Question

2NH3(g) + 3O2(g) + 2CH4(g)  2HCN(g) + 6H2O(g)

	?H°f (kJ mol-1)	S° (J K-1 mol-1)
NH3(g)	-46	193
O2(g)	0	205
CH4(g)	-75	186
HCN(g)	135.1	202
H2O(g)	-242	189

The above reaction is used in the industrial production of hydrogen cyanide. (The tabulated values ?H°_fand S° are for 25°C. For the purposes of this question assume that ?H° and ?S° are invariant with temperature. This is not actually true but would generally be a reasonable approximation over "small" temperature ranges.)

?H° is -939.8

?S° is 165

Calculate ?G° at 1005°C, for this equation.

Are the following statements about this process True or False?

At temperatures significantly lower than 1000°C this reaction is spontaneous.
The high temperature required for this process is needed for thermodynamic reasons.
The equilibrium position for this reaction is further to the right at higher temperatures.
Thermodynamically, this reaction is spontaneous only below a certain temperature.
This reaction is endothermic at room temperature.

Answer 1

$\Delta$ G^o = $\Delta$ H^o - T $\Delta$ S^o

$\Delta$ G^o = -939.8 - 1278*165

$\Delta$ G^o= - 211.8 KJ

1) True ------------- Spontaneous, due to $\Delta$ G^o is negative.

2) True -------------At higher temperature $\Delta$ G^o is negative, so it is a spontaneous reaction at higher temperatures.

3) True -------------- At higher temperatures this reaction is spontaneous, so the equilibrium shifts towards the right.

4) False ------------- At above certain temperatures the reaction is spontaneous, due to negative $\Delta$ G^o value.

5) False -------------- At room temperature, the reaction is spontaneous, so no need for higher temperature.