Question

How to calculate the number of phase components and degrees of freedom of the following system or reaction :N2(g)+3H2(g) = 2NH3(g) and this reaction is in equilibrium

Answer 1

All chemical reactions are broadly classified into 2 types:

1) Irreversible Reactions : Zn + H2SO4 −−−−−> ZnSO4 + H2↑

2) Reversible reactions:

(a) Homogeneous reversible reactions

Eg: N2(g ) + 3H2(g) 2NH3(g)

(b) Heterogeneous reversible reactions

Eg: CaCO3(s) CaO(s) + CO2(g)

The reversible reactions are represented by 2 arrows in the opposite directions.

The homogeneous reversible reactions can be studied by the law of mass action and the

heterogeneous reversible reactions using the phase rule, given by Willard Gibbs (1874)

which is defined as,

PHASE RULE:

If the equilibrium between any numbers of phases is not influenced by

gravitational/electrical/magnetic forces but is influenced by pressure, temperature and

concentration, then the number of degrees of freedom (F) is related to the number of

components (C) and the number of phases (P) as: F = C − P + 2

Answer 2

Concept:

Gibbs phase rule: The equality gives this rule, which applies to non-reactive multi-component heterogeneous systems in thermodynamic equilibrium.

Given:

The reaction is: N₂(g)+3H₂(g) ⇄ 2NH₃(g)

Find: How to calculate the number of phases, components, and degrees of freedom of the following system or reaction: N₂(g)+3H₂(g) ⇄ 2NH₃(g) and this reaction is in equilibrium?

Solution:

Gibbs phase rule can be expressed as:

            P + F - C = 2

                   P + F = C + 2

where P is the number of phases in thermodynamic equilibrium.

F is the number of degrees of freedom.

C is the number of components.

In the given reaction system,

P = 1, all the species are in the gaseous form.

F =?

C = 2, it is so because if the reactants and the products are in equilibrium then the number of components gets reduced by one.

Now, degrees of freedom can be calculated as follows below:

F + P = C+ 2

F + 1 = 2 + 2

    F = 4 - 1

    F = 3

Therefore, the degrees of freedom is 3.

Hence, the number of phases, components, and degrees of freedom of the given reaction system are 1, 2, and 3 respectively and this reaction is in equilibrium.

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