Le Chatelier's Principle

 

This is part of the HSC Chemistry course under the topic Factors that Affect Equilibrium

HSC Chemistry Syllabus

• Investigate the effects of temperature, concentration, volume and/or pressure on a system at equilibrium and explain how Le Chatelier’s principle can be used to predict such effects, for example:

– heating cobalt (II) chloride hydrate

– interaction between nitrogen dioxide and dinitrogen tetroxide

– iron(III) thiocyanate and varying concentration of ions (ACSCH095) 

• Explain the overall observations about equilibrium in terms of the collision theory (ACSCH094)
  

• Examine how activation energy and heat of reaction affect the position of equilibrium

What is Le Chatelier's Principle?

This video will explore what Le Chatelier's Principle is and discuss its relevance regarding understanding how equilibrium positions are manipulated. 

 

Le Chatelier's Principle Explained

• Le Chatelier’s principle predicts the outcome of an equilibrium system when a change is imposed on it. It states:

Effect of Concentration Changes on Equilibrium

• Concentration directly influences the rate of collision between molecules and atoms which in turn affects the rate of reaction.
  
  

Figure: addition of a chemical increases its concentration and rate of collision with other molecules which in turn increases the reaction rate. This disturbs the equilibrium as rates of forward and reverse reactions are no longer equal.

 

• When concentration of reactant(s) increases, by Le Chatelier’s principle, the equilibrium position will shift to the product side to oppose the change by reducing the concentration of reactants. Therefore, when the reaction reaches a new equilibrium, the concentration of products would have increased.

• When concentration of reactant(s) decreases, by Le Chatelier’s principle, the equilibrium position will shift to the reactant side to oppose the change by increasing the concentration of reactants. Therefore, when the reaction reaches a new equilibrium, the concentration of products would have decreased.

 

Let's consider the following equilibrium system:

$$N_2O_4(g) \rightleftharpoons 2NO_2(g)$$

• If additional moles of `N_2O_4` is added to the system, `[N_2O_4]` is increased. By LCP, the equilibrium position will shift to the right, decreasing `[N_2O_4]` (opposing the change) and increasing `[NO_2]`.

• If additional moles of `NO_2` is added to the system, `[NO_2]` is increased. By LCP, the equilibrium position will shift to the left, decreasing `[NO_2]` (opposing the change) and increasing `[N_2O_4]`. 

Effect of Pressure/Volume Changes on Equilibrium

• Pressure and volume affect the collision rate between gas molecules. The effect on aqueous, liquid and solid-state compounds and molecules is negligible, thus it is generally disregarded.

 

Figure: reduction in volume increases pressure on gases and the collision rate between them. Solids and liquids are not affected by changes in pressure and volume (the effect is negligible compared to gases).

 

• At constant temperature, pressure is inversely proportional to volume (Boyle's law)

 

Figure: changes in pressure or volume directly impact the collision rate and thus, the rate of reaction. An increase in pressure increases reaction rate while a decrease in pressure decreases reaction rate.

 

• Changes in pressure and volume have greater effect on collision rate and reaction rate when there are more gas molecules present. Thus, when the stoichiometric ratio between gaseous reactants and products is not 1:1, changes in pressure and volume will cause forward and reverse reaction rates to become different, hence disturbing the equilibrium. 

 

Figure: changes in pressure and volume have greater effect when there are more gas molecules. In this example, there are more reactants (blue), so changes in pressure exert greater effects on the forward reaction rate.

 

• When pressure is increased (volume is reduced), by Le Chatelier's principle, the equilibrium position will shift in a direction to reduce the number of gas molecules in the system in order to oppose the change by reducing the pressure. This is because pressure is directly proportional to the number of gas molecules (Avogadro's law).

• When pressure is decreased (volume is increased), by Le Chatelier's principle, the equilibrium position will shift in a direction to increase the number of gas molecules in the system in order to oppose the change by increasing the pressure. Again, this is due to Avogadro's law. 

• Changes in pressure or volume does not affect equilibrium position if a reaction has no gaseous species or the stoichiometric ratio between gaseous reactants and products is 1:1.

 

Let's consider the following equilibrium again:

$$2NO_2(g) \rightleftharpoons N_2O_4(g)$$

 

• The forward reaction consumes 2 molecules of `NO_2` to produce 1 molecule of `N_2O_4`. In other words, there are more gas molecules on the reactant side and less on the product side.

• An increase in pressure (decrease in volume) increases the rate of the forward reaction more than the reverse. By LCP, this shifts the equilibrium position towards the right side to decrease the pressure by reducing the number of gas molecules in the system. As a result, when the reaction reaches a new equilibrium, the quantity of `N_2O_4` and `NO_2` would have increased and decreased respectively.

• A decrease in pressure (increase in volume) decreases the rate of forward reaction more than the reverse. By LCP, this shifts the equilibrium position towards the left side to increase the pressure by increasing the number of gas molecules in the system. As a result, when the reaction reaches a new equilibrium, the quantity of `N_2O_4` and `NO_2` would have decreased and increased respectively. 

Effect of Changes in Temperature on Equilibrium

Figure: Maxwell-Boltzmann distribution curves demonstrating the effect of temperature on the molecular energy distribution. 

• In collision theory, temperature affects rate of collision as well as the energy of molecules during collisions. Higher temperature means there is more energy in the system and therefore, more molecules will have energy greater than the activation energy barrier. This increases the rate of reaction. 

• Changes in temperature affect the rate of both forward and reverse reactions. However, it has a greater impact on the reaction with a larger activation energy (E_a) which is always the endothermic reaction in a reversible reaction.

 

Figure: energy profile of an exothermic reaction. The reverse reaction is endothermic and has a larger activation energy barrier than the forward reaction.

Example 1

Solution:

Before beginning the question, it is important to note that this is not a rate versus time graph but instead it is a concentration versus time graph.

(a) At `t_1`, only `[N_2]` increased instantaneously which suggests this is not a change in pressure/volume as it would otherwise affect concentrations of all chemical species in the reaction. This is also not a change in temperature as changes in temperature do not change concentrations of species instantaneously. Therefore, the change at `t_1` is likely to be addition of nitrogen gas. 

(b) At `t_2`, concentrations of all chemical species of the reaction have increased instantaneously. This suggests an increase in pressure on the system (or reduction in volume of vessel). This can be confirmed by examining the equation of the reaction:

$$N_{2(g)}+3H_{2(g)} \rightleftharpoons 2NH_{3(g)}\;\;\; \Delta{H} = -92\, kJmol^{-1}$$

Since there are more moles of gases on the reactant (left) side, rate of forward reaction is favoured by an increase in pressure. This causes greater quantities of `NH_3` (ammonia) to be produced, which is shown as a gradual change right after the pressure change at `t_2`.

(c) No chemical species at have changed their concentrations instantaneously at `t_3`. This means the change is associated with temperature. The concentrations of `N_2` and `H_2` gradually increased, and that of `NH_3` decreased. This indicates that the temperature change favours the reverse reaction. Since the reverse reaction is endothermic, the change must an increase in temperature. 

 

Previous section: Collision Theory in Equilibria

Next section: Practical Investigations of Le Chatelier's Principle (Cobalt (II) Chloride Equilibrium)

 

BACK TO MODULE 5: EQUILIBRIUM AND ACID REACTIONS