Qualitative Changes in Equilibrium Systems

Le Châtelier’s Principle

• Le Châtelier’s Principle – chemical systems at equilibrium shift to restore equilibrium when a change occurs that disturbs the equilibrium

Factors disturb equilibrium

• Concentration, pressure and temperature or a combination of them

• Why would we care? Le Chatelier feeds the world

Le Châtelier’s Principle and changes in concentration

• Equilibrium shift – a change in concentrations of reactants and products in order to restore an equilibrium state

• What do you predict would happen if I added more SCN anion to the reaction?

• Or what about more FeSCN?

p. 440

-Adding a reactant causes a shift to the other side ofthe equilibrium

What if we removed a reactant?

Removing CO(g) shiftsequilibrium to the right

Explaining the effect of concentration on equilibria with collision theory

• Greater concentration of a reactant means higher probability of collision leading to higher probability in shift to product.

• It goes in both directions!

Applying the concentration change principle

• If you run a reaction in which you continually add a reactant and/or remove a product the equilibrium will continuously shift towards the product side of the equation

• Used by chemical engineers to design industrial processes for reversible reactions

• E.g. production of hydrogen gas uses high pressure and temperature to shift reaction to product

Le Châtelier and energy changes

• Adding or subtracting energy can also shift a reaction

Endothermic reactions

Cooling reactionShifts it to the left

Heating reactionShifts it to the right

Exothermic reactions

Cooling the reaction shifts it to the right

p. 443

Le Châtelier and gas volume

• This is an extension of the concentration principle

• Assuming an ideal gas pressure is inversely proportional to volume and each gas exerts its own partial pressure

• Therefore changing the volume changes the pressure exerted by each gas

-Decreasing the volume will increase the pressure-This will favour a shift to the right since fewer chemicalentities will lead to a reduction in pressure

p. 444

Changing an equilibrium system without affecting equilibrium position

• Catalysts – can change the rate of forward and reverse reactions by lowering Ea but do not affect the equilibrium position (just how quickly you get there)

p. 445

• Inert Gas – adding an inert gas will increase the pressure thus increasing number of collisions but since it is inert the collisions will not cause reactions leaving the equilibrium intact

p. 445

• State of reactants – In reactions involving chemicals in different states adding or subtracting chemicals in other states will have not effect at equilibrium so long as they are not completely removed.

Assigned questions

• P. 446, 1-4