Chapter 3, Problem 1Q

Summary Introduction

To review:

The differences between the properties of water (H₂O), ammonia (NH₃), and methane (CH₄), based on the hydrogen bonding in their structure. The heat of fusion for water, ammonia, and methane is given as 6.01, 5.66, and 0.94 kJ/mol, respectively. Also, determine increase or decrease in the density of ammonia in the form of ice (if generated), as compared to liquid ammonia.

Introduction:

The molecular mass of water, ammonia, and methane is almost equal, and all of them show tetrahedral geometry. The heat of fusion is the highest in water, decreases in ammonia, and is the least in methane. Water has a highly polar structure. The three molecules (water, ammonia, and methane) are sp³ hybridized and are tetrahedral instructure. However, there is a difference in the number of lone pairs in them, leading to an overall different geometry and different physical properties.

Explanation of Solution

Water has two hydrogen atom, shich are covalently bonded to one oxygen atom. The oxygen is sp³ hybridized. Due to the presence of twolone pairs, this molecule has a bent geometry. As a whole, a water molecule is polar and acts as a dipole. The electronegativity of oxygen is more than that of hydrogen, and thus, it bears a partial negative charge, whereas two hydrogen atoms beara partial positive charge. The hydrogen atoms in asingle water molecule are electrondeficient, and thus, tend to be attracted toward the oxygen atom of another water molecule. Thus, hydrogen bonds (intermolecular)act as bridges between neighboringwater molecules. One water molecule can form hydrogen bonds with four otherwater molecules.

The structures of thethreemolecules can be represented as

An ammoniamoleculehasonelone pair (unshared electron) ofa nitrogenatom and has a trigonal pyramidal structure. There is limited hydrogen bonding (intramolecular)in case of ammoniaasnitrogen has onlyone lone pair available.

Methane exists in a gaseous statethat has one carbon atom bonded with four hydrogen atoms. The central atom, that is, carbon, forms covalent bonds with four hydrogen atoms by the sharing of electrons. This sharing completes the outer shell of both the carbon and the hydrogen atoms. The four hydrogen atoms give an overall tetrahedral shape to the molecule. No hydrogen bonds are involved in methane as there is no highly electronegative element to form a bond with hydrogen.

A hydrogen bondhaslower bond strength thana covalent bond, thus, it is weaker. However, when a largenumber of intermolecular bonds are formed, they are quite strong. This explains the differences in the heat of fusion (energy required to melt ice by breaking bonds) for water, ammonia, and methane. Due to a highernumber of hydrogen bonds, physical propertie, such asmelting point, boiling point, heat capacity, the heat of fusion, surface tensio, nnd heat of vaporization, have higher values in wate, rs compared to that of methane and ammonia.

In ammonia, the density of the solid state (ice), if generated, is expected to be more than that of liquid ammonia. This is because the hydrogen bonding between different ammonia molecules will not be as extensive as in case of water insolid state (open cage structure in ice). This is due to limited hydrogen bonding of nitrogen. Thus, as a general case, the solid form of ammonia will beless denser than its liquid form.

Conclusion

Thus, it can be concluded that water, ammonia, and methane molecules are tetrahedral in structure, but differ in their overall geometry and physical properties due to thenumber of lone pairs in them and hydrogen bonding. The heat of fusion is the highest for water moleculesbecauseofstrong intermolecular hydrogen bonds. Also, ammonia ice will not form a cage-like structure (as in water), and thus, its density is expected to be lesser than that of liquid ammonia.