Question

Consider the following reaction between oxides of nitrogen:NO 2 1 g 2 + N 2 O 1 g 2 ¡ 3 NO 1 g 2 (a) Use data in Appendix C to predict how ∆ G for the reac-tion varies with increasing temperature. (b) Calculate ∆ G at 800 K, assuming that H ° and S ° do not change with tem-perature. Under standard conditions is the reaction sponta-neous at 800 K? (c) Calculate ∆ G at 1000 K. Is the reaction spontaneous under standard conditions at this temperature?

Answer 1

(a)

We can see from the expression of free energy that the value of ∆G depends on temperature.

∆G = ∆H – T∆S_system

Sometimes, with change the temperature of reaction can change the sign of ∆G.

(b) If the Gibbs free energy of a reaction would be negative means the reaction to be spontaneous and if the Gibbs free energy of a reaction would be positive means the reaction to be non-spontaneous at given temperature.

The Standard state Gibbs free energy of a reaction is calculated by the following Gibbs Free Energy equation:

Delta G = Delta H - T(Delta S)

H º = 155.7 kJ or 155700 , S º =171.4 J/K and T = 800K

Delta G = 155700 J – 800K( + 171.4 J/K)

= 155700 J - 137120 J

= + 18580 J or + 18.581

Here the Gibbs free energy of this reaction is positive means the reaction to be non-spontaneous.

(c)

The Standard state Gibbs free energy of a reaction is calculated by the following Gibbs Free Energy equation:

Delta G = Delta H - T(Delta S)

H º = 155.7 kJ or 155700 , S º =171.4 J/K and T = 1000K

Delta G = 155700 J – 1000 K( + 171.4 J/K)

= 155700 J - 171400 J

= - 15700 J or - 15.71

Here the Gibbs free energy of this reaction is negative means the reaction to be spontaneous.