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Effect Of Volume Changes On Gas-phase Equilibria

Le Chatlier's Principle states that when a system at equilibrium undergoes a change in temperature, volume, or amount of moles present in a reactant or product, the system will respond in order to reach equilibrium. Think of a system at equilibrium as a balanced scale (equal weights on both sides) and when one side gains more weight, the scale will have to adjust the other side in order to reach equilibrium.

Introduction

In terms of volume changes within a system at equilibrium, the following applies:

  • When there is a decrease in volume, the equilibrium will shift to favor the direction that produces fewer moles of gas.
  • When there is an increase in volume, the equilibrium will shift to favor the direction that produces more moles of gas.

It's important to remember that these rules only apply to equations in which gases  are involved. If only solids and aqueous solutions are present, volume changes will have no effect on the equilibrium.

Conceptual Questions

  1. What happens when you increase the volume in a system at equilibrium that has equal moles of reactants and products and you increase the volume, in which direction will a net change occur in order to restore equilibrium? Decrease?
  2. What happens when you increase the volume in an equilibrium system in which there are more moles of reactants than products?
  3. What happens when you decrease the volume in an equilibrium system in which there are more moles of products than reactants?
  4. What happens when you increase the volume in an equilibrium system in which there are more moles of products than reactants?
  5. What happens when you decrease the volume in an equilibrium system in which there are more moles of reactants than products?

Practice Problems

How could one increase the amount of reactants produced in the following equation: 

\(N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)\)

a) increase volume b) decrease volume

Explain what would happen in each of the following equations if there is a DECREASE in volume:

\(2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)\)

\(H_2(g) + I_2(g) \rightleftharpoons 2HI(g)\)

\(CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)\)

\(AgCl(s) + \rightleftharpoons Ag^{+1}(aq) + Cl^-(aq)\) 

Summary

Le Chatlier's Principle applies to changes in the following:

  1. pressure changes
  2. concentration changes
  3. volume changes

When any system at equilibrium is disturbed, it will attempt to reach a new equilibrium.

Answers

  1. Because there is an equal number of moles on both sides of the reaction, an increase in volume will have no effect on the equilibrium and thus there is no shift in the direction. Similarly, when you decrease the volume there is no effect on the equilibrium.
  2. Because there are more moles of reactants, an increase in volume will shift the equilibrium to the left in order to favor the reactants.
  3. When there is a decrease in volume, the equilibrium will shift towards the side of the reaction with fewer moles. In this case, there are fewer moles of reactants and so the equilibrium will favor the reactants and shift to the left.
  4. An increase in volume always favors the direction that produces more moles of gas and because in this case there are more moles of products, the reaction will shift to the right and produce more moles products.
  5. Because a decrease in volume always favors the direction that produces fewer moles, this system will shift to right and produce more moles of products.

Answers

  1. In order to increase the amount of reactants formed, increase the volume so that it will shift to the right (towards the side with more moles of gas).
  2. Effects of a Decrease in Volume
  • The products are favored; shift to right.
  • Same number of moles on each side of equation; no effect.
  • The reactants are favored; shift to left.
  • Because no gases are present there is no change.

 Contributors

  •   Leah Hughes