Consider the reaction under standard state conditions: NH_4NO_3(s) rightarrow 2H

ID: 524674 • Letter: C

Question

Consider the reaction under standard state conditions: NH_4NO_3(s) rightarrow 2H_2O(g) + N_2O(g) (a) Calculate Delta H degree, Delta S degree, and Delta G degree for the reaction at 25.00 degree C. (b) The reaction is allowed to proceed until all substances involved have reached their equilibrium concentrations. Under those conditions, what is Delta G for the reaction? (c) What can be said about how Delta H degree, Delta S degree, and T affect the reaction's spontaneity? Discuss in detail. (d) Is NH_4NO_3 thermodynamically stable? Explain.

Explanation / Answer

NH4NO3 (s) -> 2 H2O (g) + N2O (g)

T = 298.15 K

Hfo

Gfo

kJ/mol

J/mol-K

NH4NO3

-365.6

-183.9

151.1

H2O

-241.8

-228.6

188.8

N2O

81.6

103.7

220

Hro = Standard heat of formation of products - Standard heat of formation of reactants

= 2 * (241.8) + (81.6) – (365.6) kJ/mol

= -36.4 kJ/mol

Gro = Standard Gibbs free energy of products - Standard Gibbs free energy of reactants

= 2 * (-228.6) + (103.7) – (-183.9) kJ/mol

= -169.6 kJ/mol

Gro = Hro - T Sro

-169.6 = -36.4 – 298.15 * Sro

Sro = 0.447 kJ/mol-K

Under equilibrium conditions, Gr = 0

For a spontaneous reaction, Gr < 0

Or Hr - T Sr < 0

Or Sr > Hr / T

At T = 25 °C, Hr is negative (exothermic in nature) and Sr is positive (entropy increases), the reaction is spontaneous.

Since at standard conditions of pressure and temperature, the reaction is spontaneous, ammonium nitrate (NH4NO3 (s)) is thermodynamically unstable.

Hfo

Gfo