
The major industrial use of hydrogen is in the production of ammonia by the Haber process:
3H2(g)+N2(g)→2NH3(g)
a. Using data from Appendix 4, calculate ∆H°, ∆S°, and ∆G° for the Haber process reaction.
b. Is the reaction spontaneous at standard conditions?
c. At what temperatures is the reaction spontaneous at standard conditions? Assume ∆H° and ∆S° do not depend on temperature.
(a)

Interpretation: The values of ΔS∘,ΔH∘ and ΔG∘ is to be calculated at 25 °C for the given reaction. If the reaction is spontaneous is to be stated. The temperature at which the reaction is spontaneous is to be calculated.
Concept introduction:
The expression for ΔS∘ is,
ΔS∘=∑npΔS∘(product)−∑nfΔS∘(reactant)
The expression for ΔH∘ is,
ΔH∘=∑npΔH∘(product)−∑nfΔH∘(reactant)
The expression for ΔG∘ is,
ΔG∘=∑npΔG∘(product)−∑nfΔG∘(reactant)
Explanation of Solution
The stated reaction is,
3H2(g)+N2(g)→2NH3(g)
Refer to Appendix 4.
The value of ΔS∘(J/K⋅mol) for the given reactant and product is,
Molecules | ΔS∘(J/K⋅mol) |
NH3(g) | 193 |
N2(g) | 192 |
H2(g) | 131 |
The formula of ΔS∘ is,
ΔS∘=∑npΔS∘(product)−∑nfΔS∘(reactant)
Where,
- ΔS∘ is the standard entropy of reaction.
- np is the number of moles of each product.
- nr is the number of moles each reactant.
- ΔS∘(product) is the standard entropy of product at a pressure of 1 atm.
- ΔS∘(reactant) is the standard entropy of reactant at a pressure of 1 atm.
Substitute all values from the table in the above equation.
ΔS∘=∑npΔS∘(product)−∑nfΔS∘(reactant)=[2(193)−{3(131)+(192)}] J/K=−199 J/K_
The value of ΔH∘(kJ/mol) for the given reactant and product is,
Molecules | ΔH∘(kJ/mol) |
NH3(g) | −46 |
N2(g) | 0 |
H2(g) | 0 |
The formula of ΔH∘ is,
ΔH∘=∑npΔH∘(product)−∑nfΔH∘(reactant)
Where,
- ΔH∘ is the standard enthalpy of reaction.
- np is the number of moles of each product.
- nr is the number of moles each reactant.
- ΔH∘(product) is the standard enthalpy of product at a pressure of 1 atm.
- ΔH∘(reactant) is the standard enthalpy of reactant at a pressure of 1 atm.
Substitute all values from the table in the above equation.
ΔH∘=∑npΔH∘(product)−∑nfΔH∘(reactant)=[2(−46)−{3(0)+(0)}] kJ=−92 kJ_
The value of ΔG∘(kJ/mol) for the given reactant and product is,
Molecules | ΔG∘(kJ/mol) |
NH3(g) | −17 |
N2(g) | 0 |
H2(g) | 0 |
The formula of ΔG∘ is,
ΔG∘=∑npΔG∘(product)−∑nfΔG∘(reactant)
Where,
- ΔG∘ is the standard Gibb’s free energy of reaction.
- np is the number of moles of each product.
- nr is the number of moles each reactant.
- ΔG∘(product) is the standard Gibb’s free energy of product at a pressure of 1 atm.
- ΔG∘(reactant) is the standard Gibb’s free energy of reactant at a pressure of 1 atm.
Substitute all values from the table in the above equation.
ΔG∘=∑npΔG∘(product)−∑nfΔG∘(reactant)=[2(−17)−{3(0)+(0)}] kJ=−34 kJ_
(b)

Interpretation: The values of ΔS∘,ΔH∘ and ΔG∘ is to be calculated at 25 °C for the given reaction. If the reaction is spontaneous is to be stated. The temperature at which the reaction is spontaneous is to be calculated.
Concept introduction:
The expression for ΔS∘ is,
ΔS∘=∑npΔS∘(product)−∑nfΔS∘(reactant)
The expression for ΔH∘ is,
ΔH∘=∑npΔH∘(product)−∑nfΔH∘(reactant)
The expression for ΔG∘ is,
ΔG∘=∑npΔG∘(product)−∑nfΔG∘(reactant)
Explanation of Solution
A reaction is said to be spontaneous if the value of ΔG is negative.
Since, the value of ΔG for the given reaction is −34 kJ, therefore, the given reaction is spontaneous.
The formula of ΔG is,
ΔG=ΔH−TΔS
Where,
- ΔH is the enthalpy of reaction.
- ΔG is the free energy change.
- T is the temperature.
- ΔS is the entropy of reaction.
Since, the value of ΔH is negative, therefore, the reaction is spontaneous at lower temperature.
(c)

Interpretation: The values of ΔS∘,ΔH∘ and ΔG∘ is to be calculated at 25 °C for the given reaction. If the reaction is spontaneous is to be stated. The temperature at which the reaction is spontaneous is to be calculated.
Concept introduction:
The expression for ΔS∘ is,
ΔS∘=∑npΔS∘(product)−∑nfΔS∘(reactant)
The expression for ΔH∘ is,
ΔH∘=∑npΔH∘(product)−∑nfΔH∘(reactant)
The expression for ΔG∘ is,
ΔG∘=∑npΔG∘(product)−∑nfΔG∘(reactant)
Answer to Problem 54E
Solutions are as follows.
Explanation of Solution
The value of ΔS∘ for the given reaction is −199 J/K.
The value of ΔH∘ for the given reaction is −92 kJ.
The negative value of ΔH∘ suggests that the give reaction is exothermic. But ΔS∘ favors opposite process. These opposite tendencies cancel out that means that the reaction has achieved equilibrium. The value of ΔG∘ at equilibrium is zero.
The formula of ΔG is,
ΔG∘=ΔH∘−TΔS∘
Where,
- ΔH∘ is the enthalpy of reaction.
- ΔG∘ is the free energy change.
- T is the temperature.
- ΔS∘ is the entropy of reaction.
Substitute the values of ΔS∘,ΔG∘ and ΔH∘ in the above equation.
ΔG∘=ΔH∘−TΔS∘T=ΔH∘ΔS∘−ΔG∘=−92×103 J−199 J/K−0=462.3 K_
The given reaction will be spontaneous below 462.3 K_.
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