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Wize University Chemistry Textbook > Equilibrium

Le Chatelier's Principle [more detail]

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Changes to Concentration Time Graphs Based on Changes To The System

N2O4(g) + heat −>2NO2(g)N_2O_4\left(g\right)\ +\ heat\ ->2NO_2\left(g\right)_{_{ }}

1. Temperature Changes
a) Temperature is increased at 2 minutes
  • The equilibrium would shift (left/right/no shift):
    right
  • We would get (more/less)
    less
    reactants and (more/less)
    more
    products
  • What should the concentration time graph look like if equilibrium is established at 4 mins?



b) Temperature is decreased at 2 minutes
  • The equilibrium would shift (left/right/no shift):
    left
  • We would get (more/less)
    more
    reactants and (more/less)
    less
    products
  • What should the concentration time graph look like if equilibrium is established at 4 mins?




2. Concentration Changes
  • What will an increase or decrease look like on the concentration time graph?


  • What should the concentration time graph look like if [NO2] was increased and the new equilibrium is established at 4 mins?
  • The equilibrium would shift (left/right/no shift):
    left



3. Changes in Total Pressure
  • What will an increase in pressure or decrease in volume look like on the concentration time graph?
  • When total pressure increases, the concentration of every gas in the container initially increases at that moment (since the # of moles/unit volume increases), then the equilibrium will shift whichever way it needs to to counteract the stress
  • **Pressure or volume changes will result in vertical lines on the concentration time graphs
  • If we increase the pressure at time 2 minutes, it would look like this:





  • Fill in the rest of the concentration time graph after the pressure was increased:
  • When pressure is increased, eqm shift to side with (more/less)
    less
    moles, which is (left/right)
    right




4. Adding a catalyst
  • results in no changes to equilibrium concentrations so you wouldnt see any changes to the concentration graph!

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Le Châtelier's Principle - Change in Concentration

2SO2(g)+O2(g)⇌2SO3(g)+energy2SO_2 (g)+O_2 (g)⇌2SO_3 (g)+energy
  • SO2 is added to the reaction at some point in time. Thus the [SO2] is suddenly INCREASED. The equilibrium will shift to the RIGHT in order to counteract the sudden increase in the [SO2]. Thus [SO2] and [O2] will decrease and the [SO3] will increase

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Le Châtelier's Principle - Change in Volume/Pressure

2SO2(g)+O2(g)⇌2SO3(g)+energy2SO_2 (g)+O_2 (g)⇌2SO_3 (g)+energy
  • The volume of the container is suddenly DECREASED. Thus the concentrations of ALL gases initially increase. In order to counteract the imposed pressure increase, the equilibrium will shift to the side with LESS moles of gas. In this case, this would be a shift to the RIGHT where [SO2] and [O2] will decrease and the [SO3] will increase.

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Le Châtelier's Principle - Change in Temperature

2SO2(g)+O2(g)⇌2SO3(g)+energy2SO_2 (g)+O_2 (g)⇌2SO_3 (g)+energy
  • At some time, the temperature is decreased. We know that the equilibrium will shift to the RIGHT, towards the heat term in order to counteract the imposed change. During this shift to the right, the [SO2] and [O2] will decrease and the [SO3] will increase. This is not instant, but takes place gradually, until a NEW equilibrium is established.

  • A change in temperature will cause a change in the value of K!
  • For exothermic reactions, the equilibrium constant decreases with increasing temperature
  • For endothermic reactions, the equilibrium constant increases with increasing temperature
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Le Châtelier's Principle - Other

Addition of an inert gas

  • Addition of an inert gas at constant volume will cause an increase in total pressure. However, the partial pressures of the gases will remain the same. As such, the equilibrium concentrations and equilibrium constant values will not change.

Catalysts

  • Catalysts speed up the rate of a reaction, but they do not have an effect on equilibrium. They will cause a reaction to reach equilibrium faster, but equilibrium concentrations and equilibrium constants would not be affected.