Factors Which Effect Equilibrium and Le Chatelier’s Principle

Le Chatelier’s Principle

Le Chatelier’s principle is used to describe how an equilibrium system will respond to changes that the system is subjected to. Le Chatelier’s states that:

When a system that is at equilibrium experiences any changes, the system will shift to minimise the impact of those changes. In shifting, the system will establish a new equilibrium position.

Some of the main changes that may impact an equilibrium system include concentration, temperature and volume/pressure. In response to these changes, reactants or products may be used up or made in establishing a new equilibrium. If we understand the chemical system and the changes that it is exposed too, we can predict the impact of those changes on the system.

When talking about equilibrium systems the terminology of shift to the right or shift to the left is common.

  • A shift to the right means the reaction favours the formation of products (reactants are used up). We may also say that the reaction moves/proceeds in the forward direction.
  • A shift to the left means the reaction favours the formation of reactants (products are used up). We may also say that the reaction moves/proceeds in the reverse direction.

Factors Which Impact Equilibrium Systems

Below is a brief overview of factors which impact equilibrium systems and how shifts in equilibrium can be predicted as a result of these changes. This is consolidated with the examples at the end of this section.

Concentration

The concentration of a reactant or product can be changed in a number of ways. For example:

  • The reactant or product could be added or removed directly
  • A reactant or product could be added in the form of another species (this is common with ions)
  • If a reactant or product is an ion it may be removed as a precipitate if another species is added which is not related to the equilibrium system

When there is a change in concentration of a reactant or product, the equilibrium will shift to minimise this change:

  • If the concentration of reactants is increased, the system will shift to the right to use them up and create more product
  • If the concentration of products is increased, the system will shift to the left to use them up and create more reactant
  • If the concentration of reactants is decreased, the system will shift to the left to create more reactant and use up products
  • If the concentration of products is decreased, the system will shift to the right to create more product and use up reactants

Temperature

Heat energy can be added or removed from an equilibrium system by changing the temperature. If the temperature is increased, heat energy is added to the system and vice versa.

The impact of temperature on an equilibrium system depends on the enthalpy change, that is, the endothermic and exothermic nature of the system.

Recall the following:

Exothermic reaction:

A ⇌ B (ΔH is negative, −ΔH)

A ⇌ B + heat energy

Endothermic reaction:

A ⇌ B (ΔH is positive, +ΔH)

A + heat energy ⇌ B

An exothermic reaction releases energy, while an endothermic reaction absorbs energy. If the temperature (hence, energy) of the system is increased, then according to Le Chatelier’s principle, the system will favour the reaction that absorbs the energy. Conversely, if the temperature (hence, energy) of the system is decreased, then according to Le Chatelier’s principle, the system will favour the reaction that releases the energy. In summary:

  • An increase in temperature will favour the endothermic direction
  • A decrease in temperature will favour the exothermic direction

Another way to visualise energy changes to equilibrium systems:

  • If the temperature of the system is increased, the equilibrium will shift away from the heat that may be stated in the equation.
  • If the temperature of the system is decreased, the equilibrium will shift toward the heat that may be stated in the equation.

Volume/Pressure

In a closed system, pressure and volume are related. They are inversely proportional:

  • If volume is increased, pressure decreases
  • If volume is decreased, pressure increases

The pressure of an equilibrium system is dependent on the gases in it. The system can adjust the pressure by shifting to a side which contains a greater or fewer number of gas moles. A shift to the side with more gas moles will increase the pressure and a shift to the side with fewer gas moles will decrease the pressure.

  • If the volume of an equilibrium system is increased (decrease in pressure) the system will shift to the side with more gas moles in order to increase the pressure.
  • If the volume of an equilibrium system is decreased (increase in pressure) the system will shift to the side with less gas moles in order to decrease the pressure.

Graphing Changes to Equilibrium.

Interpreting graphs is an important part of being able to determine and predict what may be happening in an equilibrium system. We have seen previously that graphs of concentration vs time can be useful in determining the amount of product or reactant that is present and also indicating when a system is at equilibrium:

Changes to equilibrium systems can be represented on these graphs and it is important to understand how to determine what impact results in which change on a graph. For the purpose of this section we will use a fictional equation to analyse each change:

A(g)  ⇌  2B(g)  + C(g)      −ΔH (exothermic reaction)

Concentration:

Changes in concentration result in an immediate increase or decrease to the concentration of that substance. All reactants/products will then change over time in response to this change in concentration until a new equilibrium is reached.

