Chemical Equilibrium Reactants Products Reactants Products As the time increases… [Reactants] decrease, so the rate of forward reaction decreases; [Products] increase, so the rate of backward reaction increases; At one particular time, rate of forward rxn = rate of backward rxn

When this happens we say that the reaction has reached “Chemical Equilibrium”. This is a “Dynamic Equilibrium”, means the reactions do proceed in both directions, but [reactants] and [products] do not change. At Equilibrium; Rate of forward rxn = Rate of backward rxn but [Reactants] = [Products]

The Law of Mass Action For For J A + k B  l C + m D J A + k B  l C + m D The law of mass action is represented by the equilibrium expression: The law of mass action is represented by the equilibrium expression:

Equilibrium Expression 4NH 3 (g) + 7O 2 (g)  4NO 2 (g) + 6H 2 O(g) 4NH 3 (g) + 7O 2 (g)  4NO 2 (g) + 6H 2 O(g)

Time [Reactants] Conc Time [Reactants] Conc Products More reactants in the system at equilibrium More products in the system at equilibrium

Notes on Equilibrium Expressions (EE) 4 The Equilibrium Expression for a reaction is the reciprocal of that for the reaction written in reverse. 4 When the equation for a reaction is multiplied by n, EE new = (EE original ) n

N 2 + 3H 2 2NH 3 Forward rxn Backward rxn

1/3N 2 + H 2 2/3NH 3 (2) N 2 + 3H 2 2NH 3 (1)

N 2 + 3H 2 2NH 3 The equilibrium concentrations of the above system is [N 2 ]=0.399 M, [H 2 ]=1.197 M and [NH 3 ]=0.203 M. Find the K? Important Note: K has no units

The size of K and the time required to reach the equilibrium are not directly related. The time required to reach equilibrium depends on reaction rate, which is determined by the size of activation energy The size of K depends on the difference in free energy of reactants and products

If the concentration of one of the reactants or products is zero, then the reaction will shift in the direction of producing missing component. But what happens when all the initial concentrations are non-zero? Then the direction of shift is determined by “Reaction Quotient” (Q). Then the reaction quotient “Q” is determined by applying the Law of mass action to the initial concentrations. If Q>K, then we have more products and the reaction proceeds to the left. If Q<K, then we have more reactants and the reaction proceeds to the right.

The equilibrium constant for is 6.0x If [H 2 ] 0 = 2.0x10 -3 M, [N 2 ] 0 = 1.0x10 -5 M and [NH 3 ] 0 = 1.0x10 -3 M, predict the direction in which the system will shift to reach equilibrium. N 2 + 3H 2 2NH 3 There is more NH 3 in the system, so the reaction will shift to the left to reach the equilibrium

If the system in equilibrium contains gases, then concentrations of the components can be replaced by the partial pressures of gases. So the law of mass action for the above equilibrium can be written as……

2NO (g) + Cl 2 (g) 2NOCl (g) At 25 o C, P NOCl = 1.2 atm, P NO = 5.0x10 -2 atm, P Cl2 = 3.0x10 -1 atm. Calculate K p.

The relationship between K and K p : R = gas constant = T = absolute temp. (K) Δn = sum of coefficients of gaseous products minus sum of coefficients of gaseous reactants.

2NO (g) + Cl 2 (g) 2NOCl (g) The K p of this reaction at 25 o C is 1.9x10 3. Calculate the K. R= T= =298K Δn=2-(2+1)=-1

Heterogeneous Equilibria: If a system in equilibrium contains a liquid or solid, then they are ignored from Law of mass action expression as the [liquid] and [solid] are constant (never change). CaCO 3 (s ) CaO (s) + CO 2 (g)

Le Chatelier’s Principle If a stress is applied to a system at equilibrium, the position of the equilibrium will shift to reduce the stress. 3 Types of stress Changing [Reactants] or [Products] Changing the pressure of the system Changing the temperature of the system

The effect of a Change in concentration; Adding product makes Q>K Removing reactant makes Q>K Adding reactant makes Q<K Removing product makes Q<K Determine the effect on Q, will tell you the direction of shift. Changing the concentration will not affect the Value of “K”.

The effect of a change in Pressure; Only applicable to the system containing gases You can change the pressure of the system by three ways; (i) Add or remove a gaseous reactant or product (ii) Add an inert gas (one not involved in the reaction) (iii) Change the volume of the container Adding an inert gas does not change the concentration of reactant or product, so equilibrium will not shift in any direction.

When the pressure in the system is increased, the equilibrium will shift in the direction of less moles of gases. On the other hand reduction of pressure will shift the equilibrium of more molecules of gases. N 2 (g) + 3H 2 (g) 2NH 3 (g) 4 moles of gases2 moles of gases P Equilibrium Right P Equilibrium left Change in Pressure will not change the value of “K”.

The Effect of a change in Temperature; Some important points; (i)If the forward reaction is exothermic, then the backward reaction is endothermic and vice versa. (ii) The change in temperature will affect the value of “K” (iii) If the shift occurs in the forward direction then the new “K” > old “K”. (iv) If the shift occurs in the backward direction then the new “K” < old “K”.

The increase in temperature means increase in energy of the system; so the equilibrium will shift in the direction of absorbing the energy (ENDOTHERMIC). Similarly, the decrease in temperature would shift the equilibrium in the direction of increasing the energy of the system (EXOTHERMIC). T Shift in the direction of endothermic T Shift in the direction of exothermic

Predict how the value of “K” changes when the temperature is decreased for the following reaction; N2 (g) + O2 (g) 2NO (g) ΔH o = 181 kJ The reaction is endothermic in the forward direction. When the temperature is decreased, the system will respond to increase the energy by shifting in the exothermic direction (left) by releasing energy. So the value of K would decrease.