Equilibrium (II)

• A dynamic equilibrium is the equilibrium that exists in a closed system when the rate of the forward reaction is equal to that of the reverse reaction.
• Le Chatelier's Principle states that when a system in dynamic equilibrium is subject to change, the equilibrium position will shift to minimise the change.
• Kc is the equilibrium constant in terms of concentration at a constant temperature.
• Kp is the equilibrium constant in terms of partial pressure at a constant temperature.
• Partial pressure is the pressure that would be exerted by a gas in a mixture of gases if it occupied the same volume on its own at the same temperature.
• Mole fraction is the amount in moles of a component in a mixture divided by the total amount of all components in moles in the mixture.
• Homogenous equilibrium is when reactants and products are present in the same phase, e.g. everything is a gas, or everything is in solution.
• Heterogenous equilibrium is when reactants and products are present in more than one phase, e.g. reactions including a variety of solids, liquids and/or gases.
• What is a closed system?

  A system isolated from its surroundings so no energy/particles enter and affect the reaction.
• Describe dynamic equilibrium in terms of rates.

  The rate of the forward reaction is equal to the rate of the reverse reaction.
• Describe dynamic equilibrium in terms of concentrations.

  The concentrations of the reactants and products do not change.
• If you disrupts a dynamic equilibrium, it will do everything it can to return to how it was before the disruption by doing the opposite of what the disruption was.
• Factors that may affects the position of equilibrium include:

  • concentration of a solution
  • pressure of gasses
  • temperature
• If you make a dynamic equilibrium hotter, it will try to get colder.
• If you try to increase the pressure of a dynamic equilibrium, the reaction will try to decrease the pressure.
• If you try to decrease the pressure of a dynamic equilibrium, the reaction will try to increase the pressure.
• Reaction: A + B ⇌ C + D

  Describe and explain the effect of increasing the concentration of A on the position of equilibrium.

  Equilibrium will shift to the right to decrease the concentration of A, increasing the amount of C and D.
• Reaction: A + B ⇌ C + D

  Describe and explain the effect of decreasing the concentration of A on the position of equilibrium.

  Equilibrium will shift to the left to increase the concentration of A, increasing the amount of A and B.
• Reaction: A + B ⇌ C + D

  Describe and explain the effect of increasing the concentration of C on the position of equilibrium.

  Equilibrium position will shift to left to decrease the concentration of C, increasing the amount of A and B.
• Reaction: A + B ⇌ C + D

  Describe and explain the effect of decreasing the concentration of D on the position of equilibrium.

  Equilibrium position will shift to the right to increase the concentration of D, increasing the amount of C and D.
• A reaction is brown in the left reaction and colourless in the forward reaction. The reaction system reached equilibrium and was a pale brown colour. Explain why the system had a constant colour at equilibrium.

  The colour remains constant as the rate of the forward is equal to the rate of the reverse reaction, so the products are produced at the same rate.
• Consider the reaction: A + B ⇌ C + D, ΔH = -53 kJmol-1.

  What does ΔH = -53 kJmol-1 mean?

  The forward reaction is exothermic. 52kJ of energy is released for each mole of A (or B) reacted.
• Consider the reaction: A + B ⇌ C + D, ΔH = -53 kJmol-1.

  What would happen if the temperature of the system was increased?

  The equilibrium position will shift to the left to absorb heat energy because the reverse reaction if endothermic. As a result, the amount of A & B will increase.
• Consider the reaction: A + B ⇌ C + D, ΔH = -53 kJmol-1.

  What would happen if the temperature of the system was decreased?

  The equilibrium position will shift to the right to release heat energy because the forward reaction is exothermic. As a result, the amount of C & D will increase.
• What gases are involved in the equilibrium reaction described in the study material?

  Brown gas NO2 and colourless gas N2O4
• What is the significance of the equilibrium position in the reaction between NO2 and N2O4?

  It indicates the balance between the concentrations of NO2 and N2O4 in a closed system
• How can temperature changes affect the equilibrium position of the NO2 and N2O4 reaction?

  • Increasing temperature favors the endothermic reaction, shifting equilibrium to the left (more NO2).
  • Decreasing temperature favors the exothermic reaction, shifting equilibrium to the right (more N2O4).
• What is the first step to investigate the effect of temperature on the equilibrium position of NO2 and N2O4?

  Place two sealed tubes containing the equilibrium mixture in water baths at different temperatures
• What observation would you expect from the tube placed in the warm water bath?

  The tube will change to a darker brown colour
• Why does the tube in the warm water bath change to a darker brown colour?

  Because the endothermic reaction speeds up to absorb the extra heat
• What happens to the equilibrium position when the temperature is increased in this reaction?

  The equilibrium shifts to the left
• What observation would you expect from the tube placed in the cool water bath?

  The tube will lose colour
• Why does the tube in the cool water bath lose colour?

  Because the exothermic reaction speeds up to try and replace the lost heat
• What happens to the equilibrium position when the temperature is decreased in this reaction?

  The equilibrium shifts to the right