Question

What effect does polarity have on the melting point of a pure compound?

Use electronegativity values to classify the bonds in each of the following compounds as ionic, polar covalent, or nonpolar covalent.

(a) MgCl2		(select)nonpolar covalentionicpolar covalent
(b) CO2		(select)ionicpolar covalentnonpolar covalent
(c) H2S		(select)polar covalentnonpolar covalentionic
(d) NO2		(select)nonpolar covalentpolar covalentionic

Answer 1

• The melting point of a pure compound depends on the polarity of that compound. So if polarity increases the effect of Hydrogen bonding become more stronger and high amount of heat required to break a bond and hence Melting point increases.
• Electronegativity chart :

Element	Symbol	Electronegativity
Hydrogen	H	2.2
Carbon	C	2.55
NItrogen	N	3.04
Oxygen	O	3.44
Magnesium	Mg	1.31
Sulphur	S	2.58
Chlorine	Cl	3.16

On the basis of electronegativity difference :

Ionic Compound = EN difference > 1.7

Polar- covalent Compound = EN difference > 0.4 but EN difference < 1.7

Non-polar Covalent Compound = EN difference < 0.4

Sl No	Compound	Electronegativity difference	Nonpolar Covalent ; Ionic ; Polar Covalent
1	MgCl2	3.16 - 1.31 = 1.85	Ionic Bond
2	CO2	3.44 - 2.55 = 0.89	Polar Covalent Bond
3	H2S	2.58 - 2.2 = 0.38	Non polar Covalent Bond
4	NO2	3.44 - 3.04 = 0.40	Polar Covalent Bond