chemical kinetics - Állatorvostudományi egyetem · chemical kinetics kinetics is thearea of...

27
1 Chemical Kinetics Kinetics is the area of physical chemistry concerned with the - rate which a given chemical reaction occurs - investigation of the reaction mechanisms Practical aspects: processes in the living organisms and industry A B

Upload: others

Post on 09-Jun-2020

18 views

Category:

Documents


2 download

TRANSCRIPT

Page 1: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

1

Chemical Kinetics

Kinetics is the area of physical chemistry concerned with the

- rate which a given chemical reaction occurs

- investigation of the reaction mechanisms

Practical aspects: processes in the living organisms and industry

A B

Page 2: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

2

Rate of Chemical Reactions

The reaction rate is the change in amount or concentration (at fixed volume)

of a reactant or product in a given unit of time, e.g.

H2(g) + I2(g) 2 HI(g)

v = rate (velocity) of the reaction in moldm3s1

(a function of time)

d[H2]dt

= d[I2]

dtd[HI]2 dt

=v(t) =

[A] – concentration of A in mol/dm3

At a constant volume:

[A] = [A]2 [A]1 and = d if 0.

‹v› = [H2]t

- an average rate (a number)

Page 3: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

3

Reaction Rate and Concentration

e.g. NH4NO2.aq N2(g) + 2 H2O

Determining the volume of nitrogen formed within a unit time, V(N2) in cm3/s:

if [NO2] then V(N2) keeping [NH4

+] const. with high conc. of NH4Cl

if [NH4+] then V(N2) keeping [NO2

] const. with high conc. of NaNO2

In quantitative form:

v = k [NH4+][NO2

], where k is the rate constant

reactants products

mechanism

Page 4: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

4

v = k[A]a[B]b[C]c…

a, b, c, … reaction order for a given reactant A, B, C, …

a + b + c +… overall reaction order: sum of the exponents that

occur in the rate law

Rate Law

The k rate constant

- characteristic for each reaction

- depends on the temperature exclusively

Rate law: must be determined experimentally, it cannot be predicted

from the stoichiometry of the chemical equation.

The reaction rate is proportional to the concentrations of reactants:

Page 5: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

5

2 N2O5 = 4 NO2 + O2 v = k1.[N2O5] first order

2 NOBr(g) = 2 NO(g) + Br2(g) v = k2.[NOBr]2 second order

CHCl3(g) + Cl2(g) = CCl4(g) + HCl(g)

v = k.[CHCl3].[Cl2]1/2 fractional (1.5) order (indicates radical mechanism)

3 CH3OH + 2 H2CrO4 + 6 HCl = 3 CH2O + 2 CrCl3 + 8 H2O

v = k4.[CH3OH].[H2CrO4].[HCl]2 fourth order

2 N2O(g) = 2 N2(g) + O2(g) v = k0 zeroth order(on the solid Pt wire)

H2(g) + I2(g) = 2 HI(g) v = k2.[H2].[I2] k2

’.[HI]2

H2(g) + Br2(g) = 2 HBr(g) v = (k.[H2].[Br2]1/2)/(k’ + [HBr]/[Br2])

Often the order cannot be determined (i.e. it has not got sense):

Examples

Page 6: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

6

First order reactions:

v = - = k1[A]d[A]

dt

integration ofd[A]/[A] = - k1dt

ln[A]t = ln[A]0 – k1t

k1 = — ln1

t

[A]0

[A]t

First and Second Order Reactions

t

ln[A]t

tg = - k1

(unit: 1/s)

Second order reactions:

v = - = k2[A][B]d[A]

dt

if [A] = [B], so

v = - = k2[A]2d[A]dt

1/[A]t = 1/[A]0 + k2t

1/[A]t

1/[A]0

tg = k2

t

k2 = — ( — - — )1

t [A]t [A]0

1 1

(unit: dm3/mols)

integration ofd[A]/[A]2 = - k2dt

Page 7: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

7

is the time required for the concentration (with fixed volume) of the reactant to

decrease to halfway between its initial and final values (i.e. the time for [A]0 ½[A]0).

Half-life (t1/2) of the Reactions

First order reaction:

independenton the initial concentration

t1/2 = ln 2k1

Second order reaction:

reciprocal dependenceon the initial concentration

t1/2 = 1k2[A]0

Generally for r order reaction: ax = b (a > 1, b > 0)

x = logab (loga1 = 0)

a = 2 lb, e ln, 10 log

Page 8: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

8

Effective collision: produces chemically new substance(s)

Ineffective collision: the molecules rebound unchanged (elastic collision)

Collision Theory and Transition State

A2 + B2

A A

B B

2 AB

reactants

transition state(activated complex)

product(s)

E

reaction coordinate (progress of rxn.)

