Chapter 22
description
Transcript of Chapter 22
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Chapter 22
Reaction Rate & Chemical Equilibrium
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Stability of Compounds
In 2 TiO 2 Ti + O2 Overall energy change is (+)
does not spontaneously decompose @ room temp.
Thermodynamically Stable
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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
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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.
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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.
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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.
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Reversible Rxns. & Equilibrium
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.
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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
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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
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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.
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Factors Affecting Reaction Rate
Nature of reactants
Concentration
Temperature
Catalysis
Surface Area
Pressure gases only
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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
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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
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Concentration
In General for mA + nB C rate = k[A]m[B]n
exponents are “order of the expression Rate Laws are determined experimentally
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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.
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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
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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
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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
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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.
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Catalysis
Inhibitors - “tie up” a reactant or catalyst in a complex so it will not react. does not slow down rxn. - stops it
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Reaction Mechanism
Most rxns. occur in a series of steps. usually involves collision of only 2 particles
rarely involve 3 or more particles
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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
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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.
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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
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Reaction Mechanism
N2 + 3 H2 2 NH3 R = k [N2] [H2]3 4th Order
Sum of the exponents is the Order of the Expression
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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.
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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.)
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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
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Equilibrium Constant
kf = constant
kr
Equilibrium Constant - Keq = kf
kr
Solve for kf / kr
Keq = [HI]2
[I2] [H2]
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Equilibrium Constant
General eqn for mA + nB sP + rQ Keq = [P]s [Q]r = [Prod.]
[A]m [B]n [React]
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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.
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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
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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
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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).
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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.
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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
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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
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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
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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
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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.
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Optimum Conditions
4. Catalyst should be used to lower activation energy
5. High press. should be maintained.