Cliffs AP Chemistry, 3rd Edition

Answer: A

Entropy is a measure of the randomness or disorder of a system. The greater the disorder of a
system, the greater its entropy.

In H 2 O(g) →H 2 O(,), the reaction is going from a disordered state (g) to a more ordered state
(,); low entropy, ∆S< 0.

In 2 HCl(g) →H 2 (g) + Cl 2 (g), the change in energy will be very small since there are two
moles of gas molecules on each side of the equation.

In SiO 2 (s) →Si(s) + O 2 (g), the system is becoming more disordered, apparent from the pres-
ence of gas molecules on the product side; high entropy, ∆S> 0.

6. Given for the reaction Hg(,) →Hg(g) that ∆H°= 63.0 kJ ⋅mole–1and

∆S°= 100. J ⋅K–1⋅mole–1, calculate the normal boiling point of Hg.

A. 6.30 K

B. 63.0 K

C. 630 K

D. 6.30 × 103 K

E. cannot be determined from the information provided

Answer: C

At equilibrium Hg(,) ↔Hg(g), which represents the condition of boiling, and at equilibrium
∆G°= 0. The word normal in the question refers to conditions at 1 atm of pressure, which is
reflected in the notation for standardized conditions for ∆S°and ∆H°. Therefore, using the
Gibbs-Helmholtz equation, ∆G°= ∆H°−T∆S°, we can substitute 0 for ∆G°and solve for T.

∆S mole

, mole

100

63 000

T ∆H 630

JK

J

1 1 K

1

::

:

==% =

%

Part II: Specific Topics