Cliffs AP Chemistry, 3rd Edition

Part II: Specific Topics

Samples: Free-Response Questions

1. Given the equation N 2 O 4 (g) →2 NO 2 (g) and the following data:

Species ∆H°f(kJ ⋅mole–1)G°f(kJ ⋅mole–1)

N 2 O 4 (g)9.16 97.82

NO 2 (g) 33.2 51.30

(a) Calculate ∆G°.

(b) Calculate ∆H°.

(c) Calculate the equilibrium constant Kpat 298 K and 1 atm.

(d) Calculate Kat 500°C and 1 atm.

(e) Calculate ∆S°at 298 K and 1 atm.

(f) Calculate the temperature at which ∆G°is equal to zero at 1 atm, assuming that ∆H°

and ∆S°do not change significantly as the temperature increases.

Answer

1. Given: ∆Hf°and ∆Gf°information for the equation
   N 2 O 4 (g) →2 NO 2 (g)

(a) Restatement: Calculate ∆G°.

∆G°= Σ∆Gf°products −Σ∆Gf°reactants

= 2(51.30) −(97.82) = 4.78 kJ ⋅mole–1

(b) Restatement: Calculate ∆H°.

∆H°=Σ∆Hf°products −Σ∆Hf°reactants

= 2(33.2) −9.16 = 57.2 kJ ⋅mole–1

(c) Restatement: Calculate the equilibrium constant Kpat 298 K and 1 atm.

Kp P

PNO

NO

2

2

24

= (where Prepresents the partial pressure of a gas in atmospheres)

∆G°= −2.303 RTlogK (R= 8.314 J ⋅K–1)

p.

..

.

logK 2 303∆G RT

2 303 0 008314 298

478

kJ K K

kJ mole

1

1

:

:

=- =

%

_ i_ i

= −0.838

Kp= 0.145 (at standard temperature of 298 K)

Part II: Specific Topics