Molecular Architecture Review Questions
1.  What factors determine which of two reacting atoms donates electrons and which accepts electrons?
2. Given the following data:     Solubility in Water       Melting Point
                                     (g/100 g H2O at 25oC)         (oC)
                                      NaF              4.2                 988
                                      NaCl           35.7                 801
                                      NaBr         116                   755
                                      NaI            184                   651

a)   Explain the decreasing melting point from NaF to NaI.

b)   Explain the increasing solubility in the same order.

c)   Cesium fluoride has a melting point of 682oC and a solubility of 367 g/100 g of H2O at 25oC. Explain the difference between the properties of CsF and NaF.
3. Lattice energy is related to melting point. What factors explain that sodium sulphide melts at 1180oC while magnesium sulphide melts at a temperature over 2000oC? 
4. Arrange the following in order of decreasing ionic properties.  LiI, BaO, AlCl3 , CsF,  RbBr , K2S , CaO,  ClF,  P2S3 , F2
5. What tests might you perform to determine whether a solid substance contains ionic bonds?
6. What is the reason for the generally low melting points of molecular solids?
7. Why do NH3, H2O, and HF have abnormally high boiling points when compared to the analogs of PH3, H2S, and HCl?
8. The three compound shown below are fairly similar. Thus we might expect them to have similar boiling points. Explain the increase in boiling points of the compounds that follow:
Formula            Common name         Chemical Name     b.p.(oC) 
C3H7OH           Rubbing alcohol         Propanol                  82
C3H6(OH)2       Antifreeze                 Ethylene glycol       189
C3H5(OH)3       Glycerin                    Glycerol                 290
9. a)  Which compound in Question 8 is the most viscous?

b)  Which is the most fluid?
10. How do network crystals differ from molecular crystals 
a) in structure, b) in hardness, and c) in melting points.
11. Classify these substances as ionic, solid molecular, solid covalent (non-polar or polar), or solid metallic
a) lattice composed of positive ions sharing electrons with neighbouring positive ions.
b) lattice composed of atoms bonded covalently to neighbouring atoms.
c) a solid only at extremely low temperatures.
d) a good conductor of heat and electricity.
e) extremely hard, yet very soluble in polar fluids.
f) extremely hard, not workable, and a poor conductor of heat and electricity.
g) a poor electric conductor even when melted.
12. Explain why C20H40 is a solid at 25oC while C4H6 is a gas at 25oC.
13. Germanium (Ge) is a solid whose atoms are all covalently bonded to each other, much like those of carbon atoms in a diamond crystal. Glycerol, C3H5(OH)3 is an alcohol. Potassium chloride, KCl, is a white crystalline solid. Methane, CH4, is a gas which can be liquified only under high pressure and low temperatures. Rubidium metal is very malleable and is an excellent conductor of heat and electricity.   Which has
a) hydrogen bonding;
b) the greatest hardness in the solid phase;
c) the highest melting point;
d) conductivity of electricity when melted;
e) the lowest heat of vaporization;
f) particles held together primarily by van der Waals forces;
g) nondirectional bonds?
14. a) Explain why pure carbon in the form of graphite is soft and has a high melting point.
b) Explain why pure carbon in the form of diamond is hard and has a high melting point.
c) Why is diamond a poor conductor of an electric current and graphite a good conductor?
15. Elements A, B, C, and D have consecutive atomic numbers. Element D is a monatomic gas with low melting and boiling points. All efforts to form compounds of D in the laboratory have failed.
a) Which of the remaining elements A, B, or C has the strongest affinity for an extra electron?
b) A compound of an alkali metal M with compound C has a formula of MC. Does this compound have ionic or covalent bonds? Predict other properties of MC such as melting point and solubility.
c) Write the formula for the hydrides of A, B, and C.
d) Predict the shape of each hydride molecule.
e) Predict the conductivity of solid B.
16. Elements X, Y and Z have consecutive atomic numbers. X is the only good conductor of electricity and heat, Y has an extremely high melting point, and Z has a low melting point and a molecular formula of Z4.
a) What type of bonding exists between the atoms of element Y?
b) Which of the three elements is malleable?
c) Write formula for the hydrides of these elements.
d) Which of these elements could be cut with a knife?
e) Which oxide of these elements dissolves in water and forms an acid?
17. a) Which is the better conductor of a direct electric current, liquid KCl or metallic aluminum?
b) How is the electric charge transported in each case?
c) What effect does the passage of an electric current have on each?
18. Which compound in each of these has the higher boiling point? Explain your choice?
a) NH3 or PH3 b) C2H6 or C4H10 c) S8 or Cl2
19. Which compound in each of these pairs has the higher melting point?
a) CaO or KI b) KCl or KI c) RbCl or ICl

          Answers