1. Chemistry Tutorial 5
Question 12
Wang Jia 12S7F

2. The standard enthalpy change of formation, ∆HfΘ, and the standard free energy change of formation, ∆GfΘ, of CO(g) and of CO2(g) are as follows:
CO(g)
CO2(g)
∆HfΘ /kJ mol -1
-110.5
-393.5
∆GfΘ /kJ mol -1
-137.2
-394.4

3. (a) Calculate the standard entropy change of formation, ∆SfΘ, in J mol-1, of CO(g) and of CO2(g).
∆HfΘ /kJ mol -1
-110.5
-393.5
∆GfΘ /kJ mol -1
-137.2
-394.4
∆GfΘ = ∆HfΘ -T ∆SfΘ
∆GfΘ [CO(g)] = ∆HfΘ [CO(g)] -T ∆SfΘ [CO(g)]
-137.2= –(298) ∆SfΘ [CO(g)]
∆SfΘ [CO(g)] = kJ mol-1
= J mol-1 (3s.f.)

4. (a) Calculate the standard entropy change of formation, ∆SfΘ, in J mol-1, of CO(g) and of CO2(g).
∆HfΘ /kJ mol -1
-110.5
-393.5
∆GfΘ /kJ mol -1
-137.2
-394.4
∆GfΘ = ∆HfΘ -T ∆SfΘ
∆GfΘ [CO2(g)] = ∆HfΘ [CO2(g)] -T ∆SfΘ [CO2(g)]
-394.4= –(298) ∆SfΘ [CO2(g)]
∆SfΘ [CO2(g)] = +3.02J mol-1 (3s.f.)

5. (b) Using the ∆HfΘ values given and your answers in (a), show that the reaction: C(s) + CO2 2CO(g) is not feasible at 298Kand calculate the minimum temperature at which reaction becomes feasible.
CO(g)
CO2(g)
∆HfΘ /kJ mol -1
-110.5
-393.5
∆GfΘ /kJ mol -1
-137.2
-394.4
∆SfΘ /J mol -1
+89.60
+3.020

6. To show that the reaction between C(s) and CO2(g) is not feasible at 298K
Calculate the standard Gibbs Free Energy of this reaction, ∆GΘ
Positive value  not feasible
Values that we need for the calculation:
∆HrΘ & ∆SrΘ

7. C(s) + CO2(g) 2CO(g) 2C(s) + O2(g) By Hess’s Law,
∆HrΘ
C(s) CO2(g) CO(g)
2C(s) + O2(g)
∆HfΘ [CO2(g)] =-393.5kJ mol-1
2∆HfΘ [CO(g)]=2( )kJ mol-1
By Hess’s Law,
∆HrΘ = -(-393.5) + 2(-110.5)
= kJ mol-1

8. C(s) + CO2(g) 2CO(g) 2C(s) + O2(g) By Hess’s Law,
∆SrΘ
2∆SfΘ[CO(g)]=2( J mol-1
∆SfΘ [CO2(g)]
= J mol-1
C(s) CO2(g) CO(g)
2C(s) + O2(g)
By Hess’s Law,
∆SrΘ = (89.60)
= J mol-1

9. ∆GΘ = ∆HΘ -T ∆SΘ ∆GΘ = +172.5- 298(176.2/1000) = +120 kJ mol-1
Hence, the reaction is not feasible at 298K as the Gibbs Free energy of the reaction is positive at 298K.

10. For the reaction to be feasible,
∆G < 0
∆H -T ∆S < 0
– T (176.2/1000) < 0
T > 979 K

11. Thank you !