When ignited, solid ammonium dichromate decomposes in a fiery display. This is t
ID: 510744 • Letter: W
Question
When ignited, solid ammonium dichromate decomposes in a fiery display. This is the reaction for a "volcano" demonstration. The decomposition produces nitrogen gas, water vapor, and chromium(III) oxide. The temperature is constant at 25°C.
Substance
Hf° (kJ/mol)
S° (kJ/mol K)
Cr2O3(g)
–1147
0.08118
H2O(l)
–242
0.1187
N2(g)
0
0.1915
(NH4)2Cr2O7(s)
–22.5
0.1137
Determine S° reaction (in kJ/mol K).
A.
0.7475
B.
0.1590
C.
0.2777
D.
0.8612
E.
0.6338
Determine G° (in kJ/mol).
A.
–1555.4
B.
1903.6
C.
–191.4
D.
–38.9
E.
–2281.4
Substance
Hf° (kJ/mol)
S° (kJ/mol K)
Cr2O3(g)
–1147
0.08118
H2O(l)
–242
0.1187
N2(g)
0
0.1915
(NH4)2Cr2O7(s)
–22.5
0.1137
Explanation / Answer
Decomposition of ammonium chromate is
(NH4)2(Cr2O7) --> Cr2O3 + N2 + 4 H2O
Change in entropy= entropy of products - entropy of reactant
So = (0.08118+ 4*0.1187+0.1915)-(0.1137)
=0.6338 kJ/mol K
We know that G= H-TS
Ho =[ (-1147) + 0+4*(-242)]-[-22.5]
=-2092.5
T = 25 + 273 = 298K
Go= (-2092.5) - (298)(0.6338)
=-2,281.4 kJ/ mol
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