Chapter 18

1. Chapter 18 Rate of Reaction & Equilibrium

2. Which would cause more damage when …? 35 mph 80 mph Same model

3. Which would cause more damage when …? Same speed

4. Chemical rxn Collision Theory (Chemical rxn) • According to the collision theory, atoms, ions, and molecules must collide with each other in order to react. • the result of ‘effective’ collisions of reactant particles to form product(s) • A + B → C reactants Reactant particles are always in motion → K.E.

5. Collision

6. Effective Collision • Chemical rxn may not occur as a result of collision (no product formed)→ bounce off …. • 2 other factors …

7. Collision Theory Collision with Sufficient Energy & correct orientation products reactants

8. Collision Theory Insufficiency energy or/and incorrect orientation 2 types of reactants No product formed

9. Head-on Collision Rear End Collision

10. Lock and Key

11. Quick-write The 3 conditions required for an chemrxn to occur: (60 seconds) 1. 2. 3.

12. Compared with car collision Chemical reaction Damage in Car collision ‘Effective’ collision → Damages/injuries enough K.E. correct orientation ‘Effective’ collision → chemical rxn → products • enough K.E. • correct orientation

13. Collision Theory In order for rxn to take place, 3 Conditions 1. Reactant particles must collide. 2. with enough energy. 3. with the correct orientation.

14. Rate of dissolving sugar Suggest ways to speed up the dissolving process 1. 2. 3. Speed up means ……

15. Expressing Reaction Rates • The rxn rate • How much reactants ‘disappeared’ per unit time. • How much products ‘formed’ per unit time. − [reactants] Rxn Rate = time A + B → C [ reactants] decreasing [products] increasing

16. Factors Affecting Rxn Rate Rxn rate are affected by • Temperature • [ ] of reactants • Particle size • Pressure (for gases only) • Catalysts

17. How Temperature Affects Rxn Rate Temperature increase → heat ____________________ → _________ energy for collision → _____________ movement of reactant particles → ____________ frequency of effective collision → _______________ rate increases

18. Factors Affecting Rxn Rates Factor 1: Temperature • Increase in temperature would …. • Decrease in temperature would …. • Explanations: • absorb heat →K.E. → move faster • increases the collision frequency→ effective • Activation energy…

19. C. B. A.. D. E. Energy Graph of an __F._______________ Reaction

20. I. K. H. J. G. Energy Graph of an __L.___________________ Reaction

21. Exothermic and Endothermic Rxns • An exothermicrxn releases heat, and • an endothermic • rxn absorbs heat. ∆H = -ve • The exothermic rxn gives off heat because the products are at a lower EL than the reactants. rxn

22. Exothermic and Endothermic Rxns The endothermic rxn absorbs heat because products are at a higher EL than the reactants. ∆H = +ve

23. Exothermic and Endothermic Rxns • In an exothermic rxn, • heat released comes from the change from reactants at higher energy to products at lower energy. • In an endothermic rxn, • heat absorbed comes from the opposite change.

24. Expressing Reaction Rates • The reaction rate of a chemrxn • is stated as the change in concn of a reactant or product per unit time, expressed as mol/(L∙s). • e.g, [NO2] represents the molar concn of NO2.

25. Collision Theory • Rates of chem rxns are often measured as a change in the # of moles during an interval of time.

26. Expressing Reaction Rates • Rxn rates is always +ve. • When the rate is measured by the consumption of a reactant, apply a -ve sign to the caln to get a +verxn rate. ∆qty Average rxn rate = − ∆t

27. Collision Theory According to the collision theory, atoms, ions, and molecules must collide with each other in order to react.

28. Collision Theory Activation Energy (EA): The minimum energy that colliding particles must have in order to react. rxn

29. Collision Theory An activated complex • an unstable arrangement of atoms that forms momentarily • at the peak • high-energy • Ready to form products

31. Factors Affecting Reaction Rates • Factor 2: concentration of reactants • Rxns rate increases when the [reactants] are increased Explanation: more particles to collide; higher frequency (probability) of collision

32. Factors Affecting Reaction Rates Factor 3: particle size of reactants • smaller/finer the reactant particles, higher the rxn rate Explanation:finer particles → more surface area for collision

33. Factors Affecting Rxn Rates Factor 4: catalyst • catalyst is a sub (chemical) that increases the rate of a chemrxn w/o itself being consumed in the rxn. • not a reactant (A + B → C) • Explanation: By lowering EA • Enzymes are biological catalysts • Note: a catalyst does not yield more product catalyst

34. Catalyst Example:

35. Catalysts

36. Factors Affecting Rxn Rate Factor 5: pressure (affect gases only) • at high pressure, gases particles are closer to each other • Higher probability of collision • Higher rxn rate

37. Reversible Reactions and chemeqm • rate of the shoppers going up = the rate of the shoppers going down, • → # of shoppers on each floor remains constant • → dynamic eqm.

38. What is chemical equilibrium? N2 + 3H2= 2NH3 • Chemical eqm: • reversible rxn • Rxnsdo not stopped; ongoing→Dynamic • forward rxn rate = reverse rxns rate • [reactants] and [products] remain constant. • [reactants] ≠ [products] • never come to completion

39. Reversible reactions • A reversible rxn • one that can occur in both the forward and the reverse directions. Combination rxn Decomposition rxn

40. Reversible Reactions SO3 decomposes to SO2 and O2 SO2 and O2 react to give SO3 At eqm, all 3 types of molecules (reactants and products) are present. Rxns never come to completion. [SO2], [O2] and [SO3] remain constant.

41. What is equilibrium? What happens when 1 mole N2and 3 moles H2, the amts shown in the eqn, are placed in a closed rxn vessel at 723 K? N2 + 3H2 2NH3 ΔG= -33kj

42. What is equilibrium? • [H2]and [N2] decrease at first while the concn of the product [NH3]increases. N2 + 3H2 ↔ 2NH3 • all concns become constant. Eqm position

43. What is equilibrium? • The reactants, H2 and N2, are consumed to form NH3, so [H2]and [N2] decrease and [NH3] increases at the beginning. N2 + 3H2 2NH3 ΔG= -33kj • the product (NH3) decompose to form the reactants → [NH3] stops increasing and [H2]and [N2] stop decreasing. • After a period of time, [H2], [N2], and [NH3] no longer change → eqm

44. ` N2 + 3H2 ↔ 2NH3 [H2] decreasing eqm [H2] is constant [N2] decreasing [NH3] is constant [NH3] increasing [N2] is constant [NH3] begins at 0

45. What is equilibrium? All [ ] become constant after the eqm point (right side of the dotted line).

46. Reversible Reactions • Irreversible rxn. ( → ) • A rxn resulting in almost complete conversion of reactants to products (completion) • 1 way • Reversible rxns ( ) • 2 ways • do not go to completion. • Eqm ----appear to stop (actually ongoing). • reactants react to form the products • the products react to form the reactants.

47. Reversible Reactions • The reactants in the fd rxn are on the left. Reactants → products Products ←reactant • In the rev rxn, the reactants are on the right.

48. Reversible Reactions • Chemical equilibrium • a state in which Rateforwardrxn= Ratereverserxn [ ] are ≠ [ ] remain constant

49. Reversible Reactions • The NH3-forming rxn reaches a state of chemeqm At eqm, [reactants] and [products] are constant (not =)

50. Factors Affecting Eqm Position • Le Châtelier’sPrinciple • states that if a stress is applied to a system at eqm, eqm position shifts in the direction that relieves the stress. 2NO (g) + Br2 (g) 2NOBr (g)