Thermochemistry Questions
1. a) Urea (from urine), CO(NH2)2(aq), hydrolyzes slowly in the presence of water to produce ammonia and
carbon dioxide. What is the standard ΔHrxn, ΔSrxn and ΔGrxn for this reaction when 1 mole of urea
reacts with water? (Create the balanced equation first then use the summation formulas and the
formation values from the reference booklet.)
CO(NH2)2(aq)
+
H2O(l)
→
CO2(g)
+
2 NH3(g)
ΔH =
=
=
=
Σ ΔHproducts - Σ ΔHreactants
[CO2(g) + (2) NH3(g)] - [CO(NH2)2(aq) + H2O(l)]
[-393.5 kJ + (2) -45.9 kJ] - [-335.5 kJ + -285.8 kJ]
+136.0 kJ
ΔS =
=
=
=
Σ ΔSproducts - Σ ΔSreactants
[CO2(g) + (2) NH3(g)] - [CO(NH2)2(aq) + H2O(l)]
[213.7 J/K + (2) 192.8 J/K] - [104.6 J/K + 69.9 J/K]
+424.8 J/K
ΔG = ΔH - T ΔS
= +136.0 kJ – 298.15 K x +0.4248 kJ/K
= +9.3 kJ
b) At what temperature is this reaction spontaneous?
As ΔG is positive, the reaction is not spontaneous at 25°C.
ΔH
 136.0 kJ

ΔS  0.4248 kJ/K  It is spontaneous at T > 47.0 °C
 320.2 K  47.0 C
T
2. What is ΔGrxn for the combustion of liquid ethyl alcohol (C2H5OH) to give CO2(g) and H2O(g)? Is the
reaction spontaneous?
C2H5OH (l)
+
3 O2(g)
→
2 CO2(g)
+
3 H2O(g)
ΔG = Σ ΔGproducts - Σ ΔGreactants
= [(2) CO2(g) + (3) H2O(g)] - [C2H5OH(l) + (3) O2(g)]
= [(2) -394.4 kJ + (3) -228.6 kJ] - [-174.8 kJ + (3) 0 kJ]
= -1 299.8 kJ
Yes, the reaction is spontaneous at 25°C, as ΔG is negative.
3. Calculate ΔHrxn and ΔSrxn for the following reaction and decide in which direction each of these factors
will drive the reaction.
N2(g)
+
3 H2(g)
→
2 NH3(g)
ΔH =
=
=
=
Σ ΔHproducts - Σ ΔHreactants
[(2) NH3(g)] - [N2(g) + (3) H2(g)]
[(2) -45.9 kJ] - [0 kJ + (3) 0 kJ]
-91.8 kJ  spontaneous
ΔS =
=
=
=
Σ ΔSproducts - Σ ΔSreactants
[(2) NH3(g)] - [N2(g) + (3) H2(g)]
[(2) +192.8 J/K] - [+191.6 J/K + (3) 130.7 J/K]
-198.1 J/K  non spontaneous