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Equilibrium Rate of Reaction: Understanding Dynamic Equilibrium and Le Chatelier's Principle

This review explains the concept of equilibrium rate of reaction, including dynamic equilibrium, calculations using Keq, and the effects of temperature, concentration, pressure, and catalysts on equilibrium. Le Chatelier's Principle is also discussed.

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Equilibrium Rate of Reaction: Understanding Dynamic Equilibrium and Le Chatelier's Principle

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  1. Review Equilibrium

  2. Rate of reaction • The rate of the forward reaction = the rate of the reverse reaction • N02 is being used up at the same rate that it is being formed • N2O4 2 NO2 • Rate of reaction does not change because both N202 and NO2 are constant

  3. Equilibrium • When this happens we say the system is at EQUILIBRIUM • We have reached a state of DYNAMIC EQUILIBRIUM • Key points • Rate of the Forward Reaction = Rate of Reverse Reaction • Dynamic Equilibrium • Reactants are changing to products, products are changing to reactants but at a microscopic level • All observable properties are constant (macroscopic properties) • If no changes were made to the condition a system would stay at equilibrium forever • A system at equilibrium is a closed system

  4. Calculations • Equilibrium calculations • We can figure out what the concentrations are for the products and reactants when they are at equilibrium • We use: • Keq • It tells us the ratio of products: reactants when the reaction is at equilibrium

  5. Calculations • to figure out the concentration of the products and reactants at equilibrium we use: • Keq = [products] • [reactants] • A + B C + D • Keq = [C] [D] • [A] [B]

  6. Calculations • Keq Calculations • Write expressions • Find Keq given concentration at equilibrium • Find concentration at equilibrium given Keq and all other concentrations at equilibrium • Find Keq given initial concentration and 1 concentration at equilibrium • ICE • Find concentrations at equilibrium given initial concentration and Keq • ICEEE & Trial Keq

  7. Le Chatelier’s Principle • If a system that is at equilibrium is changed (temperature, concentration, pressure) processes will occur that tend to counteract that change • This means the rxn will shift in a way to try and “undo” what was added • Add heat  rxn will shift so it will use up added heat • Remove heat  rxn will shift so it can produce heat • Increase [ ]  rxn will shift so it will decrease [ ]

  8. Temperature • temperature is increased, the equilibrium will shift away from the side with the heat term • A + B + heat 2NO2 • Shift right • When the temperature is decreased, the equilibrium will shift toward the side with the heat term. • A + B + heat 2NO2 • Shift left

  9. Concentration • concentration is increased the equilibrium will shift toward the other side of • H2(g) + I2(g) 2HI(g) • Add H2 equilibrium will shift right • concentration decreased the equilibrium will shift toward that substance • H2(g) + I2(g) 2HI(g) Remove H2 equilibrium will shift left

  10. Partial Pressure • If the pressure of a substance is increased, equilibrium shifts toward the other side of the equation • H2(g) + I2(g) 2HI(g) • Increase pressure of H2 shifts right • If the pressure of a substance is decreased, equilibrium shifts toward the side of the equation with that substance • H2(g) + I2(g) 2HI(g) • Decrease pressure H2 shifts left

  11. Total Pressure • total pressure increased (or volume decreased), the equilibrium will shift toward the side with less moles of gas (as shown by coefficients) • N2(g) + 3H2(g) 2NH3(g) • shift to the right (the side with fewer moles of gas) • total pressure decreased (volume increased) the equilibrium will shift toward the side with more moles of gas (as shown by coefficients) • N2(g) + 3H2(g) 2NH3(g) • shift to the left (the side with more moles of gas)

  12. Keq • The larger the value for Keq the closer to completion the reaction is at equilibrium. • Most of the reactants have been completely converted to products • A very small value (< 1) for Keq means that there is very little product and lots of reactant at equilibrium. • Most of the reaction has not occurred most of reactants remained unchanged and there is very little product

  13. Keq • When Keq is not large or small (close to 1) • This means that there is about the same amount of products as reactants. • At equilibrium, this reaction has proceeded to "about half way" to completion

  14. Keq Temperature • When the temperature changes, the value of Keq also changes • Endothermic (heat on reactant side) • A + B + heat C • Increasing temp • Shifts right • Higher concentration C (product/numerator) • Higher Keq value • Decreasing Temp • Shift left • Higher concentration A + B (reactants/denomenator) • Lower Keq

  15. Keq + Temperature • Exothermic (heat on product side) • A + B C + heat • Increasing temp • Shifts left • Higher concentration A + B (reactants/denomenator) • Lower Keq • Decreasing Temp • Shifts Right • Higher concentration C (product/numerator) • Higher Keq value

  16. Keq • Concentration • Pressure • Cataylist • Do not change Keq significantly

  17. Graphs A + B  AB + Heat Temp Increase Shifts LEFT GRADUAL INCREASE/DECREASE A + B  AB + Heat INCREASE [A] Shifts RIGHT Sudden increase (decrease) A Gradual increase A + B  AB + Heat Incrase Pressure Shifts RIGHT (less moles) Sudden increase in both A gradually decrease

  18. Practical Applications Le Chatelier’s Principle • Haber Process • N2 + 3H2 2NH3 + heat • Able to produce large amounts of ammonia using Le Chatalier’s Principle • Increased [N2] shift RIGHT • Decreased Temp  shift RIGHT • Increased Pressure  shift RIGHT • Added a catalyst  speeds up rate of RXN

  19. Enthalpy & Entropy • Enthalpy • Energy it takes for a reaction to occur • Potential energy • Favours side of rxn where minimum enthalpy is • PCL5 + HEAT  Cl2(g) + PCl3(g) • Entropy • Disorder • Favours side of rxn where entropy is maximum • Entropy of a Solid < Entropy of a Liquid < Entropy of an Aqueous Solution < Entropy of a Gas • CaCO3(s) + 2 HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

  20. Enthalpy & Entropy • Equilibrium occurs when two opposeeach other" • Minimum enthalpy favours reactant • Maximum entropy favours product • When both tendencies favour the products, this reaction will go to completion. • When both favour the reactants, the reaction will not occur at all!

  21. Terms Solubility • The ability of a substance (solute) to dissolve • Soluble – substance will completely dissolve • Partially Soluble – substance will dissolve but there will be some precipitate left • Insoluble – will dissolve but the amount that dissolves is very small and almost ALL the substance precipitates. • Solubility Equilibrium • The rate of dissolving = The rate of precipitation

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