists only in the solid phase;
- every region between the solidification and boiling curves exists only in the liquid phase;
- Every region to the right of the boiling and sublimation curves exists only in the vapor phase.

The phase diagram explains many phenomena that happen in our daily lives. Ice skating is an example. Skating on the ice is made easier because by touching the ice it presses and the ice melts momentarily to liquid. When the pressure is gone, it becomes ice again.

The use of carbon dioxide (CO2) for ice cream preservation is also an example of the usefulness of the phase diagram. At 1atm pressure, it goes directly from solid to gas at 78 ° C below zero. It is therefore used as dry ice. It does not turn into liquid, it goes straight to the gas phase at room temperature.

Simple substances that suffer phenomena such as allotropy also have their phase diagram. This is the case of carbon, which has allotropes with different geometric shapes, graphite carbon and diamond carbon.

It is now known that it is possible to transform graphite into diamond according to studies carried out on the carbon phase diagram.

Graphite can be turned into a chromium catalyzed diamond at a temperature of 2000 ° C and a pressure of 100mil atm (equivalent to the underground pressure at a depth of 33Km). This process can be done in specialized laboratories (even in Brazil) and takes about 5 minutes.

  
Graphite carbon and diamond carbon

Freezing point

For a substance to move from liquid to solid phase, its molecules must lose kinetic energy (energy of motion).

Then there must be a decrease in temperature. Remember that temperature is linked to the shaking of molecules.

Among substances with the same type of intermolecular bond, the solidification point will be lower at the one with the lowest molar mass. This is because the smaller the molar mass of the substance, the greater the mobility of its molecules (kinetic energy).

Here are some solidification points:

Substance

PS (° C)

Molar Mass (g / mol)

n-pentane

-129,7

72

n-hexane

-95

86

propanone

-94

58

Acetic Acid

16,6

60

In molecules of different intermolecular bonds, the solidification point will be lower in the substances having the weakest bond.

Therefore, acetic acid has higher PS than the other substances presented. It has hydrogen-bridge intermolecular bonds, which is the strongest of bonds.