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Mastering Chemical Equilibrium & Reaction Rates

Learn about collision theory, rates of reactions, catalysts, Le Chatelier’s Principle, and achieving chemical equilibrium. Understand how concentration, temperature, and catalysts impact reaction rates.

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Mastering Chemical Equilibrium & Reaction Rates

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  1. Unit 9: Chemical Equilibrium • Collision theory • Rates of reactions • Catalysts • Reversible reactions • Chemical equilibrium • Le Chatelier’s Principle • Concentration • Temperature • Volume • Catalysts

  2. A. Collision Theory • Reaction rate depends on the collisions between reacting particles. • Successful collisions occur if the particles... • collide with each other • have the correct orientation • have enough kinetic energy to break bonds

  3. Exothermic Endothermic Energy Energy Time Time Activation energy: minimum energy required for a reaction to occur Activation energy Energy of reaction

  4. Ea A. Collision Theory • Activation Energy • depends on reactants • low Ea = fast rxn rate

  5. 16.2: Rates of Reactions Chemical kinetics: the study of the rate (the speed) of a reaction Rate of a chemical reaction depends on: 1. SURFACE AREA 2. CONCENTRATION of reactants 3. TEMPERATURE (T) of reactants 4. Presence/absence of a CATALYST

  6. SURFACE AREA • Surface Area • high SA = fast rxn rate • more opportunities for collisions • Increase surface area by… • using smaller particles • dissolving in water

  7. Effect of Concentration on Rate Concentration: • KMT (Kinetic-Molecular theory) states that increasing concentration of reactants results in more collisions. • More collisions result in more reactions, increasing the rate of the reaction.

  8. Effect of Temperature on Rate Temperature: • Increasing T increases particle speed. • Faster reactants means more collisions have the activation energy, which increases the rate of the reaction.

  9. Energy Time Effect of Catalysts on Rate A catalyst: • A chemical that influences a reaction, but is not consumed in the reaction. (It can be recovered unchanged at the end of the reaction.) • Lowers the activation energy of the reaction. Activation energy Activation energy with catalyst

  10. 16.1: Reversible Reactions * Thus far, we have considered only one-way reactions: A + B → C + D Some reactions are reversible: • They go forward (“to the right”) : A + B → C + D and backwards (“to the left”) : A + B ← C + D • Written with a two-way arrow: A + B ↔ C + D Examples: • Boiling & condensing • Freezing & melting • Recharging a “rechargeable battery”

  11. Examples of irreversible reactions: • Striking a match / burning paper • Dropping an egg • Cooking (destroys proteins)

  12. C C C D D D + + + + + + A A A B B B 16.3: Chemical Equilibrium For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established. • Both the forward and backward reactions continue, but there is a balance of products “un-reacting” and reactants reacting. A + B ↔ C + D

  13. * Le Chatelier’s Principle is about reducing stress – a stress applied to a chemical equilibrium Relax! Reduce stress brought on by chemical equilibrium with me, Henri Le Chatelier! (1850 – 1936)

  14. 16.4: Le Chatelier’s Principle Le Chatelier’s Principle: • When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress. • The system shifts in the direction of the reaction that is favored by the stress. • A stress is a change in: • Concentration • Temperature • Volume

  15. 16.5: Stress: Change Concentration Ex: Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O (pink) (blue) StressResult Add Cl1- Forward rxn favored Shifts forward to reduce extra Cl1- More CoCl42- will form Add H2OBackward rxn favored Shifts backward to reduce extra H2O More Co(H2O)62+ will form

  16. 16.7: Stress: Change Temperature Ex: heat + Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O (pink) (blue) This reaction is endothermic. For Le Chatelier’s principle, consider “heat” as a chemical. StressResult Increase T Forward rxn favored; shifts forward to reduce extra heat More CoCl42- will form Decrease T Backward rxn favored; shifts backward to replace “lost” heat More Co(H2O)62+ will form

  17. 16.6: Stress: Change Volume Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g) (1 + 3 = 4 moles of gas) ↔ (2 moles of gas) StressResult Decrease V Forward rxn favored; shifts forward to side with fewer moles of gas (reduces # of molecules packed into this smaller volume) Increase V Backward rxn favored; shifts backward to side with more moles of gas (to fill the larger volume with more molecules)

  18. 16.7: Catalysts & Equilibrium MnO2 Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g) • Since a catalyst increases the forward and backward rates equally, it will not shift the equilibrium.

  19. Reaction Rate Time Dissolving (forward rate) decreases… • Ex: saturated salt solution NaCl (s) ↔ Na+ (aq) + Cl- (aq) Equilibrium is established: Forward rate = Backward rate Crystallization (backward rate) increases…

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