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Equilibrium Chapter 13

Equilibrium Chapter 13. Dynamic Equilibrium – observed when opposing forces are occurring at the same time and the same rate. (only occurs when nothing is added or taken away and temperature must remain constant. Two Types of Equilibrium 1. Physical Equilibrium

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Equilibrium Chapter 13

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  1. Equilibrium Chapter 13 Dynamic Equilibrium – observed when opposing forces are occurring at the same time and the same rate. (only occurs when nothing is added or taken away and temperature must remain constant. Two Types of Equilibrium 1. Physical Equilibrium 2. Chemical Equilibrium

  2. Physical Equilibrium • Solution Equilibrium – when the number of particles dissolving is equal to the number of particles returning to crystal form. • Vapour Equilibrium – when the number of molecules vaporizing is equal to the number of molecules condensing.

  3. Chemical Equilibrium • occurs when the rate of formation of the products is equal to the rate of formation of the reactants • this only occurs when nothing is added or taken away from the system Almost all reactions are reversible. - formation of the products is the forward rxn - formation of the reactants is the reverse rxn 2 CrO42- + 2 H+ Cr2O72- + H2O yellow orange * Remember – Equilibrium does not mean that the concentration of reactants and products is equal but the rate they are forming is equal.

  4. Why are reactions reversible? During a reaction, the reactants collide with each other to form the products. It makes sense that the product molecules are also colliding with one another. → When product molecules collide with the correct orientation and sufficient energy to overcome the activation energy barrier, they will reform the reactants.

  5. Why does a system reach equilibrium? • As a reaction occurs, the amount of products builds up and the amount of reactants decreases. • As the reactants decreases, the rxn rate slows down. • With more products colliding, the rxn rate increases. • Eventually, the rates will even out. • When equilibrium is reached, there is no overall change in the properties of the system. (color, mass, pressure, conc, pH will stop changing) • If equilibrium is reached early – there will be little product. • If equilibrium is reached after a long time – there will be a great deal of product. Equilibrium Simulation

  6. Four Conditions That Apply To All Equilibrium Systems 1. Equilibrium is achieved in a reversible system when the rates of opposing changes is equal. The double arrow, , indicates reversible changes. 2. The observable properties of a system at equilibrium are constant. At equilibrium, you will not notice changes in colour, pressure, concentration, pH. 3. Equilibrium can only be reached in a closed system, where nothing can enter or escape – including energy. • Some changes are negligible, therefore equilibrium principles can be applied even if the system is not physically closed. 4. Equilibrium can be approached from either direction. • Ex. H2 (g) + Cl2 (g) 2 HCl(aq) If in a closed system, equilibrium will be reached with this system whether you started with HCl, or H2 and Cl2.

  7. The Equilibrium Constant • The Law of Chemical Equilibrium states that “At equilibrium, there is a constant ratio between the concentrations of the products and the reactants in any change.” • In proving this theory, chemists discovered a particular relationship among the products and the reactants of any chemical system. This relationship is known as the equilibrium constant. • The equilibrium constant has a symbol of Kc • The subscripted “c” represents the fact that the constant is in terms of molar concentration.

  8. Thus, for a general equilibrium reaction: aP + bQcR + dS Where a, b, c, and d are the coefficients of the balanced chemical equation. P, and Q could be any reactant, and R, and S could be any product. Then Kc = Notice: Kc =

  9. Write the equilibrium constant expression. Ex. 2 SO2 (g) + O2 (g) 2 SO3 (g) Kc = Write the equilibrium constant expression. Ex. H2 (g) + I2 (g) 2 HI (g) Kc =

  10. General Rules For Writing the Equilibrium Constant • Only include substances which can vary in concentration... INCLUDE: aqueous ions and gases DO NOT INCLUDE: liquids or solids Write the equilibrium constant expression for the following. Ex. P4 (s) + 3 O2 (g) 2 P2O3 (g) Kc = Ex. Ca3(PO4)2 (s) 3 Ca2+(aq)+ 2 PO43- (aq) Kc = Complete Problems #1-5 on page 497

  11. The Equilibrium Constant And Temperature • For a given system at equilibrium, the value of the equilibrium constant depends only on the temperature. • Changing the temperature of a system changes the rates of both the forward and reverse reactions – both by different amounts because both have different activation energies. a reacting mixture at one temperature has an equilibrium constant whose value changes if the mixture is allowed to reach equilibrium at a different temperature. • You will see a temperature value for each problem... It has nothing to do with your answer, except that if you wanted to look up the accepted value for that system at equilibrium, you will need to look it up based on temperature. Notice the chart on page 850 is based on 25oC.

  12. Calculating the Equilibrium Constant The equilibrium constant is a numerical value based on the concentration of the reactants and the products. To Determine the Equilibrium Constant • Write the equilibrium constant expression. • Simply place the concentration values in the correct places and do the math. ** Remember to multiply the values – they are not added. ** Make sure you know how to use your calculator.

