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Wize High School Grade 12 Chemistry Textbook > Equilibrium

Le Chatelier's Principle

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Le Chatelier's Principle


When any system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to counteract the effect of the applied change in order to establish a new equilibrium.


N2 (g) + 3H2 (g) ⇌ 2NH3 (g)             (ΔH = < 0)

Which direction will the equilibrium shift in the following scenarios?


1) If we add N2(g), equilibrium will shift to the
right
2) If we take away H2(g), equilibrium will shift to the
left
3) If we increase the volume of the container:
  • Does the pressure inside the container increase or decrease?
    decrease
  • If pressure is decreased, the equilibrium will want to shift to the side with
    more
    moles of gas
  • If pressure is increased, the equilibrium will want to shift to the side with
    less
    moles of gasd
  • In our example, the equilibrium will shift to the
    left
    side since it has
    more
    moles

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N2 (g) + 3H2 (g) ⇌ 2NH3 (g)             (ΔH = < 0)


4) If we add heat:
  • We want to treat temperature as a reagent
  • To do this, we need to figure out what side of the reaction temperature is on for this particular reaction
  • We are given an enthalpy value that is negative, does this mean that our reaction is endothermic or exothermic?
    exothermic
  • Based on this, does our reaction require energy or is it releasing energy?
  • It
    releases
    energy
  • So would we expect to see temperature on the left or right side of the equation? (write it above)
    right
    side
  • Now this question is asking what direction the equilibrium will shift, so treat temperature as any other reagent on the
    right
    side
  • Equilibrium will shift to the
    left
    !

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5) For the following reaction what direction would the equilibrium shift if we added more AgCl(s):

AgCl(s) ⇌ Ag+(aq) + Cl-(aq)

Remember that K or Q does not include solids or liquids.
This means that if we change the concentration of a solid or liquid it would not cause any change to the equilibrium! Q would still be equal to K.
No shift in equilibrium.

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N2 (g) + 3H2 (g) ⇌ 2NH3 (g)             (ΔH = < 0)

6) If we added a catalyst what direction would the equilibrium shift in?
  • Catalysts → Kinetics
  • Equilibrium → Thermodynamics
  • A catalyst would not affect equilibrium. A catalyst speeds up the rate of the forward and backward reactions equally. As a result, equilibrium amounts aren't changed!
  • No shift in equilibrium

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N2 (g) + 3H2 (g) ⇌ 2NH3 (g)             (ΔH = < 0)

7) If we add an inert gas to the mixture, what direction would the reaction shift:

a) If the container keeps a constant volume
  • This is a rigid container. Remember for Kp we are concerned with the partial pressures of each gas. Inert gases are non-reactive so wouldnt react with anything. Would the partial pressures ratios of the gases change?
  • Remember that partial pressure is the pressure that the gas would have if it was alone in the container, the partial pressure ratios would not change (if there was 2x NH3 than H2 that would still be the case after the inert gas was added)
  • Therefore the equilibrium would not shift



b) Low yield: If the container was a constant pressure container
  • If we add an inert gas this time, it increases the total pressure in the container
  • However, this container wants to have a constant pressure, so to avoid that increase in pressure it would change its volume....it would increase its volume so that the pressure remains constant
  • Now you can treat this scenario as an "increase in volume" scenario -> think of it as before, increase in volume means decrease in pressure, shift to the side with more moles! (shift left in our case)
Given the following exothermic reaction, decide which direction the equilibrium will shift (if any) in each scenario.

CH4(g) + 2H2S(g) ⇌ CS2(g) + 4H2(g)
The concentration of dihydrogen sulfide is decreased.
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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!

checklist
Mark Yourself Question
  1. Grab a piece of paper and try this problem yourself.
  2. When you're done, check the "I have answered this question" box below.
  3. View the solution and report whether you got it right or wrong.
Depending on the season, the skyline in busy cities can look different.
One thing that contributes to this is the following reaction:

Heat + N2O4(g) ⇌ 2NO2(g)

N2O4(g) is a colourless gas while NO2(g) is a brown gas. Can you explain why the city skyline may look one colour in the summer (with high temperatures) and a different colour in the winter? What season would the air be the most brown looking?

Practice: Le Chatelier's Principle

The reaction below is at equilibrium. You want to apply Le Chatelier’s principle to produce a higher quantity of product gases. Which change will accomplish this effect?

CO2 (g) + 2 H2O (l) ⇌ CH4 (g) + 2 O2 (g)
Extra Practice
Le Chatelier's Principle: Equilibrium constant
Consider the following reaction:
PCℓ5(g)⇌PCℓ3(g)+Cℓ2(g)PCℓ_5 (g) ⇌ PCℓ_3 (g) + Cℓ_2 (g)
The measured equilibrium constant is 5.0x10–4 at 373 K and 0.20 at 473 K. Which of the following statements about the reaction must be true?
Le Chatelier's Principle
Use the following reaction scenario to answer the following questions
SO2(g)+Cℓ2(g)⇌SO2Cℓ2(g)ΔrH°<0SO_2 (g) + Cℓ_2 (g) ⇌ SO_2Cℓ_2 (g) \qquad Δ_rH° < 0
You have prepared a container with these compounds and allowed it to come to equilibrium. You now have a choice of different ways to modify your system at equilibrium:
Le Chatelier’s Principle
In an esterification reaction, a carboxylic acid and an alcohol react to form an ester and water. This reaction establishes an equilibrium because the reverse process can also occur. If the water is allowed to boil and leave the system as water vapour, how will the equilibrium respond?