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Chapter 22

Chapter 22. Reaction Rate & Chemical Equilibrium. Stability of Compounds. In 2 TiO 2 Ti + O 2 Overall energy change is (+) does not spontaneously decompose @ room temp. Thermodynamically Stable. Stability of Compounds.

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Chapter 22

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  1. Chapter 22 Reaction Rate & Chemical Equilibrium

  2. Stability of Compounds • In 2 TiO 2 Ti + O2 • Overall energy change is (+) • does not spontaneously decompose @ room temp. • Thermodynamically Stable

  3. Stability of Compounds • If overall energy change is (-), reaction will proceed spontaneously • May be VERY slow • C6H12O6 + 6O2 6CO2 + 6H2O • @ room temp., no noticeable rxn • so slow it is Kinetically Stable

  4. Stability of Compounds \ to predict whether a spont. rxn. will be useful, must know the rate @ which rxn. occurs and @ what pt. equilibrium is established.

  5. Reversible Rxns. & Equilibrium • Many rxns. result in an equilibrium mixture • A rxn. goes to completion when all of one of the reactants is used up & rxn. stops • Completion Rxn.

  6. Reversible Rxns. & Equilibrium • Completion Rxn. • 1 or more product is removed from rxn. environment • gas is formed • PPT is formed • Water or undissociated, unionized subst. is formed.

  7. Reversible Rxns. & Equilibrium • Not all rxns. go to completion H2(g) + I2(g) 2HI(g) • H2 & I2 make HI • bond betw. HI is weak & easily decomposes to H2 & I2.

  8. Reversible Rxns. & Equilibrium • 1st rxn. goes from left to rt • H2 + I2 2 HI • 2nd rxn. goes from rt. to left • H2 + I2 2 HI • combined eqn. represents a reversible rxn. • H2 + I2 2 HI • Eventually reaches equilibrium

  9. Reaction Rate • If the product of a reversible rxn. decomposes faster than reactants form products, there will always be more reactant than product. • Reaction Rate - the rate of appearance of a product or rate of disappearance of a reactant

  10. Reaction Rate • usually units are (moles/ L) / s or M/s • actually measures rate of change of concentration • If the 2 rxn. rates are known, we can predict whether the product or reactant will be in higher concentration @ equilibrium.

  11. Nature of reactants Concentration Temperature Catalysis Surface Area Pressure gases only Factors Affecting Reaction Rate

  12. Nature of Reactants • Determines kind of rxn. that occurs • Rxns. w/ bond rearrangement or e- transfer take longer • neutral molec. • Ionic rxns. involve no e- transfer - faster • Active metals & nonmetals react faster than less active ones \ atomic structure affects rxn. rate

  13. Nature of Reactants • Formation of a new bond requires an “Effective Collision” • causes changes in e- clouds of colliding molecs. • Depends on: 1. Energy 2. Orientation • Colliding molecs. may form an Activated Complex • Unstable rxn. intermediate

  14. Nature of Reactants • Activation Energy - energy that must be attained in order for a collision betw. reactants to result in the formation of an activated complex • energy to weaken or destroy original bonds • If act. energy is high, few collisions have enough energy to form activated complex • Very slow rxn • Kinetically stable

  15. Concentration • [ ] = mol / L - quantity of matter that exists in a unit vol. - molarity (M) • For a rxn. to take place, particles must collide • If # of particles per unit vol. (conc.) is incr., the chance of effective collisions is incr. • If conc. of 1 reactant doubles, the rate may double bec. twice as many collisions

  16. Concentration • Ex) A + B + C D • If [A] is doubled, rate doubles • If [A] & [B] are doubled, rate incr. 4X • Ex) N2 + 3H2 2NH3 • Rate1 = k1 [N2] - rate varies directly w/ [N2] • Rate2 = k2 [H2]3 - rate varies directly w/ [H2] • Rate3 = k3 [NH3]2

  17. Concentration • k is specific rate constant • depends on size, speed, & kind of molecs involved • ea. rxn. has only 1 value of k @ a given temp.

  18. Concentration • The rate expression for H2O2 + 2HI 2H2O + I2 is rate = k [H2O2] [HI] • Even though 2 HI molecs. are in eqn., only 1 appears in the rate expression • Only way to be sure of rate expression is to use experimental data.

  19. Concentration • Rule of Thumb: • Rxn. rate varies directly as the product of the concen. of reactants • Not always true • To be sure, use experimental data • An incr. in press. on a gas will incr. its concen. & \ rxn. rate will incr.

  20. Concentration • Homogeneous rxn - reactants are all in the same phase • Heterogeneous rxn. - rxn. which takes place @ the interface betw. 2 phases • Ex) Zn dissolves (reacts) in H2SO4 • Rxn. takes place on the surface of Zn • \ if surface area is incr., rate of rxn. incr.

  21. Concentration • 2 H2 + O2 2 H2O • Rate of formation = k[H2]2[O2] • Find k if rate of formation = 0.6M/s; [H2] = 2.0 M; [O2] = 1.0M

  22. Concentration • In General for mA + nB C • rate = k[A]m[B]n • exponents are “order of the expression • Rate Laws are determined experimentally

  23. Temperature • Rxn. Rate is determined by frequency of collisions betw. molecs. • If freq. of collisions incr., rate incr. • for some rxns., their rate doubles for ea. 10 Co rise in temp.

