Equilibrium Some reactions proceed to completion, but many stop before completion. Macroscopically you don’t see any changes. The reaction has reached equilibrium. Microscopically the reactants are constantly converted into products and the products into reactants. The rates of the direct and reverse reactions at equilibrium are equal and that is why you don’t see any changes at the macroscopic level. It is a dynamic equilibrium.

Example 2NO 2 (g) N 2 O 4 (g) BROWN COLORLESS 2NO 2 (g) N 2 O 4 (g) 2NO 2 (g)N 2 O 4 (g) v1v1 v2v2 At equilibrium, v 1 = v 2

Conditions for Equilibrium  Closed system  Opposite reactions occur at the same rate  Equilibrium can be reached starting with reactants, with products, or with both.  Temperature is constant

Represent Equilibrium Graphically H 2 O(g) + CO(g)H 2 (g) + CO 2 (g) Conc. Moles/L Time (min) Rate Mol dm -3 s -1

Law of Mass Action (Guldberg and Waage)  Suppose the reaction is first order in both directions (reverse and forward).  The rate for the forward reaction is F.R.= K f [A] [B]  The rate for the reverse reaction is R.R.= K r [C] [D]  At equilibrium F.R. = R.R. So K f [A] [B] = K r [C] [D]  If we rearrange: K f = [C] [D] K r [A] [B] A + B C + D

…cont..Law of Mass Action (Guldberg and Waage)  At constant temperature K f and K r are constant  Then K = K f = [C] [D] is also a constant K r [A] [B]  The equilibrium constant K c is equal to the concentrations of the products, over the concentrations of the reactants, each one elevated to the power of their stoichiometric coefficients. A + B C + D

Examples 1.Write the equilibrium expression for the following reaction: 4NH 3 (g) + 7O 2 (g) 4NO 2 (g) + 6H 2 O(g) This is what we call the equilibrium expression For aA + bBcC + dD then K c = [C] c [D] d [A] a [B] b

FOCUS 1.What are the four conditions for equilibrium? 2.Write the equilibrium expression for the following reaction 4NH 3(g) + 7O 2(g) 4NO 2(g) + 6H 2 O(g )

2.The following concentrations were observed for the Haber process 3 H 2(g) + N 2 (g) 2 NH 3 (g) at 127 o [NH 3 ] = 3.1 x mol.dm -3 [N 2 ] = 8.5 x mol.dm -3 [H 2 ] = 3.1 x mol.dm -3 a. Calculate the value of K at 127 o b. Calculate the equilibrium constant for the reverse reaction c. Calculate the value of the equilibrium constant if the reaction given by: 3/2 H 2 (g) + 1/2N 2 (g) NH 3 (g)

Relationship between different ways of writing the equation and the equilibrium expression  For the reverse reaction K r = 1/K f  For a reaction that has the coefficients multiplied by a number n, then the constant K m = K n IMPORTANT  The equilibrium constant K for a given reaction at a certain temperature is always the same.  The equilibrium constant varies with the temperature.

Equilibrium position  Equilibrium position is a set of equilibrium concentrations.  Do not confuse equilibrium position with equilibrium constant. Equilibrium position  Depends on the initial concentrations  There are infinite number of equilibrium position for a system at a given temperature. Equilibrium constant  Does not depend on the initial concentration  There is only one equilibrium constant for a system at a given temperature. Do exercise 13.3, p.619 and p.649 to 653, 2,17,21,25,37,39,41,43,53

If the Equilibrium constant is greater than 1, it means that the concentration of the products is… GREATER than the concentration of the reactants Very large Kc means that the reaction goes almost to COMPLETION

Substances that do not change concentration (only change the amount) are not included in the equilibrium expression. Even they are consumed in the reaction… Pure solids: They have fixed density and constant concentration… Pure liquids, like water: They also have fixed density and constant concentration…

Le Chatelier’s Principle If a change is imposed to the conditions of a chemical equilibrium, then the position of equilibrium will readjust to counteract or minimize the change imposed.

Le Chatelier’s Principle ChangeEffectChange in Kc?

Phase Equilibrium  When a volatile liquid is stored in an open container the liquid eventually evaporates, leaving the container empty.  In contrast, if a liquid is stored in a sealed container, the liquid will evaporate while the vapor re-condenses simultaneously. When equilibrium is reached: Rate of evaporation = Rate of condensation

Phase Equilibrium = Dynamic Equilibrium  The equilibrium vapor pressure is the pressure exerted by the vapor when equilibrium between vaporization and condensation is reached.  It is a measure of the volatility of a compound, i.e. the tendency of its molecules to escape from the liquid phase and enter the vapor phase at a given temperature  When equilibrium between these two states occurs, a state of dynamic equilibrium is established.

Phase Equilibrium: Vapor pressure  Note that "vapor pressure," is the common name for the equilibrium vapor pressure.  All liquids have some fraction of molecules that have sufficient energy to break their attraction to other molecules and escape into the vapor phase.  The ability to do this depends on both the strength of those attractions and the temperature of the liquid.

Vapor pressure an temperature The kinetic energy of the molecules at the surface of a liquid varies over a range of values:  Some of the molecules have enough kinetic energy to overcome the attractive forces between the molecules.

Vapor pressure an temperature At higher T more molecules have enough energy to overcome the forces so there is more evaporation.  Vapor pressure increases when T increases.

Phase Equilibrium: Vapor pressure for different liquids The graph below shows how vapor pressure increases at temperature increases for several different substances. Refer to p.487

Phase Equilibrium  Since different liquids have different intermolecular forces, at the same temperature, different liquids have different vapor pressures. The greater the IMF, the more energy needed to leave the liquid, thus the lower the vapor pressure.  From the structures of the three substances, you should be able to rationalize the ranking of IMF for the three substances. From this graph, we can see that water has the greatest IMF, followed by ethyl alcohol and diethylether.

In other words:  The weaker the attractive forces, the greater the fraction of molecules with enough kinetic energy to escape  The greater the fraction of molecules which can escape the liquid, the greater the vapor pressure  The weaker the intermolecular forces, the greater the vapor pressure.

Vapor Pressure and Boiling Point BBOILING POINT : The temperature at which the vapor pressure of a liquid becomes equal to atmospheric pressure. At this temperature a liquid starts boiling.  The boiling point of a liquid varies with the atmospheric pressure. If atmospheric pressure is less then 760 torr then the boiling point of the liquid will decrease from its standard boiling point.  The boiling point of a liquid decreases with the decrease in pressure and vice versa

Normal Boiling Point The boiling point of a liquid when atmospheric pressure is 1.00 atm or 760 torr is referred to as NORMAL BOILING POINT.