Reaction Kinetics

Reaction Kinetics:

Rate at which reactants is converted into products

Rate at which reactant is

USED UP

Rate at which product is

FORMED

Rate Experimentations examples

1) Color intensity using a colorimeter: Br2, NO2 -> both decolorize with alkene

2) Conc. of reactant/product by titration

Quenching

slowing down/stopping a reaction to analyse

by:

1) rxn mixture to large V of cold solvent -> dec in K.E -> reduc in collision

2) rxn mixture + substance -> t recorded after quenching takes place

2H2O2 --> 2H2O + O2

Catalytic Decomposition

- Catalyst: MnO2

H2O2 STRUCTURE

Rate of disappearance of reactant

-> exponential decrease graph -> negative gradient -> rate CANNOT be negative

-> Rate = - d[R]/dt

-> To include -ve sign in front of the rate equation

Rate of formation of product

-> Exponential increase

-> Gradient = Positive : dy/dx = Rate

-> Rate = d(pX)/dt

Gradient of a concentration against time graph =

Rate

Units of rate = moldm-3s-1

Pressure (Nm-2) against time (s) ->

Rate = d(pX)/dt

Units: Nm-2s-1

Initial Rate

calculated by finding out the initial gradient of the graph

BrO3- (aq) + 5Br- (aq) + 5H+ (aq) ---> 3Br2 (aq) + 3H2O (l)

Rate at which reactants are used up:

-> exponential decrease graph -> negative gradient so - is added to make the rate positive

-> Rate = - d[BrO3-] / dt

-> Rate = -1/5 d[Br-]/dt

Rate at which products are formed:

-> Rate = + 1/3 d[Br2]/dt

-> +ve gradient -> exponential increase graph so positive gradient => positive rate

N2 (g) + 3H2 (g) ---> 2NH3 (g)....

Rate at which R is used up:

-> Rate = -d(pN2)/dt

-> Rate = -1/3 d(pH2)/dt

Rate at which P formed:

-> Rate = + 1/2 d(pNH3)

Rate of Reactants being used UP (GRAPH)

Rate of products being FORMED - Graph

Rate equation is NOT determined by the overall reaction, but by the:

- slow rate determining step

How to calc. initial rate of RXN?

Via rate eq. using initial reacnt conc + catalyst

To alter the proportionality constant (K)

- Presence/absence of catalyst

- Temperature

Constant of proportionality (K)

K is equal to the RATE of RXN when all R substances concentrations are equal to 1

Rate = k[R]

[R] = 1, SO RATE = K

Order wrt substance

the power of the conc. term of the substance in the rate equation

overall order

the sum of the powers of conc. terms in rate eq

CH3COCH3 + I2 ----> CH3COCH2I + HI

Rate = k[CH3COCH3][H+]

I2 is not involved in the R.D.S -> has no effect on the initial rate

Units of k when rate = k(pNO2)^2

Nm+2s-1

To determine the rate equation:

1) mechanism -> note the R.D.S --> slow step --> rate equation (initial reactants)

2) experimental data

NO2 + CO ---> NO + CO2

(1) 2NO2 ---> NO + NO3 (SLOW)

(2) NO3 + CO ----> NO2 + CO2 (FAST)

Rate = k[NO2] ^2

(CH3)3Cl + OH- ----> (CH3)3OH + Cl- (via reflux)

-> Tertiary halogenoalkane w/ OH- (Nucleophilic Subs.)

- SN1 MECHANISM

-> Only (CH3)3Cl involved before the transition state => 1 SPECIES INVOLVED BEFORE T.STATE

- Compound breaks down heterolytically (C-Cl)

- C--Cl bond weakens

- Carbocation forms + OH- joins

- Alcohol forms, byproduct = Cl-

Rate = k[(CH3)3Cl]

-> Slow step between compound and T.S

CH3Cl + OH- ---> CH3OH + Cl- (Primary halogenoalkane --> SN2)

BImolecular Nucleophilic Subs. -> 2 species involved before transition state

1) OH- attacks the C partial positive atom

(SLOW)

2) Both Cl and OH are attached

3) Fast: Alcohol forms and Cl- released by product

Rate = k[CH3Cl][OH]

Between EXP 1 and 2: concentration of A doubles, concentration of B is constant and rate doubles:

2:2

1:1

Order with respect to [A] = 1

BETWEEN EXP 2 AND 3: Conc of A doubles, Conc of B doubles, and rate quadruples

-> Conc A is 1st order (Conc x2, Rate x2)

-> Conc B => x2 conc, x2 rate => 1st order

BETWEEN EXP 1 AND 2: Conc of [H] doubles, [C] stays constant, rate doubles,

2:2

1:1

-> [H] is first order

EXP 2 TO 3: Conc of [H] is constant, conc of [C] doubles, rate quadruples

2: 4

1: 2

2nd order

2X + Y ---> Z

Exp 1 to 2:

- Initial X doubles, Initial Y constant, Rate quadruples

2:4, 1:2, 2nd order wrt to X

Exp 2 to 3:

-> X doubles, Y doubles, Rate quadruples

X => second order

as conc doubles, rate quadruples, so Y has no effect

Y order = O

Thus, Rate = k[X]^2

First order

concentration doubles, rate doubles (whilst other reactants are constant)

Second order

Concentration doubles, rate quadruples (whilst other reactants are constant)

Zero order

no effect on the rate of rxn

1st/2nd order graph of concentration against time

-> Exponential curve decrease graph

To decipher between 1st order and 2nd order

1st order: constant half life

First T 1/2 = 200, second T 1/2 = 200

2nd order = no constant half life

- Changing T 1/2

Graph of concentration against t for zeroth order

Negative linear graph

Rate against concentration graphs

-> second order: curve upwards

-> first order: linear upwards

-> zero order: horizontal

Rate of RXN =

depends on no. of collisions

For molecules to react

1) necessary Ea

2) favourable orientation

Ea

minimum amount of energy which must be possessed by the reactant molecules so that a collision w/ favorable orientation results in a chemical RXN

Maxwell Distribution Curve

-> inc in T -> Shifts graph to RHS -> more molecules w/ fav orientation + nec. Ea

Catalysts

substances which LOWER the activation energy by providing an alternative mechanism to speed up the rate of reaction

-> Lower Ea --> more molecules w/ necessary Ea and fav. orientation

Arrhenius Equation

k = rate constant

A = frequency factor/ pre exponential factor

Ea = activation energy

e- = base of natural log

R = Constant

T = temp

Inc in P

- no effect on Ea

- inc the no. of collisions -> inc in rate

AUTOCATALYSIS

catalysis in which one of the products of the reaction acts as a catalyst for the reaction

- Acid manganate (VII)

- Ethanedioate ions

Ethanedioic acid

primary standard -> preps for standard solns

[MnO4-] against time

[Mn2+] against time

Catalytic convertor

- mix of metals

- harmful products from car exhausts -> harmless ones

Three way catalytic convertor structure

honey comb -> inc S.A for more efficiency

cat.convertor:

CO -> CO2

Unburnt hydrocarbons (octane ) --> CO2 + H2O

NO/NO2 ----> N2

Lindlar Catalyst

Lindlar's catalyst is a poisoned palladium metal catalyst that performs partial hydrogenation of alkynes in the presence of hydrogen gas.

-> Palladium deposited on CaCO3 and poisoned with Pb and S

-> alkyne -> alkene