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Dynamic Equilibrium

Dynamic Equilibrium. Objectives. Describe chemical equilibrium in terms of equilibrium expressions Use equilibrium constants Describe how various factors affect chemical equilibrium Explain Le Chatelier’s principle. Equilibrium.

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Dynamic Equilibrium

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  1. Dynamic Equilibrium

  2. Objectives • Describe chemical equilibrium in terms of equilibrium expressions • Use equilibrium constants • Describe how various factors affect chemical equilibrium • Explain Le Chatelier’s principle

  3. Equilibrium • Some chemical systems have little tendency to react, while others go to completion • In between these extremes are chemical systems that reach a state of equilibrium with varying amounts of reactants unconsumed • For example: • N2 + 3H2 ⇔ 2NH3 • This reaction reaches equilibrium when fewer than 2 moles of ammonia produced

  4. Law of Chemical Equilibrium • At a given temp, ratio of concentrations of reactants and products has a constant value • General equation: • aA + bB ⇒ cC + dD • Equilibrium constant (Keq)

  5. Example • Hydrogen and Iodide react to produce Hydrogen Iodide • H2(g) + I2(g) ⇔ 2HI(g) • This is known as homogeneous equilibrium = all reactants & products in same physical state • Write Equilibrium Expression Keq for this reaction

  6. Example • Ammonia gas production: • N2 (g) + 3H2 (g) ⇔ 2NH3 (g) • Write equilibrium expression for this reaction

  7. Practice • Write equilibrium expressions for the following reactions: • N2O4(g) ⇔ 2NO2(g) • CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) • 2H2S(g) ⇔ 2H2(g) + S2(g)

  8. Heterogeneous Equilibrium • Not all reactants and products in same physical state • leave out reactants and products in solid or liquid state • Just include gas concentration or solute concentration

  9. Heterogeneous Equilibrium Example • C(s) + H2O(g) ⇔ CO(g) + H2(g) • Keq = [CO] [H2] [H2O]

  10. Why use equilibrium constants? • If you know the equilibrium constant for a particular chemical reaction at a particular temperature, you can determine the concentration of one of the reactants or products given the concentration of the remaining reactants and products

  11. Factors Affecting Chemical Equilibrium • When manufacturers make products, they want to minimize waste or leftover materials • Principles of chemical equilibrium can help determine the conditions that favor the most cost effective and environmentally friendly production of a chemical product

  12. Le Châtelier’s Principle • If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress

  13. How can we apply Le Châtelier’s Principle? • For example - we are trying to produce methane using the following reaction: • CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat • Unfortunately, using our current manufacturing techniques, at equilibrium we produce only 0.05900 mol of CH4 - way too low a yield to be cost effective • What can we do to increase our yield of methane?

  14. What is a stress? • Any kind of a change in a system that upsets equilibrium • Le Châtelier’s principle = predict how system will change • Going back to CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heathow to stress system to produce more product?

  15. Stresses that affect equilibrium • pressure/volume • temperature • concentration

  16. Changes in concentration • CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat • If we increase [CO] or [H2], system will respond by producing more product (shift to the right). If we decrease [CH4] or [H2O], system will respond by producing more product. • If we increase [CH4] or [H2O], system will respond by producing more reactant (shift to the left)

  17. Change in Volume/Pressure • N2 (g) + 3H2 (g) ⇔ 2NH3 (g) • Increase pressure (reduce volume) → reduce number of moles (shift to the right) • Reduce pressure (increase volume) → increase number of moles (shift to the left)

  18. Changes in Temperature • CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat • Increase temperature, system will respond in endothermic direction (shift to the left) • Decrease temperature, system will respond in exothermic direction (shift to the right)

  19. Practice Problems • Use Le Châtelier’s Principle to predict how each of these changes would affect CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat • Increase Temperature • Remove Hydrogen gas • Increase Volume • Increase Pressure • Decrease Temperature • Remove water vapour

  20. Practice problems • How would decreasing the volume of the reaction vessel affect each of these equilibria? • 2SO2(g) + O2(g) ⇔ 2SO3(g) • H2(g) + Cl2(g) ⇔ 2HCl(g) • 2NOBr(g) ⇔ 2NO(g) + Br2(g)

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