Equilibrium is dynamic condition where rates of opposing processes are equal. Types of Equilibrium: Physical Equilibrium (Phase equilibrium) Physical Equilibrium (Solution Equilibrium) Chemical Equilibrium

 Rate of one phase change is equal to the rate of the opposing phase change.  Occurs when two phases exist at the same temperature.  Example: Rate vaporization = Rate condensation H 2 O (l)  H 2 O (g)

 Adding a solute to solvent (making a solution), lowers the vapor pressure which in turn 1. Lowers the Freezing Point of a solution 2. Raises the Boiling Point of a solution The more solute added, the greater the change in freezing point and boiling point

 Changing the container size  Adding more of the solvent to the container

 Rate of dissolving = rate of crystallization  Occurs in saturated solutions

 SO far, we have only talked about chemical reactions that proceed in one direction, from Reactants to Products and then STOP A + B  C  Equilibrium is not reached if one of the products is withdrawn as quickly as it is produced and no new reactants are added. Reaction continues until reactants are used up.  BUT most chemical reactions are able to proceed in both directions under the appropriate conditions. They are REVERSIBLE!  Example: Fe 3 O 4 (s) + 4 H 2 (g) ↔ 3 Fe (s) + 4 H 2 O (g) WHEN DOES IT STOP?

 Represented by a double arrow between reactants and products   In a closed system, as products are produced they will react in the reverse reaction until the rates of the forward and reverse reactions are equal. Rate fwd = Rate rev  This is called chemical equilibrium.

 Rate depends on concentration  The forward rate of a reaction decreases over time  The reverse rate of a reaction increases over time  Eventually the 2 rates become equal ABABABAB

 Once equilibrium is achieved, the amount of each reactant and product remains constant  CON CON REQUAL: Concentration of reactants and products are constant, not necessarily equal. Rates are Equal!

A double arrow identifies that a reaction is in equilibrium. N 2 O 4 (g) 2 NO 2 (g)

 Ratio of Products over Reactants  Only use aqueous or gaseous substances in the expression; never use solids or liquids The equilibrium expression for this reaction would be The equilibrium expression for this reaction would be K =K =K =K = [C] c [D] d [A] a [B] b aA + bB cC + dD K is the equilibrium constant

19 The Equilibrium Expression Write the equilibrium expression for the following reaction: Write the equilibrium expression for the following reaction:

21 The Equilibrium Constant K is based on the molarities (concentration) of reactants & products at equilibrium. We generally omit the units of the equilibrium constant. We generally omit the units of the equilibrium constant. [N2] =.100M [H2] =.300 M [NH3] =.200 M Insert the concentrations into the expression to solve for K

 If K >> 1, the reaction is product-favored; product predominates at equilibrium. If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium. If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium.

As you can see, the ratio of [NO 2 ] 2 to [N 2 O 4 ] remains constant at this temperature no matter what the initial concentrations of NO 2 and N 2 O 4 are. N 2 O 4 (g) 2 NO 2 (g)

Whenever stress is applied to a reaction at equilibrium, the reaction will shift its point of equilibrium to offset the stress. Stresses include: Temperature, pressure, changes in reactant or product concentrations

25 Change in Reactant or Product Concentrations Adding a reactant or product shifts the equilibrium away from the increase. Adding a reactant or product shifts the equilibrium away from the increase. Removing a reactant or product shifts the equilibrium towards the decrease. Removing a reactant or product shifts the equilibrium towards the decrease.

26 Change in Concentration If H 2 is added while the system is at equilibrium, the system must respond to counteract the added H 2 That is, the system must consume the H 2 and produce products until a new equilibrium is established. That is, the system must consume the H 2 and produce products until a new equilibrium is established. Therefore, [H 2 ] and [N 2 ] will decrease and [NH 3 ] increases. Therefore, [H 2 ] and [N 2 ] will decrease and [NH 3 ] increases.

