Le Chateliers Principle Flashcards
1
Q
Shifting the Equilibrium Position
A
- If you disturb an equilibrium, it will shift to undo the disturbance.
- The following imposed changes will affect an equilibrium system:
- Changing the concentration of any one species in the system.
- Changing the total pressure in a gaseous system.
- Changing the temperature of the system.
2
Q
Le Chatelier’s Principle
A
- Predicts how an equilibrium system will respond to these imposed changes.
- Restoring Balance: If you add more to the left side (reactants), the lever will tip to the left. In order to restore balance, the system responds by making more of what is on the right (products) and vice versa.
3
Q
Effect of Concentration
A
- If an external change is applied to a system at equilibrium, the system adjusts in such a way that the change is partially offset as the system reaches a new equilibrium position.
- Increase concentration of product(s): Shifts the Equilibrium to the left
- Decrease concentration of product(s): Shifts the Equilibrium to the right
- Increase concentration of reactant(s): Shifts the Equilibrium to the right
- Decrease concentration of reactant(s): Shifts the Equilibrium to the left
4
Q
Effect of Volume and Pressure
A
- An equilibrium system involving gases requires a closed system for it to achieve equilibrium.
- As volume is decreased pressure increases.
- Le Châtelier’s Principle: if pressure is increased the system will shift to counteract the increase.
- Consider the following simple gas-phase equilibrium system: A(g) + B(g) <-> C(g)
- If the reaction is at equilibrium, and the pressure is increased, the system will shift to the right to reduce the pressure, since the product side has only one gaseous molecule, while the reactant side has two.
- If the pressure is reduced, the system will shift to the left to
restore the lost pressure. - If both sides of the equation have equal numbers of gas
molecules, pressure will have no effect on the system.
5
Q
Increase and Decrease in Pressure and Volume
A
- An increase in pressure favours the direction that has fewer moles of gas.
- The system shifts to remove gases and decrease pressure.
- Consider the production of ammonia:
N2(g) + 3H2(g) <-> 2NH3(g) - As the pressure increases, the amount of ammonia present at equilibrium increases.
- Increase in Pressure: Shifts the equilibrium to the side with the fewest moles of gas
- Decrease in Pressure: Shifts the equilibrium to the side with the most moles of gas
- Increase in Volume: Shifts the equilibrium to the side with the most moles of gas
- Decrease in Pressure: Shifts the equilibrium to the side with the fewest moles of gas
- Addition of an inert gas does not change the equilibrium position even though its pressure increases. This is because the partial pressures of the reactants and products are not altered.
6
Q
Pressure to Colour Question Example
A
Consider: 2NO2(g) <-> N2O4(g)
Brown Colourless
- The instant the pressure increases, the system is not at equilibrium and the concentration of both gases has increased.
- The system moves to reduce the number moles of gas (i.e. the forward reaction is favoured).
- An increase in pressure (by decreasing the volume) favours the formation of colourless N2O4.
- A new equilibrium is established in which the mixture is lighter because the colourless N2O4 is favoured.
7
Q
Effect of Temperature
A
Again consider the simple equilibrium system:
A + B <-> C
- To predict the effect of temperature changes arising from transferring heat into or out of the system, we will think of heat as a substance.
- If the reaction is exothermic, we can think of the heat produced as a product. If we heat the system, it would be like adding any other product, and the system will shift to the left to absorb some of the heat added.
- If the forward reaction is endothermic, we can think of heat as a reactant, and warming the system will cause it to shift to the right.
- For an endothermic reaction, H is +ve and heat can be considered as a reactant.
- For an exothermic reaction, H is -ve and heat can be considered as a product.
- Adding heat (i.e. heating the vessel) favours away from the increase:
- if Δ is +ve (endothermic), adding heat favours the forward reaction,
- if Δ is –ve (exothermic), adding heat favours the reverse reaction.
- Removing heat (i.e. cooling the vessel), favours towards the decrease:
- if enthalpy is +ve, cooling favours the reverse reaction,
- if enthalpy is -ve, cooling favours the forward reaction.
8
Q
Increase and Decrease in Temperature
A
- K is the Equilibrium Constant
- Exothermic Reaction increase in Temperature: K decreases
- Endothermic Reaction Increase in Temperature: K increases
- Exothermic Reaction decrease in Temperature: K increases
- Exothermic Reaction decrease in Temperature: K decreases
9
Q
Temperature to Colour Question Example
A
- 2NO2(g) <-> N2O4(g) ΔH = -57 kJ
- If we heat the equilibrium mixture, the reverse reaction (endothermic) would be favoured in an attempt to remove the added heat from the system.
- The mixture will become more brown as more NO2 is formed.
10
Q
Effect of a Catalyst
A
- Adding a Catalyst:
- does not change K
- does not shift the position of an equilibrium system
- system will reach equilibrium sooner (increases rate of reaction).
- Catalyst provides an alternate pathway with a lower Ea
for both forward and reverse reactions.
11
Q
Summary
A
- Concentration: Shifts Equilibrium but does not change the Equilibrium Constant
- Pressure: Shifts Equilibrium but does not change the Equilibrium Constant
- Volume: Shifts Equilibrium but does not change the Equilibrium Constant
- Temperature: Shifts Equilibrium and changes the Equilibrium Constant
- Catalyst: Doesn’t shift Equilibrium and doesn’t change the Equilibrium Constant
