Chemical Equilibrium

NO 2 N 2 O 4

Le Châtelier’s Principle When a stress is applied to a system at equilibrium, the equilibrium shifts to relieve the stress. What are stressors?

Changing Concentration Remember the phase equilibrium: H 2 O(l) H 2 O(g). What is the effect, on the system, of increasing the temperature? Which direction does the equilibrium shift when you add or remove (change concentrations of) the reactants or products?

The global industrial production of ammonia for 2012 was anticipated to be 198,000,000 tons. N 2 + 3H 2 2NH 3

Effects of Equilibrium Shifts What happens to the concentrations of the products and reactants when the equilibrium shifts? introduce a stress  equilibrium shifts equilibrium shifts  concentrations of products and reactants change.

N 2 + 3H 2 2NH 3 When the equilibrium shifts to the right, more NH 3 molecules are being made. To make more NH 3 you need to use up more H 2. What happens to the concentrations of NH 3 and H 2 when the concentration of N 2 is increased. ACTION/ STRESS

N 2 + 3H 2 2NH 3 What happens to the concentrations of NH 3 and N 2 when the concentration of H 2 is increased. When the equilibrium shifts to the right, more NH 3 molecules are being made. To make more NH 3 you need to use up more N 2. ACTION/ STRESS

N 2 + 3H 2 2NH 3 What happens to the concentrations of H 2 and N 2 when the concentration of NH 3 is increased. When the equilibrium shifts to the left, more NH 3 molecules are being broken up into H 2 and N 2. ACTION/ STRESS

N 2 + 3H 2 2NH 3 What happens to the concentrations of NH 3 and H 2 when the concentration of N 2 is decreased. ACTION/ STRESS When the equilibrium shifts to the left, more NH 3 molecules are being broken up into H 2 and N 2.

N 2 + 3H 2 2NH 3 What happens to the concentrations of NH 3 and N 2 when the concentration of H 2 is decreased. ACTION/ STRESS When the equilibrium shifts to the left, more NH 3 molecules are being broken up into H 2 and N 2.

N 2 + 3H 2 2NH 3 What happens to the concentrations of H 2 and N 2 when the concentration of NH 3 is decreased. ACTION/ STRESS When the equilibrium shifts to the right, more NH 3 molecules are being made. To make more NH 3 you need to use up more N 2.

Changing Concentration: Common Ion Effect Consider the following equation: NaSO 4 Na + (aq) + SO 4 -2 (aq) What would be the effect on the equilibrium of adding NaCl? What would happen to the solubility of KNO 3 if KCl is added to this solution at equilibrium? KNO 3 K + (aq) + NO 3 - (aq)

Changing the temperature causes equilibrium shifts for chemical reactions as well as physical changes. The direction of shift is dependent on whether the reaction is exothermic or endothermic. Changing Temperature

Rate of Forward Reaction = Rate of Reverse Reaction N 2 + 3H 2 2NH KJ 2NO 2 N 2 O KJ 53.0 KJ + H 2 + I 2 2HI 2H 2 + O 2 2H 2 O KJ Table I

N 2 + 3H 2 2NH KJ What happens to the concentrations of H 2, N 2 and NH 3 when the temperature of the system is increased. When the equilibrium shifts to the left, more NH 3 molecules are being broken up into H 2 and N 2. ACTION/ STRESS

N 2 + 3H 2 2NH KJ What happens to the concentrations of H 2, N 2 and NH 3 when the temperature of the system is decreased. ACTION/ STRESS When the equilibrium shifts to the right, more NH 3 molecules are being made. To make more NH 3 you need to use up more N 2.

Changing pressure shifts equilibriums only when one of the products or reactants is a gas. Remember: What is the effect on the following equilibrium when pressure is increased: CO 2 (g) CO 2 (aq)? When there are gases present on the product side as well as the reactant side, another factor has to be considered when determining the direction of equilibrium shift due to pressure change: the number of moles of products and reactants. Changing Pressure

N 2 + 3H 2 2NH KJ Remember: 1 mole of a gas occupies __________ L 2 moles of a gas occupy __________ L 3 moles of a gas occupy __________ L 4 moles of a gas occupy __________ L In a container, when pressure is increased, volume _____________________. In a system at equilibrium, increasing pressure causes the equilibrium to shift in the direction of ____________ moles of gas because _________________________________. In a system at equilibrium, decreasing pressure causes the equilibrium to shift in the direction of ____________moles of gas because lower pressure means greater volume and more room for more moles of gas.

Definition of a catalyst: increases rate of reaction without being involved. A catalyst increases the rate of reaction equally in both directions. So there is no net increase in any one direction. Adding a Catalyst

HOMEWORK Equilibrium: Pg Q Entropy/Enthalpy Pg Q Kinetics and Equilibrium: Pg Q 1-46Parts A, B, C.

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