Question

Consider the following reaction between oxides of nitrogen: NO2(g)+N2O(g)?3NO(g)

1.

Calculate ?G? at 800 K, assuming that ?H? and ?S? do not change with temperature.

Express your answer using two significant figures.

2.Calculate ?G? at 1000 K.

Answer 1

In order to calculate G value we need to find H and S.

We can find H from the following data.

Hf (N2O) = 82.05KJmol^-1

Hf (NO2) = 33.18KJmol^-1

Hf (NO) = 90.25KJmol^-1

the given thermochemical reaction is

NO2(g)+N2O(g) ------------------------> 3NO(g)

H(reaction) = Hf(products) - Hf(reactants)

= 3xHf (NO) - Hf (N2O) - Hf (NO2)

= 3x90.25KJmol^-1 - 82.05KJmol^-1 - 33.18KJmol^-1 = 155.52KJ

Now we can find  S from the following data

S (N2O) = 219.17Jmol^-1K^-1

S(NO2) = 240.00Jmol^-1K^-1

S (NO) = 210.6Jmol^-1K^-1

the given thermochemical reaction is

NO2(g)+N2O(g) ------------------------> 3NO(g)

S(reaction) = Sproducts) - S(reactants)

= 3xS(NO) - S(N2O) - S(NO2)

= 3x210.6Jmol^-1K^-1 - 219.17Jmol^-1K^-1 - 240.00Jmol^-1K^-1 = 172.63 Jmol^-1K^-1 = 0.17263KJmol^-1K^-1

1. Now G at 800K can be calculated from the following formulae

  G =  H(reaction) - T S(reaction) = 155.52KJ - 800x0.17263KJmol^-1K^-1 = 17.42 KJ (answer)

2. G at 1000K

  G =  H(reaction) - T S(reaction) = 155.52KJ - 1000x0.17263KJmol^-1K^-1 = - 17.11 KJ (answer)