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Chemistry > Kinetics > Flashcards

Kinetics Flashcards

1
Q

RATE LAW

A

rate law is the expression in which rate of a reaction is given by molar conc of reactants with each term raised to some power which may or may not be equal to the stoichiometric coefficient of reacting species in a balanced chemical equation

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2
Q

rate equation for:

2NO(g) + O2(g) —-> 2NO2(g)

A

Rate = k [NO]^2 [O2]

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3
Q

rate equatn

CHCl3 + Cl2 —> CCl4 + HCl

A

Rate = k[CHCl3] [Cl2]^1/2

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4
Q

rate equatn

CH3COOC2H5 + H2O —> CH3COOH + C2H5OH

A

Rate = k [CH3COOC2H5] [H20]^0

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5
Q

Factors affecting rate of reaction

A
  1. conc of reactants
  2. temperature
  3. catalyst
  4. surface area
  5. exposure to radiation
  6. nature of reactants
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6
Q

when conc of reactants is expressed in gases in term of partial pressure then unit of rate of reaction

A

atm / s

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7
Q

READ QUESTION CAREFULLY

In a reaction, 2A —-> Products, the concentration of A decreases from 0.5 mol L-1 to 0.4 molL-1 in 10 minutes. Calculate the rate during this interval?

average rate

A
  1. Rate = -1/2 Δ[R]/Δt
  2. Δ[R] = 0.4 -0.5
    =- 0.1
  3. rate = 1/2 x 0.1 /10
    = 5 x 10^-3 mol L-1 s-1

divide carefully

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7
Q

.For the reaction R—>P, the concentration of reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds.

average rate

A
  1. Δ[R] = 0.02 - 0.03
    = - 0.01
    Δt = 25 min
    ∴ rate = - Δ[R]/Δt
    =0.01/ 25
    = 4 x 10^-4 mol L^-1 min^-1
  2. in sec
    rate = - Δ[R]/Δt
    =0.01/ 25 X60 = 6.66 x 10^-6 mol L^-1 s-1
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8
Q

order of reaction

A

sum of power of conc of reactants in the rate law expression
1. can be for elementary and complex reanx
2. experimental reacn
3. zero or even an integer
4. for complex rnx; order given by slowest step

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9
Q

ELEMENTARY REACTION

COMPLEX REACTION

A
  1. single step reaction
  2. sequence of elementary reaction (called mechanism) gives us the product
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10
Q

UNIT of rate constant

rate constant = k

A

Mol^n-1 L^1-n s-1

let order be n

  1. 0 order = mol L-1 s-1
  2. 1st order = s-1
  3. . 2nd order = mol-1 L s-1
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11
Q

MOLECULARITY

A

THE no. of reacting species taking part in an elementary reaction that must collide simultanously to bring about a chemical rnx

  1. only for elementary rnxs
  2. can’t be integer or 0
  3. theoretical value
  4. molecularity for slowest step is same as the ORDER of overall rnx

please consider REACTANTS STOICHIOMETIC NO. AND ADD THEM

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12
Q

Find Molecularity

  1. NH4NO2 —> N2 +H20
  2. 2 HI —>H2 +I2
  3. 2NO + O2 —-> 2NO2

consider ONLY reactants

A
  1. UNIMOLECULAR
  2. BI
  3. TRI
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13
Q

The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y ?

A
  1. the reaction is : X—>Y
  2. According to rate law,
    rate = k[X]2
  3. If [X] is increased to 3 times, then the new rate is
    rate’ = k[3X]2
    rate’ = 9 k [X]2 = 9 rate
    Thus, rate of reaction becomes 9 times and hence rate of formation of Y increases 9-
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14
Q

Pseudo first-order reaction

A

a second-order or bimolecular reaction that is made to behave like a first-order reaction.
This reaction occurs when one reacting material is present in great excess or is maintained at a constant concentration compared with the other substance