  24. Temperature • An incr. in temp. will incr. K.E. of molecs. & \ collisions • also incr. # of molecs. which have reached activation energy • An incr. in temp. will incr. the rate of rxn. • incr. # of activated complexes formed

  25. Catalysis • The process of increasing rxn. rates by the presence of a catalyst • Catalyst - subst. which incr. a rxn. rate w/out being permanently changed • decreases required activation energy

  26. Catalysis • Heterogeneous Catalyst - reactants & catalyst are not in the same state • has a surface on which the substs. can react. • adsorbs one of the reactants • Adsorbtion - the adherence of 1 subst. to the surface of another • ex) catalytic converters

  27. Catalysis • Homogeneous Catalyst - exists in same phase as reactants • enters into the rxn. - forms rxn. intermediate or activated complex • requires less activation energy • returns unchanged in final step of rxn.

  28. Catalysis • Inhibitors - “tie up” a reactant or catalyst in a complex so it will not react. • does not slow down rxn. - stops it

  29. Reaction Mechanism • Most rxns. occur in a series of steps. • usually involves collision of only 2 particles • rarely involve 3 or more particles

  30. Reaction Mechanism • If a rxn. consists of several steps: A B; B C; C final product One of the steps will be slower than all the others • Rate Determining Step • Faster steps will not affect the rate

  31. Reaction Mechanism • Reaction Mechanism - The series of steps that must occur for a rxn. to go to completion • @ a given temp., the rate of a rxn. varies directly w/ the product of the concentrations of the reactants in the slowest step.

  32. Reaction Mechanism 2H2 + O2 2H2O • Rate of formation = k [H2]2 [O2] • 3rd Order • A + B C R = k [A] [B] • 2nd Order • A + 2B C R = k [A] [B]2 • 3rd Order

  33. Reaction Mechanism N2 + 3 H2 2 NH3 R = k [N2] [H2]3 • 4th Order • Sum of the exponents is the Order of the Expression

  34. Reaction Mechanism • If rxn. is a single step rxn., coef., in eqn. will become exponent in rate expression • The only way to know the rate expression for sure is by examining experimental data.

  35. Equilibrium Constant H2 + I2 2 HI (Forward rxn.) • As rxn starts, lots of H2 & I2, no HI • as rxn. proceeds, there’s less & less H2 &I2 • fewer molecs. mean fewer collisions • There’s more & more HI • rxn. of 2HI H2 + I2 is incr. (reverse rxn.)

  36. Equilibrium Constant • When the rate of forward rxn. = rate of reverse rxn., we have equilibrium • rate of forward rxn. = kf [H2] [I2] • rate of reverse rxn. = kr [HI]2 • @ equilibrium: • kf [H2] [I2] = kr [HI]2

  37. Equilibrium Constant • kf = constant kr • Equilibrium Constant - Keq = kf kr • Solve for kf / kr • Keq = [HI]2 [I2] [H2]

  38. Equilibrium Constant • General eqn • for mA + nB sP + rQ • Keq = [P]s [Q]r = [Prod.] [A]m [B]n [React]

  39. Equilibrium Constant • If Keq is small (<1), very little product is formed. • Reactant is favored. • If Keq is lg. (>1), rxn. is nearly complete • much product is formed • product is favored.

  40. Equilibrium Constant • What is the equilibrium constant for the following rxn. if the final concentrations are CH3COOH = 0.302M, CH3CH2OH = 0.428M, H2O = 0.654M, and CH3CH2OOCCH3 = 0.655M? CH3COOH + CH3CH2OH H2O + CH3CH2OOCCH3

  41. Equilibrium Constant • What is the equilibrium concentration of SO3 in the following rxn. if the concentrations of SO2 and O2 are each 0.0500M and Keq = 85.0? 2SO2 + O2 2SO3

  42. Le Chatelier’s Principle • Conditions affecting equilibrium: 1. Temp. 2. Press. 3. Concentration (of prods. & reacts.) • If a condition is changed (stress) on a syst. in equilib., then the equilib. will shift to restore the original conditions (relieve the stress).

  43. Le Chatelier’s Principle N2(g) + 3H2(g) 2NH3(g) + energy 1. Conc. of reactants is incr. (either H2 or N2) • # of collisions betw. reactants incr • Incr. rxn. rate toward right (shift right) • amt. of product formed is incr.

  44. Le Chatelier’s Principle N2(g) + 3H2(g) 2NH3(g) + energy 2. Press. is incr. • Has same effect as incr. conc. of all gases in eqn. • Applies only to gases • Equilib. usually shifts to right • ck equilib. expression

  45. Le Chatelier’s Principle • Keq = [NH3]2 [N2] [H2]3 • If press. doubles, reverse rxn. must speed up by a factor of 4 • since [H2] is cubed doubling press. (which doubles conc.) speeds up forward rxn. by a factor of 16

  46. Le Chatelier’s Principle • In H2(g) + Cl2(g) 2HCl(g) • Doubling press. will not shift equilib. • Why? • Rate in ea. direction is affected the same way. • An incr. is press. will always drive a rxn. in the direction of the smaller # of moles of gas. • Press. affects only gases

  47. Le Chatelier’s Principle N2(g) + 3H2(g) 2NH3(g) + energy 3. If temp. is incr., equilib. may shift either left or right. • If heat is a product, equilib. will shift left • If heat is a reactant, equilib. will shift right

  48. Optimum Conditions • Conditions which produce hightest yield. In Haber process: 1. High conc. of H2 & N2 should be maintained. 2. NH3 should be removed as it’s formed. 3. Temp. should be high enough to maintain a reasonable rate, but low enough not to favor reverse rxn.

  49. Optimum Conditions 4. Catalyst should be used to lower activation energy 5. High press. should be maintained.

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