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Le Chatelier's Principle (DP IB Chemistry)

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Caroline Carroll

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DP Chemistry: SLDP Chemistry: HL

Le Chatelier's Principle

Position of equilibrium

  • The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.

  • When the position of equilibrium shifts to the left, it means the concentration of reactants increases

  • When the position of equilibrium shifts to the right, it means the concentration of products increases

Le Chatelier’s principle

  • Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change

  • The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Effects of concentration on the position of equilibrium

  • When the concentration of a reactant increases, the rate of the forward reaction increases and the system is no longer in equilibrium

  • When a new equilibrium is established, there will be more product and less reactant within the reaction mixture, so the equilibrium has shifted to the right

  • This shift has reduced the effect of the increase by removing some of the reactant

Effects of Concentration Table

Change

How the equilibrium shifts

Increase in concentration of  a reactant

Equilibrium shifts to the right to reduce the effect of an increase in the concentration of a reactant

Decrease in concentration of a reactant

Equilibrium shifts to the left to reduce the effect of a decrease in the concentration of a reactant 

Increase in concentration of  a product

Equilibrium shifts to the left to reduce the effect of an increase in the concentration of a product

Decrease in concentration of a product

Equilibrium shifts to the right to reduce the effect of a decrease in the concentration of a product

Effects of concentration of the value of K

  • If all other conditions stay the same, the equilibrium constant K is not affected by any changes in concentration of the reactants or products

  • For example, the decomposition of hydrogen iodide:

2HI ⇌ H2 + I2

  • The equilibrium expression is:

K space equals space fraction numerator open square brackets H subscript 2 close square brackets open square brackets I subscript 2 close square brackets over denominator open square brackets H I close square brackets squared end fraction space equals space 6.25 space cross times space 10 to the power of negative 3 end exponent

  • Adding more HI makes the ratio of [ products ] to [ reactants ] smaller

  • To restore equilibrium, [H2] and [I2] increases and [HI] decreases

  • Equilibrium is restored when the ratio is 6.25 x 10-3 again

Worked Example

1. Using the reaction below:

CH3COOH (l) + C2H5OH (l) ⇌ CH3COOC2H5 (l) + H2O (l)

Explain what happens to the position of equilibrium when:

  1. More CH3COOC2H5 (l) is added

  2. Some C2H5OH (l)  is removed

 

2. Use the reaction below:

Ce4+ (aq) + Fe2+ (aq) ⇌ Ce3+ (aq) + Fe3+ (aq)

Explain what happens to the position of equilibrium when water is added to the equilibrium mixture

  

Answer 1a:

  • The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed

  • The reaction moves in this direction to oppose the effect of added ethyl ethanoate, so the ethyl ethanoate decreases in concentration

 

Answer 1b:

  • The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed

  • The reaction moves in this direction to oppose the removal of ethanol so more ethanol (and ethanoic acid) are formed from ethyl ethanoate and water

 

Answer 2:

  • There is no effect as the water dilutes all the ions equally so there is no change in the ratio of reactants to products

Effects of pressure on the position of equilibrium

  • Changes in pressure only affect reactions where the reactants or products are gases

  • The pressure of a gas in a fixed volume increases as the number of gas molecules increases

  • Changes in pressure will cause the equilibrium to shift to reduce the effect of this change

Effects of Pressure Table

Change

How the equilibrium shifts

Increase in pressure

Equilibrium shifts in the direction that produces the smaller number of molecules of gas to decrease the pressure again

Decrease in pressure

Equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure again

Effects of pressure on the value of K

  • If all other conditions stay the same, the equilibrium constant K is not affected by any changes in pressure of the reactants and products

Examiner Tips and Tricks

  • If there are the same number of gas molecules on either side of the reaction, changes in pressure will not change the position of equilibrium

Worked Example

Predict the effect of increasing the pressure on the following reactions:

   1. N2O4 (g)  ⇌  2NO2 (g)

   2. CaCO3 (s)  ⇌  CaO (s) + CO2 (g)

Predict the effect of decreasing the pressure on the following reaction:

