Chemistry > Chemical Bonding and Molecular Structure > 7.0 Hydrogen Bonding
Chemical Bonding and Molecular Structure
1.0 Ionic Bond or Electrovalent Bond
2.0 Lattice Energy
3.0 Characteristics of Electrovalent Compounds
4.0 Covalent Bond (By Mutual Sharing of Electrons)
5.0 Characteristics of Covalent Compounds
6.0 Fajan’s Rule
7.0 Hydrogen Bonding
8.0 Coordinate Bond
9.0 Valence Shell Electron Pair Repulsion (VSEPR) Theory
10.0 Valence Bond Theory
11.0 Sigma and Pi Bonds ($\sigma $ and $\pi $ Bonds)
12.0 Hybridisation
12.1 Types of hybridization and spatial orientation of hybrid orbitals
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions
13.0 Molecular Orbital Theory
7.1 Properties Explained using Hydrogen Bonding
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions
- Strength of certain acids and bases can be explained on the basis of hydrogen bonding.
- Solubility
An organic substance is said to be insoluble in water if it does not form hydrogen bonding with water. The organic compound like alkanes, alkenes, ethers, etc., are insoluble in water as they do not form hydrogen bonding with water, while alcohols and acids are soluble because they readily form hydrogen bonds with water.
i) Melting and boiling points of hydrides of $N$, $O$ and $F$. If the melting points and boiling points of the hydrides of the elements of $IVA$, $VA$, $VIA$ and $VIIA$ groups are plotted against the molecular weights of these hydrides, we shall get the plots as shown in figure $(a)$ and $(b)$.
From these plots it may be seen that although in case of $SbH_3$, $AsH_3$, $PH_3$ ($VA$ group elements hydrides), $H_2Te$, $H_2Se$, $H_2S$ ($VI$ $A$ group elements hydrides) and $HI$, $HBr$, $HCl$ ($VIII$ group elements hydrides) there is a progressive decrease in their mp’s and b.p’s with the decrease in their molecular weights, the mp’s and b.p’s of $NH_3$, $H_2O$ and $HF$ hydrides suddenly increase with a further decrease of their molecular weights.
The sudden increase in mp’s and bp’s in these hydrides is due to the inter-molecular $H-$bonding in between $H$ and $F$ in case of $HF$, in between $H$ and $O$ in case of $H_2O$ and in between $H$ and $N$ in case of $NH_3$ respectively. The existence of $H-$bonding in these molecules gives polymerized molecules $(NH_3)_n$. Thus mp’s and bp’s of these molecules are suddenly raised.
Having no power to form $H-$bonds, the simple carbon family hydrides ($SnH_4$, $GeH_4$, $SiH_4$ and $CH_4$) show a decrease in their bp’s and mp’s with the decrease in their molecular weights.
ii) Ice has less density than water.
In the crystal structure of ice, the $O-$atom is surrounded by four $H-$atoms. Two $H-$atoms are linked to $O-$atom by covalent bonds as shown (by normal covalent bond) and the remaining two $H-$atoms are linked to $O-$atom by two $H-$bonds shown by dotted lines.
Thus in ice every water molecule is associated with four other water molecules by $H-$bonding in a tetrahedral fashion.
Ice has an open cage like structure with a large empty space due to the existence of $H-$bonds. As ice melts at $0^°C$, a number of $H-$bonds are broken down and the space between water molecules decreases so that water molecules move closer together.
The density of water increases, from $0^°$ to $4^°C$, and at $4^°C$ it is maximum. Above $4^°C$ the increase in kinetic energy of the molecules is sufficient to cause the molecules to begin to disperse and the result is that the density decrease with increasing temperature.