A2 + B2

2 AB

H

Ea Ea = energy of activation

H = enthalpy of reaction

Page 9: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

9

The Arrhenius-equation

The rate increases exponentially withincreasing temperature

The activation energy, Ea

can be determined

k - rate constant; e = 2.7182 …

Ea - energy of activation (J/mol); T - abs. temperature (K)

A - constant (characteristic for the given reaction)

R = 8.314 J/molK - universal gas constant

Page 10: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

10

Classification of Reactions

single step

unimolecular(CH3NC CH3CN)

bimolecular(CH3Br + OH CH3OH + Br)

termolecular(very rare)

H + H + H H2 + H

Molecularity of a single step in a reaction refers to the number of

molecules that participate in the step.

multistep

Page 11: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

11

Multistep Reactions

Some subsequent steps e.g.

CO(g) + NO2(g) CO2(g) + NO(g)

Rate equation determined by experiments as v = k [NO2]2 in NO2.

The proposed mechanisms with NO3 intermediate:

1) NO2 + NO2NO3 + NO v1 = k.[NO2]2

2) NO3 + CO NO2 + CO2 v2 = k’.[NO3].[CO]

1) rate-determining (slow) step, rds with Ea1

2) fast step with Ea2 and Ea1 > Ea2

The overall rate of the multistep reactions

is determined by the slowest step.

Page 12: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

12

rds

RDS

Page 13: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

13

Catalysts

A catalyst is a substance that increases the rate of a chemical reaction

without being used up in the reaction. The catalyst:

- may be recovered unchanged at the end of the reaction (theoretically)

- small amount is enough to accelerate a reaction

- cannot initiate thermodynamically not allowed reactions

- opens a new path by which the reaction can take place

- lowers the energy of activation, Ea

Sugar burns in the presence of LiCl

Page 14: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

14

Catalytic Action

Page 15: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

15

- homogeneous: 2 H2O2 2 H2O + O2

Catalyst: Br2 (l)

H2O2 + Br2 2 HBr + O2

2 HBr + H2O2 Br2 + 2 H2O

Steps of the catalysed decomposition of H2O2

Homogeneous and Heterogeneous Catalysis

- heterogeneous:

2 SO2 + O2 2 SO3 catalyst: V2O5 (s)

CH2=CH2 + H2 CH3–CH3 catalyst: Pt (s)

The reactants are adsorbed on the surface of the catalyst via physisorption and/or

chemisorption (e.g. H2 2 H on Pt). The catalysts are finely dispersed solids having a

large specific surface area (m2/g).

Page 16: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

16

Inhibitor („negative catalysts”; increases the Ea) e.g. antioxidants like flavonoids

retard the decomposition of food by oxidants.

Catalytic poison is a substance that inhibits the activity of catalyst by blocking its

„active site” (e.g. small amount of arsenic destroys the power of platinum to

catalyze the formation of SO3 from SO2).

In autocatalysis the reaction is accelerated by one of the reaction products, e.g.:

2 KMnO4 + 5 (COOH)2 + 3 H2SO4 = 2 MnSO4 + K2SO4 + 10 CO2 + 8 H2O

Mn2+ ions formed work catalytically

Autocatalysis

Page 17: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

17

Michaels-Menten Kinetics of Enzymes

The KM 1/KS is dependent on both the enzyme

and the substrate, as well as conditions such as

temperature and pH.

Page 18: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

18

Page 19: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

19

Chemical Equilibrium

Reversible reactions can proceed in both directions, e.g.

N2 + 3 H2 2 NH3

2 NH3 N2 + 3 H2

in one equation: N2 + 3 H2 ⇌ 2 NH3

The double arrow (⇌) indicates that the reaction can be read in either direction

(do not use , that is for mesomeric structures only).

The equilibrium composition is independent of the direction from which it is

approached. It is macroscopically static, but is microscopically dynamic.

Page 20: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

20

In general for any reversible reaction:

a A + b B ⇌ c C + d D

K - equilibrium constant in (mol/dm3)n, where n = 0, 1, 2, …

- independent on the concentrations

- depends on the temperature

[C], [D], [A], [B] - concentrations are in equilibrium state in mol/dm3

by convention the concentrations of the

- products in numerator (right side)

- reactants in denominator (left side)

K = [C]c [D]d

[A]a [B]b

Law of Mass Action

Page 21: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

21

All reversible processes tend to reach an equilibrium state and the

concentration of all substances involved remain constant.

rates of opposing (i.e. forward and reverse) reactions are equal: kfor = krev

phases of all reacting components can be same or different.

