Chapter 22. Reaction Rate & Chemical Equilibrium

Transcription

1 Chapter 22 Reaction Rate & Chemical Equilibrium

2 Stability of Compounds! In 2 TiO 2 Ti + O 2 n Overall energy change is (+) w does not spontaneously room temp. n Thermodynamically Stable

3 Stability of Compounds! If overall energy change is (-), reaction will proceed spontaneously n May be VERY slow n C 6 H 12 O 6 + 6O 2 room temp., no noticeable rxn w so slow it is Kinetically Stable 6CO 2 + 6H 2 O

4 Stability of Compounds to predict whether a spont. rxn. will be useful, must know the which rxn. occurs 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 n Completion Rxn.

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

7 Reversible Rxns. & Equilibrium! Not all rxns. go to completion H 2(g) + I 2(g) n H 2 & I 2 make HI 2HI (g) w bond betw. HI is weak & easily decomposes to H 2 & I 2.

8 Reversible Rxns. & Equilibrium! 1st rxn. goes from left to rt n H 2 + I 2 2 HI! 2nd rxn. goes from rt. to left n H 2 + I 2 2 HI! combined eqn. represents a reversible rxn. n H 2 + I 2 2 HI n 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 n usually units are (moles/ L) / s or M/s n 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 equilibrium.

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

12 Nature of Reactants! Determines kind of rxn. that occurs n Rxns. w/ bond rearrangement or e - transfer take longer w neutral molec. n Ionic rxns. involve no e - transfer - faster n 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 n causes changes in e- clouds of colliding molecs. n Depends on: 1. Energy 2. Orientation n Colliding molecs. may form an Activated Complex w 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 n energy to weaken or destroy original bonds n If act. energy is high, few collisions have enough energy to form activated complex w Very slow rxn w 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 n If # of particles per unit vol. (conc.) is incr., the chance of effective collisions is incr. n If conc. of 1 reactant doubles, the rate may double bec. twice as many collisions

16 Concentration! Ex) A + B + C D n If [A] is doubled, rate doubles n If [A] & [B] are doubled, rate incr. 4X! Ex) N 2 + 3H 2 2NH 3 n Rate 1 = k 1 [N 2 ] - rate varies directly w/ [N 2 ] n Rate 2 = k 2 [H 2 ] 3 - rate varies directly w/ [H 2 ] n Rate 3 = k 3 [NH 3 ] 2

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

18 Concentration! The rate expression for H 2 O 2 + 2HI 2H 2 O + I 2 is rate = k [H 2 O 2 ] [HI] n Even though 2 HI molecs. are in eqn., only 1 appears in the rate expression n Only way to be sure of rate expression is to use experimental data.

19 Concentration! Rule of Thumb: n Rxn. rate varies directly as the product of the concen. of reactants w Not always true w To be sure, use experimental data n 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 the interface betw. 2 phases n Ex) Zn dissolves (reacts) in H 2 SO 4 w Rxn. takes place on the surface of Zn w if surface area is incr., rate of rxn. incr.

21 Concentration! 2 H 2 + O 2 2 H 2 O! Rate of formation = k[h 2 ] 2 [O 2 ]! Find k if rate of formation = 0.6M/s; [H 2 ] = 2.0 M; [O 2 ] = 1.0M

22 Concentration! In General for ma + nb C n rate = k[a] m [B] n w exponents are order of the expression n Rate Laws are determined experimentally

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

24 Temperature! An incr. in temp. will incr. K.E. of molecs. & collisions n also incr. # of molecs. which have reached activation energy! An incr. in temp. will incr. the rate of rxn. n 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 n decreases required activation energy

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

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

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

29 Reaction Mechanism! Most rxns. occur in a series of steps. n usually involves collision of only 2 particles w 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 n Rate Determining Step n 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 2H 2 + O 2 2H 2 O! Rate of formation = k [H 2 ] 2 [O 2 ] n 3rd Order! A + B C R = k [A] [B] n 2nd Order! A + 2B C R = k [A] [B] 2 n 3rd Order

33 Reaction Mechanism N H 2 2 NH 3 R = k [N 2 ] [H 2 ] 3 n 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 n The only way to know the rate expression for sure is by examining experimental data.

35 Equilibrium Constant H 2 + I 2 2 HI (Forward rxn.)! As rxn starts, lots of H 2 & I 2, no HI n as rxn. proceeds, there s less & less H 2 &I 2 w fewer molecs. mean fewer collisions n There s more & more HI w rxn. of 2HI H 2 + I 2 is incr. (reverse rxn.)

36 Equilibrium Constant! When the rate of forward rxn. = rate of reverse rxn., we have equilibrium n rate of forward rxn. = k f [H 2 ] [I 2 ] n rate of reverse rxn. = k r [HI] equilibrium: n k f [H 2 ] [I 2 ] = k r [HI] 2

37 Equilibrium Constant! k f = constant k r! Equilibrium Constant - K eq = k f k r! Solve for k f / k r! K eq = [HI] 2 [I 2 ] [H 2 ]

38 Equilibrium Constant! General eqn n for ma + nb sp + rq n K eq = [P] s [Q] r = [Prod.] [A] m [B] n [React]

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

40 Equilibrium Constant! What is the equilibrium constant for the following rxn. if the final concentrations are CH 3 COOH = 0.302M, CH 3 CH 2 OH = 0.428M, H 2 O = 0.654M, and CH 3 CH 2 OOCCH 3 = 0.655M? CH 3 COOH + CH 3 CH 2 OH H 2 O + CH 3 CH 2 OOCCH 3

41 Equilibrium Constant! What is the equilibrium concentration of SO 3 in the following rxn. if the concentrations of SO 2 and O 2 are each M and K eq = 85.0? 2SO 2 + O 2 2SO 3

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 N 2(g) + 3H 2(g) 2NH 3(g) + energy 1. Conc. of reactants is incr. (either H 2 or N 2 ) n # of collisions betw. reactants incr n Incr. rxn. rate toward right (shift right) n amt. of product formed is incr.

44 Le Chatelier s Principle N 2(g) + 3H 2(g) 2. Press. is incr. 2NH 3(g) + energy n Has same effect as incr. conc. of all gases in eqn. n Applies only to gases n Equilib. usually shifts to right w ck equilib. expression

45 Le Chatelier s Principle! K eq = [NH 3 ] 2 [N 2 ] [H 2 ] 3 n If press. doubles, reverse rxn. must speed up by a factor of 4 n since [H 2 ] is cubed doubling press. (which doubles conc.) speeds up forward rxn. by a factor of 16

46 Le Chatelier s Principle! In H 2(g) + Cl 2(g) 2HCl (g) n Doubling press. will not shift equilib. w Why? w 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. n Press. affects only gases

47 Le Chatelier s Principle N 2(g) + 3H 2(g) 2NH 3(g) + energy 3. If temp. is incr., equilib. may shift either left or right. n If heat is a product, equilib. will shift left n If heat is a reactant, equilib. will shift right

48 Optimum Conditions! Conditions which produce hightest yield. In Haber process: 1. High conc. of H 2 & N 2 should be maintained. 2. NH 3 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.