Dynamic EquilibriumDefinition:• Equilibrium is dynamic (reactions doesn’t stop)• Equilibrium is achieved in a closed system• At equilibrium theres no change in macroscopic properties (no change in color and density)• Equilibrium can be reached from either direction (doesn’t matter if started with all reactants or all products)Physical Systems:• There will come a time when the rate of the forward reaction is equal to the rate of the reverse reaction. At this point, the system has reached equilibrium.Chemical Systems:• The concentrations of both reactants and products remain constant over time
Equilibrium Constant• For the hypothetical homogeneous chemical reaction: aA+bB cC+dD The equilibrium constant is defined asKC = [C]^c [D]^d , where [A] signifies the molar concentration of species [A]^a [B]^b• Note that the expression for the equilibrium constant includes only solutes and gases; pure solids and liquids do not appear in the expression.
Magnitude of the Equilibrium Constant• When Kc >> 1, the reaction goes almost to completion• When Kc << 1, the reaction hardly proceedsIf the value of Kc is large (Kc >> 1 ):• the equilibrium lies to the right hand side - i.e. there is a much greater concentration of the products than the reactantsIf the value of Kc is small (Kc << 1):• the equilibrium lies to the left hand side - i.e. there is a much greater concentration of reactants than products.
Le Chatelier’s Principle• States: when a stress is brought to bear on a system at equilibrium, the system will react in the direction that serves to relieve the stress.
Temperature• The only thing which can change the value of Kc for a given reaction is a change in temperature.• The effect of a change of temperature on a reaction will depend on whether the reaction is exothermic or endothermic• When the temperature increases, Le Chateliers principle says the reaction will proceed in such a way as to counteract this change, i.e. lower the temperature.• Endothermic reactions (Hrxn > 0 )will move forward, and exothermic reactions (Hrxn < 0 )will move backwards• The reverse is true for a lowering of temperature.
Pressure• According to Le Chatelier, the position of equilibrium will move in such a way as to counteract the change. That means that the position of equilibrium will move so that the pressure is reduced again.• Increasing the pressure on a gas reaction shifts the position of equilibrium towards the side with fewer molecules.• In the case of the same number of molecules on both sides of the equilibrium reaction, increasing the pressure has no effect whatsoever on the position of the equilibrium.
Concentration• When the concentration of a product is increased, the reaction proceeds in reverse to decrease the concentration of the products. a+b c+d , reaction moves backwards• When the concentration of a reactant is increased, the reaction proceeds forward to decrease the concentration of reactants. a+b c+d , reaction moves forward
Value of the Equilibrium ConstantEquilibrium constants arent changed if youchange the pressure of the system.Equilibrium constants arent changed if you adda catalyst.Temperature:• Equilibrium constants are changed if you change the temperature of the system• As the temperature increases, the value of Kc falls.
Catalyst• Adding a catalyst makes absolutely no difference to the position of equilibrium• Le Chateliers Principle doesnt apply to them.• A catalyst speeds up the rate at which a reaction reaches dynamic equilibrium.
The Haber Process• Ammonia: (Haber Process)• N2 + 3H2 2NH3 -92kJ/mol• 400-450 degrees Celsius is not a low temperature, which defies the Le Chatelier’s principle, as it would be expected to have a lower temperature to produce more ammonia. However, this was done to speed up the rate of the process• 200 atm is not a very high pressure, but it was used because very high pressures are expensive to achieve