Homogeneous equilibria, e.g.

gas-phase reactions: H2(g) + I2(g) ⇌ 2 HI(g) at 440 oC

or the metal-complex formation in water (see Complex Compounds):

βn = K1. K2

. K3 ... KnMLn-1 + L ⇌ MLn

[MLn]

[MLn-1][L]Kn =

[MLn]

[M][L]n=

where n = 1 – 9 unit is dm3/mol unit is (dm3/mol)n

Homogeneous Equilibria

Page 22: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

22

Equilibria between substances in two or more phases, e.g.

CaCO3(s) ⇌ CaO(s) + CO2(g)

Concentrations of pure solids and liquids are constant by definition:

[CaO] = a and [CaCO3] = b

K’ = [CO2] K = [CaO][CO2]

[CaCO3]=

a[CO2]

b

Irrespective the relative amounts of CaO and

CaCO3, at a given temperature the equilibrium

concentration of CO2 over a mixture of the solids is

a definite value.

Heterogeneous Equilibria

Page 23: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

23

In saturated solutions with a solid substances a dynamic equilibrium is attained:

dissolution ⇌ precipitation (mostly applied for slightly soluble compounds), e.g.:

[AgCl] [Ag+][Cl]

K =

AgCl(s) ⇌ Ag+ + Clas [AgCl] is constant, K[AgCl] = [Ag+][Cl], so

Ksp = K[AgCl] = [Ag+][Cl] - solubility product of AgCl

Generally for a MxAy compound:

Ksp = [My+]x[Ax]y

in (mol/dm3)x+y

The Solubility Product

saturated solution of AgCl:Ag+ and CI ions

solid AgCl

Page 24: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

24

Substance Ksp

AgCl 1.7 10-10

AgBr 5.0 10-13

AgI 8.5 10-17

Ag2CrO4 1.9 10-12

CaCO3 4.7 10-9

BaCO3 1.6 10-9

PbCl2 1.7 10-5

PbS 7.0 10-27

HgS 1.6 10-54

NaCl 37.2

The Solubility Product

Calculation of solubility, s from Ksp

PbCl2(s) ⇌ Pb2+ + 2 Cl

Page 25: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

25

A system in equilibrium reacts to a change (stress) in conditions in a way that

counteracts the applied change and establishes a new equilibrium. Each change in

the conditions (concentration, temperature, etc.) should be considered a stress.

Henry Louis Le Chatelier(1850-1936)

Karl Ferdinand Braun(1850-1918)

The Le Chatelier - Braun Principle

Page 26: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

26

Equilibrium Constant and Temperature

- exothermic forward reaction: N2 + 3 H2 ⇌ 2 NH3 ΔH = 92 kJ

[H2]3[N2]

[NH3]2

K =Temperature, oC

300 9.6400 0.5500 0.06600 0.014

If the temperature of the system

is raised, the position of the

equilibrium will shift to the left.

- endothermic forward reaction: H2 + CO2 ⇌ H2O + CO ΔH = + 41 kJ

[H2][CO2]

[CO][H2O] K =Temperature, oC

700 0.63800 0.93900 1.29

1000 1.66

If the temperature of the systemis raised, the position of the equilibrium will shift to the right.

Page 27: Chemical Kinetics - Állatorvostudományi Egyetem · Chemical Kinetics Kinetics is thearea of physical chemistry concerned with the ... The reactionrate is the change in amountor

27

System Change Result

N2 + 3 H2 ⇌ 2 NH3

4 moles → 2 moles

increase (partial) pressureby decreasing volume

Shift to the right. Counteracting the pressure increase by shifting to the side with fewer moles of gas that exert less pressure.

H2(g) + I2(g) ⇌ 2 HI(g) some argon gas is added No change since Ar is not a component of this reaction system.

NaCl(s) + H2SO4(l) ⇌ NaHSO4(s) +HCl(g)

reaction is carried out in an open container

Because HCl is a gas that can escape from the system, the reaction is forced to the right.

HCN(aq) ⇌ H+(aq) + CN–(aq)

HCN + H2O ⇌ H+(aq) + CN–(aq)

the solution is diluted Shift to right; the „reactant” concentration is increased

AgCl(s) ⇌ Ag+(aq) + Cl–(aq) some NaCl is added to the solution

Shift to left due to increase in Cl

concentration. This is known as the common ion effect on solubility.

2 H2O2 ⇌ 2 H2O + O2 a catalyst is added to speed up this reaction

No change. Catalysts affect only the rate of a reaction; the have no effect at all on the composition of the equilibrium state.