For example:  A(g)  ⇌  2B(g)  + C(g)  (−ΔH) is at equilibrium when the concentration of C is increased:

Temperature:

Changes in temperature result in a gradual change to all reactants/products. The change is dependent on the temperature change and the enthalpy nature of the system. All reactants/products will then change over time in response to this change in temperature until a new equilibrium is reached.

For example:  A(g)  ⇌  2B(g)  + C(g)  (−ΔH) is at equilibrium when the temperature is increased:

Volume/Pressure

Changes in volume and pressure result in an immediate change in concentration to all reactants/products in the system. All reactants/products will then change over time in response to this change in volume or pressure until a new equilibrium is reached.

For example:  A(g)  ⇌  2B(g)  + C(g)  (−ΔH) is at equilibrium when the pressure is increased:

Analysing Equilibrium Systems:

Heating Cobalt Chloride

When cobalt (II) chloride is dissolved in water it forms an equilibrium between the hydrated and dehydrated forms of the ion according to the following equation:

Co(H2O)62+(aq)  + 4Cl–(aq) ⇌ CoCl42-(aq) + 6H2O(l)              ∆H = positive (endo)

This reaction can be simplified to:

Co2+(aq)  + 4Cl–(aq) ⇌ CoCl42-(aq)              ∆H = positive (endo)

This equilibrium system can be easily demonstrated in a lab and simple water baths may be used to change the temperature. This system is an endothermic reaction in the forward direction, so:

  • An increase in temperature will shift the equilibrium right to produce more products. The solution will become more blue.
  • A decrease in temperature will shift the equilibrium left to produce more reactants. The solution will become more pink.

Other changes and observations with this reaction:

Additions which impact the [Cl–] are common with this reaction:

  • Addition of HCl will increase the [Cl–] and the equilibrium will shift right to favour the products, turning the solution more blue.
  • Addition of NaCl will increase the [Cl–] and the equilibrium will shift right to favour the products, turning the solution more blue.
  • Addition of AgNO3 will precipitate AgCl. This will decrease the [Cl–] and the equilibrium will shift left to favour the reactants, turning the solution more pink.
  • Adding water will shift the equilibrium to the left to form the hydrated cobalt ion, turning the solution pink.

As there are no gases in this system, changes to volume and pressure have a negligible impact on the position of the equilibrium.

Interaction between nitrogen dioxide and dinitrogen tetroxide

Nitrogen dioxide and dinitrogen tetroxide exist together in an equilibrium according to the following equation:

2NO2(g)  (brown)  ⇌  N2O4(g) (colourless)         −∆H = negative (exothermic)

Nitrogen dioxide is a brown gas and dinitrogen tetroxide is a colourless gas. When this reaction takes place in a clear vessel the colour change between brown and colourless makes equilibrium changes easy to observe.

  • Increasing the pressure will shift the equilibrium to the right and the gas mixture will become lighter in colour.
  • decreasing the pressure will shift the equilibrium to the left and the gas mixture will become darker brown in colour.

A simple demonstration of this reaction can be done with a syringe. The diagram below illustrates changes to this equilibrium as a result of pressure. Initially, the system is at equilibrium. When the syringe is initially compressed the equilibrium has not shifted and re-established itself and the appearance is darker, which is not predicted by LeChatelier’s principle. This is due to the original gas mixture being compressed into a smaller space. As the equilibrium shifts it can be seen that the colour becomes lighter as a result of a shift to the right hand side as a new equilibrium is reached.

Other changes and observations with this reaction:

Changes in temperature can also be easily observed. This reaction is exothermic in the forward direction so:

  • An increase in temperature will shift the equilibrium left and the gas mixture will become darker brown in colour.
  • A decrease in temperature will shift the equilibrium right and the gas mixture will become lighter in colour.

