In this article we will discuss trends that effect the boiling points.

Define Boiling points:

The boiling point of any liquid is the temperature at which liquid start to changes into vapors

Trends that Affect boiling points:

There are many factors which effect boiling points depends on the nature of boiling points but we will discuss five main factors.

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1. Boiling Point Trend in periodic Table:

a) Trend in Groups:

1st group elements are alkali metals (Hydrogen, Sodium, Potassium, Rubidium, Cesium and Francium). Generally the melting and boiling points of alkali metals decrease down the group due to increase the size of atoms and decrease intermolecular forces.

2nd group elements are alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra). They have high melting and boiling points than 1st group elements but when we compare alkaline earth metals with d-block elements we concluded that d-block elements have higher melting and boiling points than alkaline earth metals.

Following are some factors which effect the boiling points in groups when we see group 1,2,13 and 14 their boiling points decrease from top to bottom.

  • Due to increase of shielding effect.
  • Due to increase size and weaken intermolecular forces between nucleus and electrons.

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This table is showing the boiling points of Group 1,2 and 13 elements.

Group 1

Boiling point

ºC

Group 2

Boiling point

ºC

Group 13

Boiling point

ºC

Lithium

1347

Beryllium

1287

Boron

4196

Sodium

886

Magnesium

650

Aluminum

3013

Potassium

774

Calcium

842

Gallium

2949

Rubidium

688

Strontium

777

Indium

2626

Cesium

678

Barium

727

Thallium

2003

Francium

677

Radium

700

   

 

In group 15, 16 and 17 the boiling point increase due to strong vendor wall forces.

Group 14

Boiling points ºC

Group 15

Boiling points ºC

Group 16

Boiling points ºC

Group17

Boiling points ºC

Carbon

4827

Nitrogen

-196

Oxygen

-183

Fluorine

-187

Silicon

3280

Phosphorous

280

Sulpher

415

Chlorine

-35

Germanium

2850

Arsenic

614

Selenium

685

Bromine

59

Tin

2623

Antimony

1587

Tellurium

988

Iodine

183

Lead

1751

Bismuth

1564

Polonium

962

Astatine

336.8

 

b) Trend in period:

When we move from left to right in periods the boiling point increase till the group 14 and after group 14 it will decrease rapidly. But in periods boiling point sequence interrupted due to properties of that element. Boiling points of different compounds have been shown in this graph.

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Transition metals have an irregular pattern in case of boiling point.

2. Boiling point trend with respect of Pressure:

Boiling point of a liquid will automatically changed with the change of external pressure.

External pressure is less than 1atm Low boiling points

External pressure is 1atm Normal boiling point

External pressure is greater than Boiling point is also greater than normal

1atm

The vapor pressure of Diethyl ether, ethyl alcohol and water increase with increase of temperature. Ether boils first then alcohol and then water. Vapor pressure of water increases slowly because water has strong intermolecular forces. There are different molecules showing their vapor pressure at different temperature in given graph.

⦁	Boiling point trend with respect of Pressure

 

 

In this graph with increase of vapor pressure boiling point is also increasing.

For example:

Boiling point of water is 100ºC at atmospheric pressure but when we move at Mount Everest its boiling point will be 92ºC due to increase of elevation.

What is liquid vapor pressure?

The pressure which is exerted by a liquid when its liquid and gaseous states reached at the points of equilibrium.

3rd Trend that Afffect Boiling point is relative strength of Intermolecular forces:

Intermolecular forces play very important role in boiling points of any liquid. If there is strong intermolecular forces between the atoms increase the stability of molecules and greater the energy will be required to break the molecules so its boiling point will be high.

So we can say

Strength of intermolecular forces ∞ Boiling points

4th Trend that Affect Boiling point is Impurities / Solution:

Pure solvents have lower boiling points as compared to solutions. When we add solute into the solvents it will become a solution and solution has greater boiling points than solvents.

For example.

Water boils at 100ºC but when we add small amount of salt in it, it will become solution and its boiling point will increase.

 

5th Trend that Affect Boiling point is Molecular Mass:

Boiling points of the molecules will increase with increase of molecular mass. As increase the surface area of molecules increase it increase the surface area of molecules so molecules become strong. Due to increase the stability it’s difficult to break the molecules so their boiling points increase.

The molecules of small size can easily boil and evaporate as compared to big sized molecules. e.g. hexane C6H14 is a small sized molecule as compare to decane C1oH22 . Therefore hexane evaporate rapidly than decane

Mostly substance boils when the force of attraction between the molecules overcomes by the movement of thermal energy. When we take average of thermal energy molecules with small size have greater thermal motion than big molecules. So we can say smaller molecules has low boiling point. But some molecules break this trend which have strong intermolecular forces and boil at high temperature like water. Water has low molar mass but has high boiling points.

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