1. + Unit D: Equilibrium Focusing on Acids & Bases Textbook Reference: Chapters 15 and 16

2. + Equilibrium 1.1K: Define equilibrium and state the criteria that apply to a chemical system in equilibrium; i.e., closed system, constancy of properties, equal rates of forward and reverse reactions 1.2K: Identify, write and interpret chemical equations for systems at equilibrium 1.4K: Define K c to predict the extent of the reaction and write equilibrium-law expressions for given chemical equations, using lowest whole-number coefficients

3. + Chemical Equilibrium Not all reactions are quantitative (reactants  products) Evidence: For many reactions reactants are present even after the reaction appears to have stopped Recall the conditions necessary for a chemical reaction: Particles must collide with the correct orientation and have sufficient energy If product particles can collide effectively also, a reaction is said to be reversible Rate of reaction depends on temperature, surface area and concentration 100%

4. + Chemical Equilibrium Consider the following reversible reaction: The final state of this chemical system can be explained as a competition between: We assume this system is closed (so the reactants and products cannot escape) and will eventually reach a: DYNAMIC EQUILBRIUM - Opposing changes occur simultaneously at the same rate The collisions of reactants to form products The collisions of products to re-form reactants

5. + Modelling Dynamic Equilibrium Mini Investigation pg. 678 Volume of Cylinder #1 Volume of Cylinder #2 25.00.0 20.05.0 17.08.0 14.011.0 14.0 8.017.0 5.020.0 2.023.0 2.023.0 2.023.0 2.023.0 Assume large straw transfers 5 mL each time and the smaller straw transfers 2 mL each time

6. + Chemical Equilibrium Consider the following hypothetical system: AB + CD  AD + BC forward reaction, therefore AD + BC  AB + CD reverse reaction Initially, only AB and CD are present. The forward reaction is occurring exclusively at its highest rate. As AB and CD react, their concentration decreases. This causes the reaction rate to decrease as well. As AD and BC form, the reverse reaction begins to occur slowly. As AD and BC’s concentration increases, the reverse reaction speeds up. Eventually, both the forward and reverse reaction occur at the same rate = DYNAMIC EQUILIBRIUM

7. + 4 Conditions of Dynamic Equilibrium* 1. Can be achieved in all reversible reactions when the rates of the forward and reverse reaction become equal Represented by rather than by  2. All observable properties appear constant (colour, pH, etc) 3. Can only be achieved in a closed system (no exchange of matter and must have a constant temperature) 4. Equilibrium can be approached from either direction. This means that the equilibrium concentrations will be the same regardless if you started with all reactants, all products, or a mixture of the two

8. Types of Equilibrium 1. Phase Equilibrium: a single substance existing in more than 1 phase Example: Liquid water in a sealed container with water vapour in the space above it Water evaporates until the concentration of water vapour rises to a maximum and then remains constant 2. Solubility Equilibrium: a saturated solution Rate of dissolving = rate of recrystallization

9. Types of Equilibrium 3. Chemical Equilibrium – reactants and products in a closed system Example: The Hydrogen-Iodine Equilibrium System The rate of reaction of the reactants decreases as the number of reactant molecules decrease. The rate at which the product turns back to reactants increases as the number of product molecules increases. These two rates become equal at some point, after which the quantity of each will not change.

10. + Describing the Position of Equilibrium 1. Percent Yield- the yield of product measured at equilibrium compared with the maximum possible yield of product. % yield = product eq’m x 100 % product max The equilibrium concentration is determined experimentally, the maximum concentration is determined with stoichiometry

11. + 1. Percent Yield – Example If 2.50 mol of hydrogen gas reacts with 3.0 mol of iodine gas in a 1.00L vessel, what is the percent yield if 3.90 mol of hydrogen iodide is present at equilibrium % yield = product eq’m x 100 % product max H 2(g) + I 2(g) 2HI (g) 2.50 mol x (2 mol HI) = 5.0 mol HI 1 mol H 2(g) Describing the Position of Equilibrium % yield = 3.90 mol x 100 % 5.00 mol = 78%

12. + 2. Using an Equilibrium Constant, (K c ) This relationship only works if all concentrations are at equilibrium at a constant temperature in a closed system Think “products over reactants” If the K c > 1, the equilibrium favours products If the K c < 1, the equilibrium favours reactants Describing the Position of Equilibrium

13. + 2. Using an Equilibrium Constant, (K c ) Example #1: Write the equilibrium law expression for the reaction of nitrogen monoxide gas with oxygen gas to form nitrogen dioxide gas. Describing the Position of Equilibrium

14. + 2. Using an Equilibrium Constant, (K c ) Note: The K c value describes the extent of the forward reaction. K c reverse = 1 = The reciprocal value K c forward Example #2: The value of K c for the formation of HI (g) from H 2(g) and I 2(g) is 40, at a given temperature. What is the value of K c for the decomposition of HI (g) at the same temperature. K c reverse = 1.= 1 = 0.025 K c forward 40 Describing the Position of Equilibrium

15. + 2. Using an Equilibrium Constant, (K c ) Note: For heterogeneous equilibrium systems, DO NOT include liquids and solids in the expression. (They are assumed to have fixed concentrations) Example #3: Write the equilibrium law expression for the decomposition of solid ammonium chloride to gaseous ammonia and gaseous hydrogen chloride Example #4: Write the equilibrium law expression for the reaction of zinc in copper(II) chloride solution. Describing the Position of Equilibrium The solid is omitted from the expression The solids, as well as spectator ions (Cl - ) are omitted from the expression

16. + Describing the Position of Equilibrium

17. + PRACTICE Lab Exercise 15.B pg. 686 - complete Analysis Pgs 688 – 689 #s 1, 4 - 6