Question

Does the number of moles of reaction products increase, decrease or remain same when each of the following equilibria is subjected to a decrease in pressure by increasing the volume?

1. \(PCl_5 (g) ⇋ PCl_3 (g) + Cl_2 (g)\)
2. \(CaO (s) + CO_2 (g) ⇋ CaCO_3 (s)\)
3. \(3Fe (s) + 4H_2O (g) ⇋ Fe_3O_4 (s) + 4H_2 (g)\)

Answer 1

Concept:

Decrease in pressure (by increasing volume) shifts the equilibrium towards the side having more number of moles of gaseous species.

Solids do not affect pressure and are not counted.

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(i) PCl₅(g) ⇋ PCl₃(g) + Cl₂(g)

Number of gaseous moles: 
Reactant side = 1 mole 
Product side = 2 moles

On decreasing pressure, equilibrium shifts towards the side with more moles of gas, i.e. products.

Number of moles of products increases.

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(ii) CaO(s) + CO₂(g) ⇋ CaCO₃(s)

Number of gaseous moles: 
Reactant side = 1 mole (CO₂) 
Product side = 0 mole

On decreasing pressure, equilibrium shifts towards the side with more gaseous moles, i.e. reactants.

Number of moles of products decreases.

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(iii) 3Fe(s) + 4H₂O(g) ⇋ Fe₃O₄(s) + 4H₂(g)

Number of gaseous moles: 
Reactant side = 4 moles (H₂O) 
Product side = 4 moles (H₂)

Since number of gaseous moles is same on both sides, pressure change has no effect.

Number of moles of products remains the same.

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Final Answer: 

Reaction	Effect on number of moles of products
(i)	Increases
(ii)	Decreases
(iii)	Remains the same