92.9k views
1 vote
Nitrogen reacts with hydrogen to form ammonia

N2 + 3H2 → 2NH3
The bond energies involved:
H-H 436 kJ/mol
N-N 945 kJ/mol
N-H 391 kJ/mol
a) Calculate the enthalpy change, ΔH, for the reaction.
This reaction can be reversed.
2NH3 → N2 + 3H2
b) What would be the enthalpy change, ΔH, for this reaction?

User DsRaj
by
7.7k points

1 Answer

4 votes

Answer:

a. 2433 kJ/mol

b. 93 kJ/mol

Step-by-step explanation:

a) To calculate the enthalpy change of the reaction, we need to use the bond energies of the reactants and products. The enthalpy change is the difference between the energy required to break the bonds in the reactants and the energy released when new bonds are formed in the products. Using the bond energies given:

Reactants:

N≡N: 1 × 945 kJ/mol = 945 kJ/mol

H-H: 3 × 436 kJ/mol = 1308 kJ/mol

Products:

N-H: 2 × 391 kJ/mol = 782 kJ/mol

N-H: 2 × 391 kJ/mol = 782 kJ/mol

N-H: 2 × 391 kJ/mol = 782 kJ/mol

The balanced equation for the reaction is:

N2 + 3H2 → 2NH3

The total energy required to break the bonds in the reactants is:

945 kJ/mol + 1308 kJ/mol = 2253 kJ/mol

The total energy released when new bonds are formed in the products is:

2 × 782 kJ/mol + 2 × 782 kJ/mol + 2 × 782 kJ/mol = 4686 kJ/mol

Therefore, the enthalpy change for the reaction is:

ΔH = energy released - energy required = 4686 kJ/mol - 2253 kJ/mol = 2433 kJ/mol

b) The enthalpy change for the reverse reaction can be calculated using the same approach. The balanced equation for the reverse reaction is:

2NH3 → N2 + 3H2

The total energy required to break the bonds in the products is:

N2: 1 × 945 kJ/mol = 945 kJ/mol

H-H: 3 × 436 kJ/mol = 1308 kJ/mol

The total energy released when new bonds are formed in the reactants is:

N-H: 6 × 391 kJ/mol = 2346 kJ/mol

Therefore, the enthalpy change for the reverse reaction is:

ΔH = energy released - energy required = 2346 kJ/mol - 2253 kJ/mol = 93 kJ/mol

Note that the enthalpy change for the reverse reaction is much smaller than for the forward reaction. This means that the forward reaction is more exothermic and thus more favorable than the reverse reaction.

User Smackenzie
by
8.2k points