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15
Q

Example of psuedo first order rnx

A
  1. hydolysis of ethyl acetate
    CH3COOC2H5 + H2O = CH3COOH + C2H5OH
    R = K[CH3COOC2H5] [H2O]^0
  2. inversion of cane sugar
    C12H22012 + H20 = C6H12O6 + C6H12O6
    R = K[C12H22O12] [H2O]^0
16
Q

INTEGRATED RATE EQN OF ZERO ORDER

A

K= [R] - [R]0/ t

17
Q

EXAMPLES OF ZERO ORDER

ITHOTINHIERE

A
  1. SOME enzyme catalysed reaction and reactions which occur on metal surface
  2. decomposition of GASEOUS AMMONIA ON HOT PLATINUM SURFACE AT HIGH TEMP
    2NH3__1130K platinum_________> N2 + 3H2

rate = k[NH3]^0
= K
MOLECULARITY = 2

  1. THERmal decomposition of HI ON GOLD SURFACE
18
Q

INTEGRATED RATE EQN OF FIRST ORDER

A
  1. ln [R] = -kt +ln [R]0
    y = mx + c
  2. k = 2.303/t log [R]o/[R]
    for graph: log[R]o/[R] = kt/2.303
    y = mx + c
19
Q

examples of FIRST ORDER RNX

A
  1. hydrogention of ETHENE
    C2H4 + H2 = C2H6
    RATE = K [C2H4]
  2. ALL NATURAL AND RADIOACTIVE DECAY OF UNSTABLE NUCLEI
    RATE = K [RA]
  3. decomposition of N2O5 AND N2O`
20
Q

HALF LIFE OF 0 ORDER

A

t1/2 = [R]0/ k
* t1/2 directly proportional to initial conc and inversely prop to k

21
Q

HALF LIFE OF 1ST ORDER

lo2 = 0.301

A

t1/2 = 0.693/k
* t1/2 is independent of initial conc

22
Q

Arrhenius EQUATION

TEMP DEPENDENCE OF THE RATE OF RNX

A

K= A e ^ -Ea/RT

k = rate constant
A =* pre-exponential factor **OR arhennius factor or frequency factor
E_{a} = activation energy (in the same units as RT) (joules / mole)
R = universal gas constant (8.314)
T = absolute temperature (in Kelvin)

23
Q

WHat happens to rate when the temperature increases to 10degree

A

RATE IS DOUBLED

24
Q

ACTIVATION ENERGY

A

THE energy required to form intermediate called activated complex
* joules/ mol

25
Q

plot graph from ln k = - Ea/RT +ln A

A

ln k = - Ea/RT +ln A
y = lnk
x = 1/T
m= -Ea/R
c= ln A

26
Q

Activation Energy when 2 temperature and 2 rate constant is given

A

log k2/k2= -Ea/2.303 R [T2-T1/T1T2]

27
Q

EFFECT OF CATALYST

A
  • REDUCING** Ea** BTWN REACTANTS AND PRODUCTS and hence lower the potential energy barrier
28
Q

INHIBITOR

A

the substance when added reduce the rate of rnx

29
Q

hOW does catalyst affect GIBBS ENERGY

SIKE

A

it DOES NOT alter gibbs energy
* it catalyses spontaneous rnx and not non spontaneous rnx

30
Q

hOW does catalyst affect EQUILIBRIUM CONSTANT

SIKE AGAIN

A

it DOES NOT change Kc but helps in attaining equilibrium faster
* it catalyse forward and backward rnx to the same extend
* so equi state remain constant but reached FASTER

31
Q

COLLISON FREQUENCY (Z)

dontforgetV

A

the no. of collision per sec per unit volume of rnx mixture

32
Q

effective collision

why proper orientation needed?

A
  1. molecules should collide with sufficient kinetic energy (threshold energy)
  2. proper orientation
    to faciliate breaking of bonds between reating species and formation of new bonds to form products

e.g. for proper ori is bromomethane

33
Q

consider steric factor p and collision frequency z then RATE will be?

A

Rate = PZab e-Ea/RT

34
Q

drawback of COLLISION theory

A
  1. consider atoms and molecules as hard sphere
  2. ignores their structural aspect

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