Module 7 

Lesson 3.3  Temperature Changes in Equilibrium Systems



Key Concepts


According to Le Chatelier's Principle, changing the temperature of a chemical system at equilibrium causes the system to establish a new equilibrium.


If we increase the temperature of a system at equilibrium, the endothermic reaction will be favoured.


Conversely, if we decrease the temperature of a system at equilibrium, the exothermic reaction will be favoured.


Note that temperature changes alter the Kc value.


As you learned in Unit B, the energy associated with a chemical reaction can be written also a term in the equation. In the case of an endothermic reaction, the energy term appears on the side of the reactants. In the case of an exothermic reaction, the energy term appears on the side of the products.


Example

Consider the following equilibrium system, which is exothermic in the forward direction. The energy term may be regarded as a product.


\( \mathrm { 2SO_2 (g) + O_2 (g) \leftrightharpoons 2SO_3 (g) + energy } \)

When working with a chemical equilibrium equation, treat the energy in the system as though it were a reactant or product.

If we lower the temperature of this system (that is, remove energy), the equilibrium will shift to the right in favour of the SO3 product. This shift is illustrated on the graph below.



Fig. 1


The animation below illustrates the effect of temperature on a gaseous equilibrium mixture that is endothermic in the forward direction.


\( \mathrm { N_2O_4(g) \leftrightharpoons 2NO_2(g)~~~~~\Delta_rH = +58~\frac{kJ}{mol} } \)


Use the slider to vary the temperature and observe the effect on this mixture of dinitrogen tetroxide and nitrogen dioxide. Nitrogen dioxide is dark brown in colour; therefore, as more nitrogen dioxide is produced, the chemical system becomes darker in colour.





When the temperature of an equilibrium system changes, the temperature change is applied to both reactants and products (since they are mixed together in the same reaction vessel). Remember that an increase in temperature will increase the rate of both the forward and reverse reactions. The collision-reaction theory can be used to explain how temperature changes cause equilibrium shifts.


Recall that for a reaction to occur, the reactant molecules have to collide with sufficient energy. If the temperature is increased, the rate of a reaction increases because the reactant molecules move faster and are more likely to collide with each other. If there are more collisions, there will be more products formed.

Watch

Watch this YouTube video to further review Le Chatelier's Principle.





    Which way will the Equilibrium Shift? (Le Chatelier's Principle)
© chemistNATE


Virtual Investigation



 Read page 693 in the textbook.

Check Your Understanding


Complete "Practice" questions  2 and 3 on page 695 of the textbook. Click the link to check your work.


Page 695 Practice Question 2

Direction of shift in equilibrium Graph
a Right

b Left

c Right

d Right


Page 695 Practice Question 3

  1. Increasing the pressure increases the frequency of collisions between the gaseous particles favouring the forward reaction. The system shifts to the right.
  2. The forward reaction is exothermic; therefore, increasing the temperature will favour the reverse reaction. The system shifts to the left.
  3. A high temperature is necessary to increase the speed of the reactions so the process occurs in a reasonable amount of time.
  4. In an open system, the fuel can be removed to shift the equilibrium to the right to favour the forward reaction.