La Chatelier’s Principle & Collision theory

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

Learn.

When an external stress (Change in temperature, pressure, concentration) is applied to a system in equilibrium. The system will respond in a way which minimizing the effect of the change. This is La Chatelier’s principle.

Collision theory explain chemical reactions as successful collisions between reactant molecules, increasing the temperature or number of reactants increases the likely-hod of a successful collision occurring hence increasing the rate of reaction.

Collision theory can be applied to La Chatelier’s principle’s as follows:

Stress on systemEffect on equilibrium position
Increasing temperatureIncreases the rate of exothermic reaction and endothermic reaction, however, it increases the rate of the endothermic reaction more hence the system shifts to reduce effect of heating it. Favoring the endothermic reaction.
Decreasing temperatureDecreases the rate of exothermic reaction and endothermic reaction, however, it decreases the rate of the endothermic reaction more hence the system shifts to minimize cooling. Favoring the exothermic reaction.
Increasing reactantsIncreasing reactants pushes the equilibrium towards the products since more successful collisions can occur between reactants hence increasing the rate of the forward reaction.
Decreasing reactantsDecreasing reactants pushes the equilibrium towards the reactants as fewer successful collisions can occur between reactants hence decreasing the rate of the forward reaction.
Increasing productsIncreasing products pushes the equilibrium towards the reactants since more successful collisions can occur between products hence increasing the rate of the reverse reaction.
Decreasing productsDecreasing products pushes the equilibrium towards the products as fewer successful collisions can occur between products hence decreasing the rate of the reverse reaction.
CatalystIncrease the rate of both the reverse and forward reaction equally hence no net change in reaction rate. This occurs since less energy is required for a successful collision
Increasing pressure/ decreasing volume* This will only effect a system which contains gasses
* Increasing pressure/ decreasing volume will increase the proximity of the gasses to one another hence increasing the number of collisions which occur, this will result in the side with more moles of gas having more successful collisions per unit time hence increasing reaction rate – favoring this reaction.
Decreasing pressure/ increasing volume* This will only effect systems which contain a gas
* Decreasing pressure/ increasing volume will decrease the proximity of the gasses to one another hence decreasing the number of collisions which occur, this will result in the side with fewer moles of gas having more successful collisions per unit time hence increasing reaction rate – favoring this reaction.
Adding an inert gasNo effect on equilibrium position since it it will not effect the partial pressures of reactants and products.

Master.

Question 1.

The following system is exothermic

Predict what will happen to the K eq if the temperature of the system is decreased – explain why this occurs with respect to collision theory. [4 marks]


Question 2.

The following system is exothermic

“The above reaction will cease to occur when the system is at equilibrium and increasing the temperature will make the reverse reaction start up again.”

Evaluate the validity of the students statement making reference to collision theory [8 marks]


Question 3.

Which of the following explains in terms of collision theory what would happen to a system if reactants were increased.

a) Increasing reactants pushes the equilibrium towards the products since more successful collisions can occur between reactants hence increasing the rate of the forward reaction.

b) Increasing reactants pushes the equilibrium towards the reactants as fewer successful collisions can occur between reactants hence decreasing the rate of the forward reaction.

c) This decreases products pushing the equilibrium towards the reactants since more successful collisions can occur between products hence increasing the rate of the reverse reaction.

d) Increase the rate of both the reverse and forward reaction equally hence no net change in reaction rate. This occurs since less energy is required for a successful collision

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