U1 S2 L2 Shifting the Balance. Factors that effect equilibrium p

Upon completion of this lesson, you will be able to: explain how the forward and reverse reaction rates in a chemical equilibrium are affected by changes in the temperature, pressure, concentration of a reactant or product, or the volume of the system. explain why the addition of a catalyst and varying the surface area of a reactant or product does not cause the equilibrium to shift, but does affect the time it takes to establish an equilibrium. state Le Châtelier's principle.

The Position of an Equilibrium In a system at equilibrium, the concentrations of the reactants and products do not have to be equal; in fact, they are unequal in almost all cases. –The concentrations of reactants are either greater than or less than those of the products. The only equal thing in an equilibrium is the opposing rates of reaction. The position of an equilibrium depends on the relative concentrations of reactants and products: –if the concentration of the reactants is greater than the concentration of the products, then we say that the position of the equilibrium lies to the left. –if the concentration of the products is greater than the concentration of the reactants, then we say the position of the equilibrium lies to the right.

Changing the Position of an Equilibrium A system at equilibrium will stay at equilibrium as long as: 1.the system remains closed - nothing is added or removed, and 2.temperature and pressure are kept constant. If a system is disturbed, then the position of the equilibrium may shift. three types of disturbance that can cause a shift in equilibrium: 1.changing the concentration of a species.concentration 2.changing the volume of a system.volume 3.changing the temperature. temperature

Le Chatelier’s Principle Le Chatelier’s principle states: (p 520) –If a system at equilibrium is subjected to a stress, the equilibrium will shift in a direction to relieve the stress. (ie: the system is looking for balance)

1) Concentration From the collision theory, if you increase the concentration of a reactant then there is an increased number of collisions. Thus, the rate of forward reaction is increased and the equilibrium will shift to the right. The stress on the system is excess reactant, the system responds by trying to remove the excess reactant. As a result the reaction shifts forward producing more products thus attaining a new balance!

2) Temperature To consider the effect of temperature on equilibrium you must consider whether the reaction is endothermic or exothermic. Endothermic –The addition of heat (increase in temp) will favour the endothermic reaction, the extra heat is absorbed by the reaction. –The stress is the addition of energy, the system will respond to remove the energy. Exothermic –The removal of heat (decrease in temp) will favour the exothermic reaction, the reaction shifts to produce more energy. –The stress is the loss of energy, the system will shift to produce more energy.

3) Pressure A change in pressure only affects equilibrium systems that include gases. Why? Solids and liquid are incompressible (they can’t be squat together) According to Le chatelier’s principle, when the pressure on an equilibrium system is increased, the reaction is driven in the direction that will relieve the pressure. Therefore with an increase in pressure the system will shift to the side with the least moles of gaseous molecules. (less moles of gas = less pressure) Increased pressure –> shift to fewest moles of gas Decreased pressure –> shift to most moles of gas –If the moles of gas are equal, pressure will not alter the position of equilibrium.

Volume Changing the volume of the reaction vessel in effect changes the pressure of the system. –Reducing the volume causes an increase in pressure. –Increasing the volume reduces pressure.

Catalysts The addition of a catalyst speed up the rate of forward reaction as well as the rate of reverse reaction. diagram Therefore, a catalyst does not affect the position of equilibrium. The catalyst simply gets us to the point of equilibrium quicker.

Haber process. List all changes that could be made to maximize the production of ammonia:

Trivia!Trivia! 1)Consider chemical systems at equilibrium. Which statement is FALSE? a)They usually involve equal amounts of reactants and products. b)The rates of the forward and reverse reactions are equal. c) All visual observation appear unchanged. d)The establishment of this equilibrium requires a closed system. 2)In which reaction will a decrease in the volume of the reaction vessel favour the formation of products? a)2C (s) + O 2(g) ⇌ 2CO 2(g) b)2HBr (g) ⇌ H 2(g) + Br 2(g) c)3H 2(g) + N 2(g) ⇌ 2NH 3(g) d)CaCO 3(s) ⇌ CaO (s) + CO 2(g) 3)Which change will increase the equilibrium concentration of NH 3 (g) ? a)a decrease in the concentration of H2 (g) b)a decrease in the temperature of the system c)a decrease in the total pressure d)an increase in the volume of the system

Textbook Practice Items p 529 # 33 – 37 P 533 # page 535: items 4, 5, 6, and 7 pages : items 15, 16, 17, 38 and 43

Predict the shift in equilibrium you would expect when 1) the temperature of the reaction mixture is increased PCl 5(g) + energy ⇌ PCl 3(g) + Cl 2(g) 2) PCl 3 is removed from the system PCl 5(g) + energy ⇌ PCl 3(g) + Cl 2(g) 3) Cl 2 is added to the system PCl 5(g) + energy ⇌ PCl 3(g) + Cl 2(g) 4) the pressure of the system is increased PCl 5(g) + energy ⇌ PCl 3(g) + Cl 2(g) 5)a catalyst is added PCl 5(g) + energy ⇌ PCl 3(g) + Cl 2(g) a) Left b) Right c) No effect d) idk

Predict the direction in which each of the following systems will shift upon applications of the stresses listed. 1)2NOBr (g) ⇌ 2NO (g) + Br 2(g) Stress: Decrease in pressure 2)3Fe (s) + 4H 2(g) ⇌ Fe 3 O 4(s) + 4H 2(g) Stress: Add Fe 3)2SO 2(g) + O 2(g) ⇌ 2SO 3 (g) kJ Stress: Increase the pressure 4)2SO 2(g) + O 2(g) ⇌ 2SO 3(g) kJ Stress: Increase the temperature a)Left b)Right c) No effect d) idk