Page 50 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

34 Chapter 3

WORKED EXAMPLE

Example: Determine AGO for the reaction of N2(,) and H2(g) to form
NH,,,), given the following data. Comment on the sign of AGO.

AHfO/kJ mol- ' N2(g) HW W3(g)
-46.1 1
0
0
So/J K-' mol-' 191.61 130.684 192.45

So lution:

1. Read the question carefully.
2. Species involved: Nz(~), H2(,) and NH3(,).
3. Balanced chemical equation: 0.5N2(,) + 1 .5H2(,) -+ NH3(,).
4. AHg, = [ 1 x ( - 46.1 1 )] - [O. 5 x (0) + 1 .5 x (O)] = - 46.1 1 kJ mol - ' .

5. ASR, = [l x (192.45)] - [0.5 x (191.61) + 1.5 x (130.684)] =
-99.381 J K-' mol-'.
6. AGO = AHo - TAS"
= (-46 110) - [(298) x (-99.381)]
= - 16494.462 J mol-' = -16.49 kJ mol-'.
7. AGO -ve + spontaneous reaction.

SUMMARY OF CHAPTERS 2 AND 3 ON CHEMICAL
THERMODYNAMICS
The multiple-choice test and the accompanying three longer ques-
tions which follow act as a revision of both Chapters 2 and 3 on
thermodynamics.

MULTIPLE-CHOICE TEST

1. A gas expands from a volume of 2.5 dm3 to 3.7 dm3 against an
external pressure of 1.5 bar, while absorbing 78 J of heat. What is
the change in the internal energy of the gas (in joules), given that
1 dm3 bar = 99.98 J?
(a) + 198.0 (b) +258.0 (c) -101.96 (d) -258.0
2. Which of the following four combinations guarantees that a
reaction will proceed spontaneously: