Cliffs AP Chemistry, 3rd Edition

9. If ∆H°and ∆S°are both negative, then ∆G°is

A. always negative

B. always positive

C. positive at low temperatures and negative at high temperatures

D. negative at low temperatures and positive at high temperatures

E. zero

Answer: D

Examine the Gibbs-Helmholtz equation, ∆G°= ∆H°−T∆S°, to see the mathematical relation-
ships of negative ∆H°’s and ∆S°’s. (Refer to page 170.)

10. Determine the entropy change that takes place when 50.0 grams of compound xare

heated from 50.°C to 2,957°C. It is found that 290.7 kilojoules of heat are absorbed.

A. −461 J/K

B. 0.00 J/K

C. 230 J/K

D. 461 J/K

E. 921 J/K

Answer: C

Use the equation

∆S=2 303. CplogTT^21

where Cprepresents the heat capacity

(Cp= ∆H / ∆T = 290,700 J / 2907 K = 100.0 J/K).

Substituting into the equation yields

∆S=2 303. CplogTT 12

..

.

2 303 100 0 log 323

3230

KJ K^230

K

==cm J/K

Energy and Spontaneity