Preview Lesson Starter Objectives Predicting the Direction of Shift Reactions That Go to Completion Common-Ion Effect Chapter 18 Section 2 Shifting Equilibrium

Imagine children playing on a seesaw. Five boys are sitting on one side and five girls on the other, and the seesaw is just balanced. Then, one girl gets off, and the system is no longer at equilibrium. One way to get the seesaw in balance again is for one of the boys to move toward the girls’ side. Section 2 Shifting Equilibrium Chapter 18 Lesson Starter

When he gets to the middle, the seesaw is again at equilibrium. The stress of one girl getting off is relieved by having one of the boys move his position. How would a chemical system in equilibrium respond to removing one of the products? Section 2 Shifting Equilibrium Chapter 18 Lesson Starter, continued

Objectives Discuss the factors that disturb equilibrium. Discuss conditions under which reactions go to completion. Describe the common-ion effect. Section 2 Shifting Equilibrium Chapter 18

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. Section 2 Shifting Equilibrium Chapter 18 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.

Click below to watch the Visual Concept. Visual Concept Chapter 18 Le Chatelier's Principal Section 2 Shifting Equilibrium

Click below to watch the Visual Concept. Visual Concept Chapter 18 Factors Affecting Equilibrium Section 2 Shifting Equilibrium

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. Section 2 Shifting Equilibrium Chapter 18 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 Section 2 Shifting Equilibrium Chapter 18 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. Section 2 Shifting Equilibrium Chapter 18 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, 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. Section 2 Shifting Equilibrium Chapter 18

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. Section 2 Shifting Equilibrium Chapter 18

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. Section 2 Shifting Equilibrium Chapter 18

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. Section 2 Shifting Equilibrium Chapter 18

End of Chapter 18 Section 2 Show