Question

Consider the following reaction and calculate the standard Gibbs Free Energy for it.

2 NO + O2 --> 2 NO2

Given that delta H= -114.1 kJ and delta S= -146.5 kJ. Also comment on the spontaneity or otherwise of this reaction at 298 K. Are there any conditions in which your answer might be different? If so state this condition

Answer 1

2 NO + O₂ 2 NO₂ : G^O = ?

G^O = G^O_{f products} - G^O_freactants

        = [2xG^O_{fNO2 (g)} ] - [(2xG^O_fNO(g) )+ G^O_fO2(g) ]

        = [2x51.30] - [(2x86.57)+ 163.18]

       = -233.72 kJ

So the standard Gibbs Free Energy is -233.72 kJ      

Given that H= -114.1 kJ and S= -146.5 kJ

We know that G = H - TS

                            = -114.1 - ( 298 x (-146.5 ) )

                            = 43542.9 kJ

Since the value of G is positive the reaction is not spontaneous reaction.

The reaction is spontaneous if G is negative.