Le Chantelier's Principle

• Reversable reactions try to counteract changes
• Le Chantelier's Principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change
• The Chantelier's Principle can be used to predict the effect of any changes you make to a reaction system
• All reactions are exothermic in one direction and endothermic in the other
• If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat. This means you'll get more products for the exothermic reaction and fewer products for the endothermic reaction
• If you raise the temperature, the equilibrium will move in the endothermic direction to try to decrease it. You'll now get more products for the endothermic reaction and fewer products for the exothermic reaction
• N2 + 3H2 ⇌ 2NH3

  Here the forward reaction is exothermic - a decrease in temperature moves equilibrium to the right (more NH3)
• Changing the pressure only affects an equilibrium involving gases
• If you increase the pressure, the equilibrium tries to reduce it - it moves in the direction where there are fewer molecules of gas
• If you decrease the pressure, the equilibrium tries to increase it - it moves in the direction where there are more molecules of gas
• You can use the balanced symbol equation for a reaction to see which side has more molecules of gas
• N2 + 3H2 ⇌ 2NH3

  There are 4 molecules on the left but only 2 on the right. So, if you increase the pressure, the equilibrium shifts to the right (more NH3)
• If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium. So the system responds to bring itself back to equilibrium again
• If you increase the concentration of the reactants the system tries to decrease it by making more products
• If you decrease the concentration of the products the system tries to increase it again by reducing the amount of reactants
• N2 + 3H2 ⇌ 2NH3

  If more N2 or H2 is added, the forward reaction increases to produce more NH3