EQUILIBRIUM TIER 4 Apply LeChatelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant State and explain the effect of a catalyst on an equilibrium reaction

Predicting the Direction of Shift Changes in pressure, concentration, or temperature can alter the equilibrium position and thereby change the relative amounts of reactants and products. Le Châtelier’s principle states that if a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress. This principle is true for all dynamic equilibria, chemical as well as physical. Changes in pressure, concentration, and temperature illustrate Le Châtelier’s principle.

Predicting the Direction of Shift, continued Changes in Pressure A change in pressure affects only equilibrium systems in which gases are involved. For changes in pressure to affect the system, the total number of moles of gas on the left side of the equation must be different from the total number of moles of gas on the right side of the equation. An increase in pressure is an applied stress. It causes an increase in the concentrations of all species. The system can reduce the total pressure by reducing the number of molecules.

Predicting the Direction of Shift, continued Changes in Pressure, continued the Haber process for the synthesis of ammonia 4 molecules of gas 2 molecules of gas When pressure is applied, the equilibrium will shift to the right, and produce more NH 3. By shifting to the right, the system can reduce the total number of molecules. This leads to a decrease in pressure.

Predicting the Direction of Shift, continued Changes in Pressure, continued Even though changes in pressure may shift the equilibrium position, they do not affect the value of the equilibrium constant. The introduction of an inert gas, such as helium, into the reaction vessel increases the total pressure in the vessel. But it does not change the partial pressures of the reaction gases present. Increasing pressure by adding a gas that is not a reactant or a product cannot affect the equilibrium position of the reaction system.

Predicting the Direction of Shift, continued Changes in Concentration An increase in the concentration of a reactant is a stress on the equilibrium system. An increase in the concentration of A creates a stress. To relieve the stress, some of the added A reacts with B to form products C and D. The equilibrium is reestablished with a higher concentration of A than before the addition and a lower concentration of B.

Predicting the Direction of Shift, continued Changes in Concentration, continued Changes in concentration have no effect on the value of the equilibrium constant. Such changes have an equal effect on the numerator and the denominator of the chemical equilibrium expression. The concentrations of pure solids and liquids do not change, and are not written in the equilibrium expression. When a solvent, such as water, in a system involving acids and bases, is in an equilibrium equation, it is not included in the equilibrium expression.

Predicting the Direction of Shift, continued Changes in Concentration, continued High pressure favors the reverse reaction. Low pressure favors the formation of CO 2. Because both CaO and CaCO 3 are solids, changing their amounts will not change the equilibrium concentration of CO 2.

Predicting the Direction of Shift, continued Changes in Temperature Reversible reactions are exothermic in one direction and endothermic in the other. The effect of changing the temperature of an equilibrium mixture depends on which of the opposing reactions is endothermic and which is exothermic. The addition of energy in the form of heat shifts the equilibrium so that energy is absorbed. This favors the endothermic reaction. The removal of energy favors the exothermic reaction.

EXOTHERMIC REACTIONS: A + B  C + D +HEAT Increasing the temperature is like adding something to the product side of the equation, so the equilibrium position will shift back to the reactant side to relief the stress Therefore decreasing the temperature is like removing something on the product side of the equation, so the equilibrium position will shift toward the product side to relief the stress

ENDOTHERMIC REACTION: A + B + HEAT  C + D Increasing the temperature is like adding something to the reactant side of the equation, so the equilibrium position will shift toward the product side to relief the stress Therefore decreasing the temperature is like removing something on the reactant side of the equation, so the equilibrium position will shift toward the reactant side to relief the stress

Predicting the Direction of Shift, continued Changes in Temperature, continued A rise in temperature increases the rate of any reaction. In an equilibrium system, the rates of the opposing reactions are raised unequally. The value of the equilibrium constant for a given system is affected by the temperature.

Predicting the Direction of Shift, continued Changes in Temperature, continued The synthesis of ammonia by the Haber process is exothermic. A high temperature favors the decomposition of ammonia, the endothermic reaction. At low temperatures, the forward reaction is too slow to be commercially useful. The temperature used represents a compromise between kinetic and equilibrium requirements.

Predicting the Direction of Shift, continued Changes in Temperature, continued Catalysts have no effect on relative equilibrium amounts. They only affect the rates at which equilibrium is reached. Catalysts increase the rates of forward and reverse reactions in a system by equal factors. Therefore, they do not affect K.

PROBLEM: EXAMINE THE FOLLOWING REACTION AND APPLY Le CHATELIER’S PRINCIPLE TO PREDICT WHICH DIRECTION IF ANY THE EQUILIBRIUM POSITION WILL SHIFT IF THE DESIGNATED STRESS IS APPLIED 2CO(g) + O 2 (g)  2CO 2 (g) + 167kJ (A.) Adding more carbon dioxide (B)Increasing the pressure on the closed system (C)Adding an inert gas such as helium (D)Increasing the temperature (E)Adding more oxygen gas (F)Adding a catalyst

ANSWER: A SHIFTS BACK TOWARD THE REACTANTS OR SHIFTS LEFT B.SHIFTS TOWARD THE PRODUCTS SINCE (SHIFTS RIGHT) THERE ARE LESS GAS MOLECULES ON THE PRODUCT SIDE C.DOES NOT AFFECT EQUILIBRIUM D.This does not effect the equilibrium E.This has not affect on equilibrium F.SHIFTS BACK TOWARD THE REACTANTS OR SHIFTS LEFT G.SHIFTS TOWARD THE PRODUCTS H.HAS NO AFFECT ON EQUILIBRIUM

The end