Chemical Equilibrium

Equilibrium A state where the reactants and products remain constant over time. For some reactions, the equilibrium position favors the products and the reaction appears to have gone to completion (amount of reactants is negligible)-”Equilibrium lies to the right” (in direction of the products) Example: 2 H2 + O2  2 H2O Other reactions only occur to a small extent with the product virtually undetectable-”Equilibrium lies to the left” (in direction of the reactants) Example: 2 CaO  2Ca + O2

Equilibrium is not static Because the concentrations do not change, it appears that the reaction has stopped. Instead, equilibrium is highly dynamic with the reactions continuing to occur in both directions at the same rate.

Eventually, the rates become equal. As the concentration of the reactants decreases, the forward reaction slows down. As the concentration of the products increases, the rate of the reverse reaction increases Eventually, the rates become equal. products reactants

Equilibrium Constants (Application from the lab) The equilibrium constant is a ratio of the amount of product to the amount of reactant. At a given temperature: 1) K always has the same value regardless of the starting concentrations 2) the equilibrium concentrations will not always be the same (The set of equilibrium concentrations is called the equilibrium position.) There is only one equilibrium constant at a particular temperature, but there is an infinite number of equilibrium positions. The specific equilibrium position depends on the starting concentrations, but the equilibrium constant does not.

The Extent of a Reaction The tendency for a reaction to occur is indicated by the magnitude of the equilibrium constant. A value of K much larger than 1 means that at equilibrium the reaction will consist mostly of product-the equilibrium lies to the right. (The reaction goes to completion) A value of K much smaller than 1 means that at equilibrium the reaction will consist mostly of reactants-the equilibrium lies to the left. (The reaction does not occur to any given extent) The size of K and the time required to reach equilibrium are not directly related.

Equilibrium Constant Through experimentation and observation, the Law of Mass Action was proposed. The law suggests that for a reaction of the following type: jA + kB  lC + mD the following equilibrium expression is used to represent the reaction. K = [C]l [D]m [A]j [B]k K is the equilibrium constant [ ] represent the concentrations at equilibrium Coefficients become the exponents

Le Chatelier’s Principle Le Chatelier’s Principle states that if a change is imposed on a system at equilibrium, the position of equilibrium will shift in a direction that tends to reduce that change.

Effect of a Change in Concentration If a component is added to a reaction system at equilibrium (at constant T and P), the equilibrium position will shift in the direction that lowers the concentration of that component (away from). If a component is removed from the system, the system will shift in the direction that increases the concentration of that component (towards)

Copy the following equation: As4O6(s) + 6C(s) < -- > As4(g) + 6CO (g)

In which direction will the equilibrium position shift if CO is added? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

In which direction will the equilibrium position shift if C is removed? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

In which direction will the equilibrium position shift if CO is removed? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Effect of a Change in Pressure Adding an inert gas has no effect on the equilibrium position (has no effect on the concentrations or partial pressures of the reactants or products). Increasing the volume of the container decreases the pressure. The system responds by increasing the pressure through the production of more gaseous molecules (a shift toward the side with the greatest number of gas molecules). Decreasing the volume of the container results in the opposite occurrence.

Left Right No shift will occur What shift in the equilibrium position will occur if the volume is reduced for the following process: P4(s) + 6Cl2(g) < -- > 4PCl3(l) 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Left Right No shift will occur What shift in the equilibrium position will occur if the volume is reduced for the following process: PCl3 (g) + Cl2 (g) < -- > PCl5 (g) 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Left Right No shift will occur What shift in the equilibrium position will occur if the volume is reduced for the following process: PCl3(g) + 3NH3(g) < -- > P(NH2)3(g) + 3HCl(g) 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Effect of a Change in Temperature Changing the temperature changes the value of K. In exothermic reactions (heat is given off and is therefore written as a product), the equilibrium position will shift to the left if the temperature is increased and to the right if the temperature is decreased. For endothermic reactions, the opposite will occur.

Increases Decreases Remains the same For the following reaction, predict how the value of K changes as the temperature is increased. N2(g) + O2(g) < -- > 2NO(g) ΔH = 181 kJ 10 Increases Decreases Remains the same 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Increases Decreases Remains the same For the following reaction, predict how the value of K changes as the temperature in increased. 2SO2(g) + O2(g) < -- > 2SO3(g) ΔH = -198 kJ 10 Increases Decreases Remains the same 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Copy the following equation N2(g) + 3H2(g) < -- > 2NH3(g) + 92.94 kJ

How will the equilibrium position shift if the temperature increases? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

How will the equilibrium position shift if the volume increases? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

How will the equilibrium position shift if NH3 is removed? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

How will the equilibrium position shift if N2 is added? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

How will the equilibrium position shift if some Ar(g) is added? 10 Left Right No shift will occur 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30