Iron(III) thiocyanate and varying concentration of ions

The two different coloured ions, Fe3+(aq) (yellow) and Fe(SCN)2+(aq) (red), exist together in equilibrium according to the following equation:

Fe3+(aq) + SCN– (aq) ⇌ Fe(SCN)2+(aq)                  ∆H = negative (exo)

The concentrations of the ions in this equilibrium can be altered in a variety of ways. Some examples include:

  • The addition of Iron(III) nitrate, Fe(NO3)3(aq), will increase the concentration of Fe3+ The equilibrium will shift right and the solution will become more red in colour. This will be similar for other solutions that contain the Fe3+ ion.
  • The addition of a solution of hydroxide ions will precipitate Fe3+ ions as Fe(OH)3(s). This will reduce the concentration of Fe3+ The equilibrium will shift left and the solution will become more yellow in colour.
  • The addition of a solution of thiocyanate ions, for example, KSCN(aq) and NH4SCN(aq) will increase the concentration of SCN– The equilibrium will shift right and the solution will become more red in colour. This will be similar for other solutions that contain the SCN– ions.
  • The addition of a silver ions solution will precipitate SCN– ions as AgSCN(s). This will reduce the concentration of SCN– The equilibrium will shift left and the solution will become more yellow in colour.

Other changes and observations with this reaction:

Changes in temperature can also be easily observed. This reaction is exothermic in the forward direction so:

  • An increase in temperature will shift the equilibrium left and the solution will become more yellow in colour.
  • A decrease in temperature will shift the equilibrium right and the solution will become red in colour.

As there are no gases in this system, changes to volume and pressure have a negligible impact on the position of the equilibrium.

Equilibrium and Collision Theory

Remember that collision theory describes how the rate of reaction is a result of three main factors:

  • Rate of collision
  • Activation energy
  • Molecular orientation

Changes to equilibrium systems can be explained with reference to collision theory:

Concentration: Changing the concentration of reactants or products in a system directly influences how many particles are present.

  • If concentration is increased (more particles are present) this increases the chance that particles will collide and react successfully.
  • If concentration is decreased (less particles are present) this decreases the chance that particles will collide and react successfully.

Temperature: (This is detailed further in the next section) Changes in temperature influences the kinetic energy that particles can have.

  • If the temperature is increased, particles have more kinetic energy and the chance of a successful collision between particles increases.
  • If the temperature is decreased, particles have less kinetic energy and the chance of a successful collision between particles decreases.

These changes are a result of the number of particles which have enough energy to overcome the activation energy for the reaction to proceed. It is important to note that changes in temperature influence both the forward and reverse reaction. However, an increase in temperature which gives more particles the energy required to overcome the activation energy has a greater impact on the endothermic reaction. (Increase in temperature favours the endothermic reaction and decreases in temperature favour the exothermic reaction)

Volume/Pressure: Changes to volume and pressure influence the rate of collision between particles.

  • When the volume is decreased the particles will be closer together and this will increase the rate of collision. This impacts both the forward and reverse reaction. However it has the greatest impact on the side with more molecules.
  • If the volume is increased the particles will be further apart and this will decrease the rate of collision. This also impacts both the forward and reverse reaction. However it has the greatest impact on the side with less molecules.

Example 1:

The following reaction is at equilibrium in a closed system: H2(g) + I2(g) ⇌ 2HI(g)    (-∆H). Predict the impact of the following changes:

a) pressure is increased

b) some HI is removed

c) temperature is decreased

Answers:

a) no change as there are equal numbers of gas moles on each side of the equation

b) equilibrium will shift right to produce more product, HI

c) equilibrium will shift right to produce more product, HI

Example 2:

The graph below represents the following equilibrium system: N2(g) + 3H2(g) ⇌ 2NH3(g)    (-∆H)

Use the graph to determine what changes have occurred at T1, T2 and T3:

Answers:

at T1: pressure was increased (all concentrations increase) and the equilibrium shifts right to the side with less gas moles.

at T2: some NH3 was removed (concentration of NH3 increases whilst the others change as a result) and the equilibrium shifts right to produce more NH3.

at T3: temperature is increased as the equilibrium shifts left and it is an exothermic reaction (no immediate change but all species gradually change as a result)