Question, Ammonia, phosphine, intermolecular, phosphine, physical, bonding

 QUESTION 6:

             The boiling point of ammonia is higher than of phosphine. Explain

 SOLUTION:        

            Ammonia, NH3 have strong hydrogen bonding whereas phosphine, PH₃ have simple vander waals forces as intermolecular forces. Thus more energy is required to overcome hydrogen bonding present in ammonia. As a result ammonia has high boiling point than phosphine.

 QUESTION 7:

             An element X conducts electricity and melts at 660 ⁰C. It reacts vigorously with chlorine, after initial warning, to give a solid chloride of x. The solid chloride sublimes at 180 ⁰C. The chloride vapour has a relative molecular mass of 267 but at much higher temperatures it is known to disdociate to a single compound of relative molecular mass 133.5. When an aqueous solution of the chloride is shaken with ether and the ethereal solution separated some of the chloride may be recovered by carefully boiling off the solvent. Using the physical data quoted, what conclusion can you deduce about the type of structure present in the element x and its chloride?

 SOLUTION:

             This questions to relate the physical properties of a substance to the type of bonding and structure present in the compound.

Conducts electricity	Element x possesses mobile electrons. It may be a metal or graphite
Melting point of x is 660 0C	Since graphite has a macromolecular structure  possesses a much higher melting point , thus it a metal.
Chloride sublines at 180 0C	This suggests that the binding forces in the chloride are weak. The bonding in the chloride, therefore, cannot be ionic. It could be covalent
Relative molecular masses of 267 and 133.5	This indicates that the chloride undergoes dimerisation, possibly through dative covalent bond
Chloride dissolves in ether	This further suggests that the chloride has a simple molecular covalent bonding.

 QUESTION 8:

             Conduction of electricity is due to mobile electros not due to delocalized electrons. Explain?

 SOLUTION:

             Ability of metals and graphite to conduct electricity is due to availability of mobile electrons not due to delocalized electrons. There are delocalized electrons in the benzene molecule but the benzene molecule cannot conduct electricity. More accurately, the electrical conductivity of metals and graphite must be attributed to the pressure of mobile electrons not due to delocalized electrons.

 QUESTIONS 9:

             Discuss each of the following observations as fully as you can.

 (a)       Carbon dioxide is a gas whereas silicon dioxide is a high melting point solid at r.t.p.

(b)       The boiling point of beryllium chloride is lower than that of magnesium chloride [B.p of BeCl₂, 548 ⁰C; MgCl2, 1420 ⁰C].

(c)       Sulphur is a solid whereas oxygen is a gas at room temperature and pressure.

(d)       The melting point of magnesium oxide (2850 ⁰C) is higher than that of sodium oxide (1275 ⁰C).

 SOLUTION:

 (a)       CO₂ exist as discrete molecules held together by weak Van der Waals forces. On the other hand, SiO₂ has a macromolecular structure held together rigidly by strong covalent bonds.      

 (b)       Size and high charge density in its compounds predominantly charged into covalent compounds. Hence Berrylium chloride is a covalent substance held together by Van der waals forces. Whereas magnesium chloride is an ionic compound. Oppositely charged ions held together by strong electrostatic forces of attraction to form a giant ionic lattice.

 (c)       Both sulpher and oxygen are non-polar covalent molecules i.e; S₈ and O₂. They are held by weak Vander waals forces.  The difference in physical states between sulpher and oxygen is due to strength of ven dar waals, forces. Sulpher (S₈) molecules are bigger that the oxygen (O₂) molecules. Hence, bigger sulphur molecules have more polarisability and have stronger intermolecular forces.

 (d)       Both magnesium oxide MgO and sodium oxide, Na₂O are ionic compounds. Due to high charge density ionic radius of Mg²⁺ is smaller than Na¹⁺. Smaller and highly charged ions have stronger electrostatic forces of attraction in its crystal lattice. Therefore, more energy is needed to break down the ionic lattice of magnesium oxide.