Le Chatelier Shifts in Ammonia Synthesis

Given

• Reaction: N2(g) + 3H2(g) ⇌ 2NH3(g), delta H < 0 (exothermic)

• Gas amounts: 4 mol of reactant gas ⇌ 2 mol of product gas

• The system starts at equilibrium; each stress is applied separately.

Find

The direction of the equilibrium shift (toward more NH3 or less NH3) for each of the four stresses.

Principle

Le Chatelier's principle states that a system at equilibrium responds to a stress by shifting in the direction that relieves it. More gas moles on one side means higher pressure favors the side with fewer moles; heat can be treated as a product in an exothermic reaction, so warming favors the reactants; adding a reactant or removing a product pulls the equilibrium toward the products.

Formula

Solution

1. Note the two key facts: the left side has 4 moles of gas and the right side has 2, and heat is effectively a product because delta H < 0 (write it as N2 + 3H2 ⇌ 2NH3 + heat).

2. (a) Increase pressure (smaller volume): the system relieves the stress by moving to the side with fewer gas moles, 2 < 4, so the equilibrium shifts right, making more NH3.

3. (b) Increase temperature: adding heat to an exothermic reaction favors the side that consumes heat — the endothermic reverse direction — so the equilibrium shifts left, making less NH3.

4. (c) Add N2: raising a reactant concentration drives the forward reaction to consume the added N2, so the equilibrium shifts right, making more NH3.

5. (d) Remove NH3: lowering a product concentration pulls the forward reaction to replace it, so the equilibrium shifts right, making more NH3.

Unit check

This is a qualitative prediction problem, so no numerical units appear. The reasoning is consistent in moles of gas (4 vs 2) and in the sign of delta H (heat as a product), which are the two quantities Le Chatelier's principle compares.

What this means physically

Industrially, ammonia is made at high pressure (to push the equilibrium right) but not at low temperature (the equilibrium would love the cold, yet the reaction would be too slow). This example shows the practical tension: Le Chatelier tells you where equilibrium goes, while kinetics tells you how fast you get there.