Chapter 22

Chapter 22

Reaction Rate & Chemical Equilibrium

Stability of Compounds

In 2 TiO 2 Ti + O2 Overall energy change is (+)

does not spontaneously decompose @ room temp.

Thermodynamically Stable


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


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

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.


Completion Rxn. 1 or more product is removed from rxn.

environment gas is formed PPT is formed Water or undissociated, unionized subst. is

formed.


Not all rxns. go to completionH2(g) + I2(g) 2HI(g)

H2 & I2 make HI

bond betw. HI is weak & easily decomposes to H2 & I2.


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

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

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.

Factors Affecting Reaction Rate

Nature of reactants

Concentration

Temperature

Catalysis

Surface Area

Pressure gases only

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

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

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

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

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

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.

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.

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.

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.

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

Concentration

In General for mA + nB C rate = k[A]m[B]n

exponents are “order of the expression Rate Laws are determined experimentally

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.

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

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

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

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.

Catalysis

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

Reaction Mechanism

Most rxns. occur in a series of steps. usually involves collision of only 2 particles

rarely involve 3 or more particles

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

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.

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

Reaction Mechanism

N2 + 3 H2 2 NH3 R = k [N2] [H2]3 4th Order

Sum of the exponents is the Order of the Expression

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.

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.)


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


kf = constant

kr

Equilibrium Constant - Keq = kf

kr

Solve for kf / kr

Keq = [HI]2

[I2] [H2]


General eqn for mA + nB sP + rQ Keq = [P]s [Q]r = [Prod.]

[A]m [B]n [React]


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.


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


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

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).


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.



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


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


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



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

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.

Optimum Conditions

4. Catalyst should be used to lower activation energy

5. High press. should be maintained.

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Chapter 22