How do I solve this Chemistry problem on Phase Changes?

Question

The molar heat of fusion of sodium metal is 2.60 kJ/lol, but it's heat of vaporization is 97.0 kJ/lol

 

A. Why is its heat of vaporization so much larger than the heat of fusion?

 

B. What quantity of heat would be needed to melt 1.00 g of sodium at its normal melting point?

 

C. What quantity of heat would be needed to vaporize 1.00 g of sodium at its normal boiling point?

 

D. What quantity of heat would be  evolved if 1.00g of sodium condensed at its normal boiling point? 

Answer 1

(A). The molar heat of vaporization is much larger than molar heat of fusion because it is breaking the intermolecular forces and going from liquid to gas as opposed to going from solid to liquid (fusion). Put another way, heat of fusion separates molecules of a solid, but there are still forces of attraction in the resulting liquid. Vaporization separates molecules of the liquid to the point where there is essentially no remaining forces in the gas phase, so it takes much more energy.

(B)  q = m∆H_f = 1.00 g Na x 1 mole/23 g x 2.60 kJ/mole = 0.113 kJ of heat

(C)  q = m∆H_v = 1.00 g Na x 1 mole/23 g x 97.0 kJ/mole = 4.22 kJ of heat

(D)  Condensation is the same process as vaporization but in the reverse direction, so the quantity of heat will be the same as in (C), i.e. 4.22 kJ of heat evolved, as opposed to being added.