Question

Explain how to recognize the conditions under which changes in pressure would affect systems at equilibrium.

Short answer

• If the number of moles of gas on product side is higher than on reactant side:
  The increase in pressure will shift equilibrium to the left (reactant side)
  The decrease in pressure will shift equilibrium to the right(product side)
• If the number of moles of gas on reactant side is higher than on product side:
  The increase in pressure will shift equilibrium to the right (product side)
  The decrease in pressure will shift equilibrium to the left(reactant side)
• If the number of moles of gas on reactant side is equal to the number of moles of gas on product side:
  Therefore, the change in pressure will not have an effect on equilibrium.

Step-by-step solution

Step 1:

Let us explain the conditions under which changes in pressure would affect systems at equilibrium.

• Le Châtelier’s principle - if a system at equilibrium is disturbed, the equilibrium will shift so as to tend to counteract the effect of the disturbance.
• The change in pressure will have a effect only on systems with gases and only if there are different numbers of moles of gas on the reactant and product sides of the equilibrium.
• If the number of moles of gas on product side is higher than on reactant side:

• The increase in pressure will shift equilibrium to the left (reactant side)
• The decrease in pressure will shift equilibrium to the right(product side)
• If the number of moles of gas on reactant side is higher than on product side:

• The increase in pressure will shift equilibrium to the right (product side)
• The decrease in pressure will shift equilibrium to the left(reactant side)
• If the number of moles of gas on reactant side is equal to the number of moles of gas on product side:

The change in pressure will not have an effect on equilibrium.