27 Volume & Pressure are inversely related. Volume & Pressure are inversely related. Increasing pressure (decrease in volume) favors the direction that has fewer moles of gas. Increasing pressure (decrease in volume) favors the direction that has fewer moles of gas. Decreasing pressure (increase in volume) favors the direction that has greater moles of gas. Decreasing pressure (increase in volume) favors the direction that has greater moles of gas. In a reaction with the same number of product and reactant moles of gas, pressure has no effect. In a reaction with the same number of product and reactant moles of gas, pressure has no effect. Effects of Volume and Pressure on Gaseous Equilibrium

28 Change in Volume & Pressure If the volume of the container increases while the system is at equilibrium(pressure of the gas decreases), the system must respond to counteract the decreased pressure That is, the system will shift to the right ;more moles of product will form until a new equilibrium is established.- this raises the pressure back to where it originally was That is, the system will shift to the right ;more moles of product will form until a new equilibrium is established.- this raises the pressure back to where it originally was Therefore, [NO 2 ] will increase and [N 2 O 4 ] decreases. Therefore, [NO 2 ] will increase and [N 2 O 4 ] decreases. colorlessbrown

29 Effects of Volume and Pressure An increase in pressure (by decreasing the volume) favors the formation of colorless N 2 O 4. An increase in pressure (by decreasing the volume) favors the formation of colorless N 2 O 4. The instant the pressure increases, the system is not at equilibrium and the concentration of both gases has increased. 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: shifts left The system moves to reduce the number moles of gas: shifts left A new equilibrium is established! A new equilibrium is established! colorlessbrown

30 The equilibrium constant is temperature dependent. The equilibrium constant is temperature dependent. For an endothermic reaction, For an endothermic reaction,  H > 0 or  H is (+); heat can be considered as a reactant. as a reactant. For an exothermic reaction, For an exothermic reaction,  H < 0 or  H is (-) heat can be considered as a product. a product. Adding heat (i.e. heating the vessel) favors away from the increase: Adding heat (i.e. heating the vessel) favors away from the increase: – if Endothermic, adding heat favors the forward reaction, – if exothermic, adding heat favors the reverse reaction. Effect of Temperature Changes

31 Effect of Temperature Changes Removing heat (i.e. cooling the vessel), favors towards the decrease: Removing heat (i.e. cooling the vessel), favors towards the decrease: – if endothermic, cooling favors the reverse reaction, – if exothermic, cooling favors the forward reaction.

32 Effect of Temperature Changes  H =+50KJ ; If the temperature is lowered, the reaction will shift to the left and the solution will change to pink making more Co(H 2 O) 6 2+ pale pink blue

N 2 (g) + 3 H 2 (g)  2 NH 3 (g) + heat a) increase [N 2 ] b) decrease [H 2 ] c) increase [NH 3 ] d) decrease [NH 3 ] e) increase pressure f) increase volume g) increase temperature h) decrease temperature

N 2 (g) + 3 H 2 (g)  2 NH 3 (g) + heat a)  [N 2 ]shift towards products (right) b)  [H 2 ]shift towards reactants (left) c)  [NH 3 ]shift towards reactants (left) d)  [NH 3 ]shift towards products (right) e)  pressureshift towards products (right) f)  volumeshift towards reactants (left) g)  temp.shift towards reactants (left) h)  temp.shift towards products (right)

 Concentration increase shift away from increase  Concentration decrease shift toward decrease   pressure shifts in direction of fewer gas molecules.   pressure shifts in direction of more gas molecules   temperature favors endothermic reaction ◦ Shift away from heat   temperature favors exothermic reaction ◦ Shift towards heat

Addition of catalysts increases the rate of both the forward and reverse reactions. There is no change in concentrations but equilibrium is reached more rapidly. Does not change the value of K

 Application of LeChatelier’s Principle N 2 (g) + 3 H 2 (g)  2 NH 3 (g) + 92 kJ increase pressureShift  decrease Temp Shift  remove NH 3 add N 2 and H 2 Shift  ****Maximum yields of NH 3 occurs under high pressures, low temperatures and by constantly removing NH 3 and adding N 2 & H 2