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Ch 18: Chemical Equilibrium

Ch 18: Chemical Equilibrium. Section 18.2 Shifting Equilibrium. Standards and Objectives. Standard 9 Students know how to use LeChatelier’s principle to predict the effect of changes in concentration, temperature, and pressure.

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Ch 18: Chemical Equilibrium

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  1. Ch 18: Chemical Equilibrium

  2. Section 18.2Shifting Equilibrium

  3. Standards and Objectives • Standard 9 • Students know how to use LeChatelier’s principle to predict the effect of changes in concentration, temperature, and pressure. • Students know equilibrium is established when forward and reverse reaction rates are equal. • Objective • We will discuss the factors that disturb equilibrium, discuss reactions that go to equilibrium, & describe the common ion effect.

  4. Reversible Reactions • Reversible Reactions: one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. • The double arrow tells you that the reaction is reversible. 2 SO2(g) + O2(g) ⇄ 2 SO3(g)

  5. Chemical Equilibrium: when the rates of the forward and reverse reactions are equal, the reaction has a reached a state of balance. • At chemical equilibrium, no net change occurs in the actual amounts of the components of the system.(Escalator example) • The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction.

  6. Factors Affecting Equilibrium: Le Châtelier’s Principle • Le Châtelier’s Principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

  7. CONCENTRATION • If you add more of a reactant, the reaction goes toward the products. • If you take away some of a reactant, the reaction goes toward the reactants. • If you add more of a product, the reaction goes toward the reactants. • If you take away some of a product, the reaction goes toward the products.

  8. TEMPERATURE • If you add heat (a product) the reaction shifts to the reactants • If you take away heat (a product), the reaction shifts toward the products. PRESSURE – ONLY GASES!! • If you increase pressure, the reaction shifts towards the side with less moles. • If you decrease pressure, the reaction shifts towards the side with more moles.

  9. Reactions to Completion • A reaction is considered to “go to completion”, when almost all of the ions are removed from the solution. • This depends on the solubility of the product formed, and if it is soluble, then on its degree of ionization.

  10. Formation of a Gas • Gases are not very soluble, so when a gas is formed and the reaction container is open to the air, the gas will escape and the reaction will go almost to completion.

  11. Formation of a Precipitate • If a product is insoluble (a precipitate), then when the product forms, it cannot dissolve to allow the reaction to go in the reverse direction. • NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s) • Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq)  Na+(aq) + NO3-(aq) + AgCl(s)

  12. Formation of a Slightly Ionized Product • This occurs with the neutralization reactions of acids and bases. • HCl(aq) + NaOH(aq) • H3O+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) Na+(aq) + Cl-(aq) + 2H2O(l) • H3O+(aq) + OH-(aq) 2H2O(l) • Water only slightly ionizes, so it exists as mainly H2O molecules.

  13. Common Ion Effect • This is the phenomenon in which the addition of an ion common to two solute brings about precipitation or reduced ionization. • If you have a NaCl solution and you add HCl acid, the extra Cl-ions from the acid will mix with the Na+ ions to form NaCl precipitate.

  14. Homework • 18.2 pg 604 #1-5

  15. Section 18.1 The Nature of Chemical Equilibrium

  16. Standards and Objectives • Standard 9.c. • Students know how to write and calculate an equilibrium constant expression for a reaction. • Objective • We will write chemical equilibrium expressions and carry out calculations involving them.

  17. Equilibrium Expressions • Equilibrium Constant: Keq is the ratio of product concentrations to reactant concentrations at equilibrium. nA + mB ⇄ xC + yD Keq = [C]x [D]y [A]n [B]m

  18. Write Equilibrium Expressions: • H2 + I2↔ 2HI • 2HgO ↔ 2Hg + O2 • 2SO2 + O2↔ 2SO3 • N2 + 3H2↔ 2NH3

  19. Equilibrium Constants • To find an Equilibrium Constant, plug in the concentrations of the reactants and products into the equilibrium expression and solve! • Keq> 1, products favored at equilibrium • Keq < 1, reactants favored at equilibrium • Keq does not have any units.

  20. Calculating Keq • A liter of a gas mixture at equilibrium at 10°C contains 0.0045 mol of N2O4 and 0.030 mol of NO2. Write the expression for the equilibrium constant and calculate Keq. N2O4(g) ⇄ 2NO2(g) Keq = [NO2]2 = (0.030 mol/L)2 [N2O4] = 0.0045 mol/L Keq = 0.20

  21. Homework • 18.1 pg 595 #1, 3, 6-9

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