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Wize University 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

PAGE BREAK
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)

Practice: Le Chatelier's Principle (Direction of Equilibrium Shift)

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.

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
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?
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)
Consider the following reaction:
2 As(s) + 6 NaOH(aq) → 2 Na3AsO3(aq) + 3 H2(g)
If the pressure of the reaction is increased, how will this change the equilibrium? Why?
Consider the following reaction:
2 As(s) + 6 NaOH(aq) → 2 Na3AsO3(aq) + 3 H2(g)
If the produced H2 gas is removed from the reaction upon being produced, which direction will this drive the equilibrium? Why?
Look at the following reaction.
BaCrO4(s) <----> Ba2+(aq) + CrO42-(aq)
If PbCrO4 is added to the solution, which direction would we expect the equilibrium to shift?
Le Chatelier's Principle
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)
Le Chatelier's Principle
Consider the following reaction at equilibrium in a closed container:
2HgO(s)⇌2Hg(l)+O2(g)ΔH°=+181.4kJ2HgO(s) \rightleftharpoons 2Hg (l) + O_2 (g) \qquad \Delta H° = +181.4kJ
The partial pressure of O2 may be increased by:
Le Chatelier's Principle
Consider the information provided in the table on the right for the following reaction:
H2(g)+Cl2(g)⇌2HCl(g)H_2(g) +Cl_2(g)\rightleftharpoons 2HCl(g)
Which of the following statement is true?
Le Chatelier's Principle
Which of the following changes would allow the formation of more CrCI3? Assume the system is at equilibrium before these changes are done.
4Cr(s)+3CCl4(g)⇌4CrCl3(g)+3C(s)ΔH=26.9kJ4Cr(s)+3CCl_4 (g)\rightleftharpoons4CrCl_3 (g)+3C(s)\qquad ΔH = 26.9 kJ
Le Chatelier's Principle
For the equilibrium process 2 NO (g) ⇌ N2 (g) + O2 (g), Kc = 2.1x1030 at 20oC and ΔHo = -413 kJ/mol. If O2 is injected into the mixture at equilibrium in a sealed rigid vessel, which of the following changes will occur?
Le Chatelier's Principle
For the equilibrium process 2 BrCl (g) ⇌ Br2 (g) + Cl2 (g), which of the following changes will cause additional Cl2 to be formed.
1. Addition of Br2
ii. Addition of BrCl
For the following reaction at equilibrium, list three ways other than concentration to shift the reaction to favor the formation of products?
2NOBr(g)⇌2NO(g)+Br2(g)                 ΔHorxn=30KJ/mol2NOBr(g)\rightleftharpoons 2NO(g) +Br_2(g) \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \Delta H^orxn=30KJ/mol
Le Chatelier's Principle
Determine the shift in equilibrium when the following changes are introduced to the following equilibrium:
(NH4)(H2NCO2)(S)⇌2NH3(g)+CO2(g)(NH_4)(H_2NCO_2)(S) \rightleftharpoons 2NH_3(g)+CO_2(g)
Volume of the container is expanded by 10 L.
Le Chatelier's Principle
Consider the information provided in the table on the right for the following reaction:
H2(g)+Cl2(g)⇌2HCl(g)H_2(g) +Cl_2(g)\rightleftharpoons 2HCl(g)
Which of the following statement is true?
Le Chatelier's Principle
Which ONE of the following changes will, at least in principle, increase the amount of C6H12O6 (s) at equilibrium in the reaction:
6 CO2 (g) + 6 H2O (l) ⇌ C6H12O6 (s) + 6 O2 (g) ΔHo is positive in the forward direction
Le Chatelier's Principle
Which of the following changes would allow the formation of more CrCI3? Assume the system is at equilibrium before these changes are done.
The equation for the reaction is:
Cr2O3(s)+3CCl4(l) ⇌ 2CrCl3​(s)+3COCl2​(g)
One of the demonstrations in this course was about the effect of temperature in chemical equilibrium. NO2 (a brown gas) was kept in a flask in equilibrium with N2O4 (a colourless gas). When the flask was immersed in liquid nitrogen, the mixture inside became colourless. On the other hand, when the flask was immersed in hot water, the mixture inside was brown. What can you say about the equilibrium: N2O4 (g) ⇌ 2 NO2 (g) from these observations?
Le Chatelier's Principle
Consider the equilibrium 4 NH3 (g) + 5 O2 (g) ⇌ 4 NO (g) + 6H2O (g). Which one of the following statements correctly describes what happens when the described change is made in the reaction vessel? Interpret the arrow symbol → to mean “has the result that … “
Le Chatelier's Principle
For the process: 2 ICl (s) ⇌ I2 (s) + Cl2 (g)
At equilibrium, which of the following changes will cause additional Cl2 to be formed?
Le Chatelier's Principle
For the equilibrium process 2 NO (g) = N2 (g) + O2 (g), Kc = 2.1x1030 at 20oC and ΔHo = -413 kJ/mol. If the temperature of a mixture at equilibrium is increased from 20oC to 30oC, which of the following will occur?
Which of the following statement is correct
Which of the following equilibrium will not respond to a change in container volume?
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?
The Haber-Bosch process is the process used to produce industrial quantities of ammonia, most of which is used to make fertilizer.
N2(g)+3 H2(g)⇌2 NH3(g)N_{2(g)}+3\ H_{2(g)}\rightleftharpoons 2\ NH_{3(g)}
The equilibrium shifts left when,
(Duplicated)