   3. 2NO2 (g)  ⇌  2NO (g)  + O2 (g)

 

Answer 1:

  • The equilibrium shifts to the left as there are fewer gas molecules on the left

  • This causes a decrease in pressure

 

Answer 2:

  • The equilibrium shifts to the left as there are no gas molecules on the left but there is CO2 on the right

  • This causes a decrease in pressure

 

Answer 3:

  • The equilibrium shifts to the right as there is a greater number of gas molecules on the right

  • This causes an increase in pressure

Effects of temperature on the position of equilibrium

  • When the temperature changes, the equilibrium will respond by moving in the direction which will absorb or release energy

Effects of Temperature Table

Change

How the equilibrium shifts

Increase in temperature

Equilibrium shifts in the endothermic direction, absorbing energy to reverse the change

Decrease in temperature

Equilibrium shifts in the exothermic direction, releasing energy to reverse the change


Effects of temperature on the value of K

  • Changes in temperature change the equilibrium constant K

  • For an endothermic reaction such as:

2HI (g) ⇌ H2 (g) + I2 (g)

K space equals space fraction numerator open square brackets straight H subscript 2 close square brackets open square brackets straight I subscript 2 close square brackets over denominator open square brackets HI close square brackets squared end fraction

  • An increase in temperature:

    • [H2] and [I2] increases

    • [HI] decreases

  • Because [H2] and [I2] are increasing and [HI] is decreasing, the equilibrium constant K increases

  • For an exothermic reaction such as:

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

K space equals space fraction numerator open square brackets SO subscript 3 close square brackets squared over denominator open square brackets SO subscript 2 close square brackets squared open square brackets straight O subscript 2 close square brackets end fraction

  • An increase in temperature:

    • [SO3] decreases

    • [SO2] and [O2] increases

  • Because [SO3] decreases and [SO2] and [O2] increases the equilibrium constant K decreases

Worked Example

  1. Using the reaction below:

              H2 (g)  + CO2 (g)  ⇌  H2O (g) +  CO (g)          ∆H = + 41.2 kJ mol-1

    Predict the effect of increasing the temperature on this reaction

  2. Using the reaction below:

              Ag2CO3 (s)  ⇌  Ag2O (s) + CO2(g)

    Increasing the temperature increases the amount of CO2(g) at constant pressure. Is this reaction exothermic or endothermic? Explain your answer.

 

Answer 1:

  • The reaction will absorb the excess heat and since the forward reaction is endothermic, the equilibrium will shift to the right

 

Answer 2:

  • The reaction will absorb the excess heat and since this causes a shift of the equilibrium towards the right (as more CO2(g) is formed) this means that the reaction is endothermic (because endothermic reactions favour the products)

Effects of catalysts

  • A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)

  • Catalysts only cause a reaction to reach its equilibrium faster

  • Catalysts therefore have no effect on the position of the equilibrium or on the value of K

Examiner Tips and Tricks

  • When conditions of industrial processes are chosen, Le Chatelier's principle can be used to predict the conditions that would cause the equilibrium to lie towards the products, giving a high equilibrium yield

  • However, the kinetics of the reaction must also be considered as the rate of reaction needs to be sufficiently fast.

  • For example, consider a reversible reaction whose forward reaction is exothermic

    • According to Le Chatelier's principle, lower temperatures would produce a higher equilibrium yield

    • However, higher temperatures will give a faster rate of reaction

    • A compromise temperature is used which gives a lower yield of product but is made more quickly

Heterogeneous equilibria

  • Le Chatelier's principle can also be applied to heterogeneous equilibria

  • For example, in a fizzy drink bottle, an equilibrium exists between the dissolved CO2 and gaseous CO2:

CO2 (g) ⇌ CO2 (aq)

  • When the bottle is opened, some CO2 (g) escapes, the equilibrium shifts to the left to reduce the effect of this change and bubbles of CO2 (g) are observed

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    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.