  13. Ex. A mixture of nitrogen and chlorine gases was kept at a certain • temperature in a reaction flask. • N2 (g) + 3 Cl2 (g) 2 NCl3 (aq) • The [N2] = 1.4 x 10-3 mol/L, [Cl2] = 4.4 x 10-4 mol/L, [NCl3] = 1.9 x 10-1 mol/L • Solution: • Kc= • = • = 3.0 x1011 • Practice Problems #6-10 page 499

  14. Qualitatively Interpreting the Equilibrium Constant • A large Kc indicates that the concentration of the products is much greater than the concentration of the reactants at equilibrium. it is said that the position of the equilibrium lies to the right, or that it favours the products. • A small Kc indicates that the concentration of the reactants is much greater than the concentration of the products at equilibrium it is said that the position of the equilibrium lies to the left, or that it favours the reactants.

  15. Generally, • When K > 1, the products are favoured. • Equilibrium lies to the right. • Reactions where K is greater than 1010 are usually regarded as going to completion. • When K 1, there are approximately equal concentrations of reactants and products at equilibrium. • When K <1, the reactants are favoured. • Equilibrium lies to the left. • Reactions where K is smaller than 10-10 are usually regarded as not taking place at all.

  16. Problems #20-24 on Page 512

  17. Further Equilibrium Calculations The equilibrium constant, Kc, is calculated by substituting equilibrium concentrations into the equilibrium expression. When you are given the initial concentrations (not at equilibrium), you will need to use ICE tables to solve the problems. Sample Problem Page 505

  18. Problems 11-15 on page 508

  19. The Meaning Of a Small Equilibrium Constant • Understanding the meaning of a small equilibrium constant can simplify the math in this unit. • A small Kc value means the equilibrium favours the reactants so only a small amount of the reactants will become products – an amount so small that it can be ignored. • When Kc is small compared to the initial concentration, the value of x (on the reactant side) in the ICE tables is small enough to be ignored.

  20. In general, we apply the 5% Rule smallest initial conc. = greater than 500 Kc You can ignore x smallest initial conc. = between 100-500 Kc You may be justified in ignoring x smallest initial conc. = less than 500 Kc You are not justified in ignoring x – solve fully (quadratic)

  21. Sample Problem Page 513-514 Problems 25-29 Page 515

  22. 13.1 Predicting the Direction of A Rxn • When systems are at equilibrium, we can get equilibrium constant expressions, calculate equilibrium constants and figure out which rxn is favoured. • When solutions are not at equilibrium, we can calculate another expression – the Reactant Quotient, Qc. This expression can tell us in which direction the reaction must proceed to reach equilibrium. • The reactant quotient is identical to the equilibrium constant expression, except that its value is calculated using concentrations that are not necessarily at equilibrium. Qc =

  23. The Relationship Between Kc and Qc • When Qc < Kc – the products are favoured - equilibrium lies to the right • When Qc = Kc – the system is at equilibrium • When Qc > Kc – the reactants are favoured - equilibrium lies to the left

  24. Sample Problem Page 518 Problems 30-32 Page 519

  25. Le Chatelier`s Principle • ’A system at equilibrium tends to respond so as to relieve the effect of any change in the conditions that affect equilibrium.’ • If a stress is applied to any system at equilibrium, the system will respond in an equal and opposite reaction so as to regain equilibrium. • There are two types of responses: • Favouring the forward rxn – shift to the right • Favouring the reverse rxn – shift to the left Stresses That Can Be Applied To A System: - Concentration Effects - Temperature Effects - Pressure Effects - Catalytic Effects

  26. 2 HI (g) H2 (g) + I2 (g) + 25.7 kJ • Concentration Effects • ↑ [reactants] will favour the forward rxn • ↓ [reactants] will favour the reverse rxn • ↑ [products] will favour the reverse rxn • ↓ [products] will favour the forward rxn • Temperature Effects • (think of the enthalpy value as a reactant or product) • ↑ temperature will favour a shift in the endothermic direction. • ↓ temperature will favour a shift in the exothermic direction

  27. N2 (g) + 3H2 (g) 2 NH3(g) • Pressure Effects • ↑ pressure (by decreasing volume) favours the reaction that will produce the fewest number of particles. • ↓ pressure (by increasing the volume) favours the reaction that will produce the most particles. • Addition of an Inert Gas has no effect on the system. • Addition Of a Catalyst • has no net effect on a system because both the forward and the reverse rxns are favoured. • Equilibrium is reached more quickly.

  28. Sample Problem Page 528 Problems #33-37 on Page 529-530

  29. Online Lab

  30. The Haber Process N2(g) + 3 H2(g) 2 NH3(g) + 92 kJ • Haber’s Process drives the forward rxn in 5 different ways • used lower temperatures • increased pressure • removed the NH3 as it was produced • took the unused N2 and H2 and fed it back into the original tank • used a catalyst

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