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Unit 9: Chemical Equilibrium

Unit 9: Chemical Equilibrium. Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration Temperature Volume Catalysts. A. Collision Theory. Reaction rate depends on the collisions between reacting particles.

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Unit 9: Chemical Equilibrium

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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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