For the following reaction predict the change in K when a catalyst is added.
(Duplicated)
For the following reaction predict the direction the equilibrium will shift when the total pressure is decreased?
(Duplicated)
For the following reaction predict the direction the equilibrium will shift when steam is added to the reaction?
(Duplicated)
For the following reaction predict the change in K when the temperature is increased?
(Duplicated)
For the following reaction predict the direction the equilibrium will shift when CO2 is selectively removed?
For the following reaction predict the direction the equilibrium will shift when the pressure of O2 is increased?
Le Chatelier's Principle
Of the following equilibria, only ______ will shift to the left in response to a decrease in volume.
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)
For the equilibrium process 2 NO (g) ⇌ N2 (g) + O2 (g), Kc = 2.1x1030 at 20oC and ΔHo = -413 kJ/mol. If O2 is injected into the mixture at equilibrium in a sealed rigid vessel, which of the following changes will occur?
Le Chatelier's Principle
For the equilibrium process 2 BrCl (g) ⇌ Br2 (g) + Cl2 (g), which of the following changes will cause additional Cl2 to be formed.
1. Addition of Br2
2. Addition of BrCl
Le Chatelier's Principle
For the equilibrium process 2 NO (g) = N2 (g) + O2 (g), Kc = 2.1x1030 at 20oC and ΔHo = -413 kJ/mol. If the temperature of a mixture at equilibrium is increased from 20oC to 30oC, which of the following will occur?
Le Chatelier's Principle
Which of the following changes would allow the formation of more CrCI3? Assume the system is at equilibrium before these changes are done.
Le Chatelier's Principle: Percent dissociation
A solution made by dissolving 0.1 mol of acetic acid in 1.0 L of water is 4.1% dissociated. Which of the modifications below would cause the % dissociation of the acetic acid in the container to increase? (Ka for acetic acid is 1.8×10-5.)
Which of the changes below would result in a shift towards the products if applied to this system at equilibrium?
CO2(g)+C(graphite)⇌2CO(g)CO_2 (g)+C(graphite)⇌2CO(g)
Le Chatelier's Principle
How many of these reactions will shift towards the products, if the size of the reaction container is decreased after equilibrium has been reached?
Le Chatelier's Principle
For the reaction of barium carbonate shown here, which of these changes suggested below will result in a decrease in pH at equilibrium? Assume that the reaction is at equilibrium before making any change, and that there is excess BaCO3 and H2O present.
BaCO3(s)+2H+(aq)⇌Ba2+(aq)+CO2(g)+H2O(l)BaCO_3 (s)+2H^+ (aq)⇌Ba^{2+} (aq)+CO_2 (g)+H_2 O(l)
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)\qquadΔ_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. Assume that during each of these changes, all other parameters remain constant. Select all that apply.
For the reaction X+Y⇌2ZX+Y \rightleftharpoons2Z, a system at equilibrium is initially prepared. The equilibrium concentrations are given by the following bar chart:
If half of the amount of Z is removed from the system, which of the following bar charts represents the concentrations in the system when it returns to equilibrium?
Over time, quicklime (calcium oxide) reacts with carbon dioxide in the air to produce calcium carbonate, as shown here:
CaO(s)+CO2(g)⇌CaCO3(s)CaO(s)+ CO_2(g)⇌ CaCO_3(s)
Suppose that you want to increase the yield of CaCO3(s) produced by shifting the equilibrium towards the products. Which of these suggested changes would accomplish this? Select all that apply
Which of the following reactions will shift towards the products if the size of the reaction container is increased after equilibrium was reached? Select all that apply.
Use the following information for both of the next two questions
We can easily observe shifts in the equilibrium of the following reaction because the reactants and products have different colours.
2NO2(g)⇌N2O4(g)Dark redColourless\def\arraystretch{1.5} \begin{array}{ccc} &2NO_2(g)&⇌&N_2O_4(g) \\ &\rm Dark \ red && \rm Colourless \end{array}
Consider the endothermic reaction: NH4Cl(s)⇌NH4+(aq)+Cl−(aq)NH_4Cl(s)\rightleftharpoons NH_4^+(aq)+Cl^-(aq). Which of the following does not occur when the reaction temperature is decreased?
Suppose you know that the reaction A⇌BA \rightleftharpoons B is endothermic. What happens to the rates of the forward and reverse reactions when the reaction temperature is increased?
Le Chatelier's Principle: Exothermic Reaction
Carbon disulfide, CS2, and chlorine gas react according to the following exothermic equilibrium reaction:
CS2(g)+3Cl2(g)⇌S2Cl2(g)+CCl4(g)CS_2 (g)+3Cl_2 (g)⇌S_2 Cl_2 (g)+CCl_4 (g)
If a vessel containing a mixture of these compounds starts at equilibrium and is exposed to a change, which of the following statements is TRUE?
Le Chatelier's Principle
Over time, quicklime (calcium oxide) reacts with carbon dioxide in the air to produce calcium carbonate, as shown here:
CaO(s)+CO2(g)⇌CaCO3(s)CaO(s) + CO_2(g) ⇌ CaCO_3(s)
Suppose that you want to increase the yield of CaCO3(s) produced by shifting the equilibrium towards the products. Which of these suggested changes would accomplish this?