Kinetics Flashcards
RATE LAW
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
rate equation for:
2NO(g) + O2(g) —-> 2NO2(g)
Rate = k [NO]^2 [O2]
rate equatn
CHCl3 + Cl2 —> CCl4 + HCl
Rate = k[CHCl3] [Cl2]^1/2
rate equatn
CH3COOC2H5 + H2O —> CH3COOH + C2H5OH
Rate = k [CH3COOC2H5] [H20]^0
Factors affecting rate of reaction
- conc of reactants
- temperature
- catalyst
- surface area
- exposure to radiation
- nature of reactants
when conc of reactants is expressed in gases in term of partial pressure then unit of rate of reaction
atm / s
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
- Rate = -1/2 Δ[R]/Δt
- Δ[R] = 0.4 -0.5
=- 0.1 - rate = 1/2 x 0.1 /10
= 5 x 10^-3 mol L-1 s-1
divide carefully
.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
- 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 - in sec
rate = - Δ[R]/Δt
=0.01/ 25 X60 = 6.66 x 10^-6 mol L^-1 s-1
order of reaction
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
ELEMENTARY REACTION
COMPLEX REACTION
- single step reaction
- sequence of elementary reaction (called mechanism) gives us the product
UNIT of rate constant
rate constant = k
Mol^n-1 L^1-n s-1
let order be n
- 0 order = mol L-1 s-1
- 1st order = s-1
- . 2nd order = mol-1 L s-1
MOLECULARITY
THE no. of reacting species taking part in an elementary reaction that must collide simultanously to bring about a chemical rnx
- only for elementary rnxs
- can’t be integer or 0
- theoretical value
- molecularity for slowest step is same as the ORDER of overall rnx
please consider REACTANTS STOICHIOMETIC NO. AND ADD THEM
Find Molecularity
- NH4NO2 —> N2 +H20
- 2 HI —>H2 +I2
- 2NO + O2 —-> 2NO2
consider ONLY reactants
- UNIMOLECULAR
- BI
- TRI
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 ?
- the reaction is : X—>Y
- According to rate law,
rate = k[X]2 - 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-
Pseudo first-order reaction
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
Example of psuedo first order rnx
- hydolysis of ethyl acetate
CH3COOC2H5 + H2O = CH3COOH + C2H5OH
R = K[CH3COOC2H5] [H2O]^0 - inversion of cane sugar
C12H22012 + H20 = C6H12O6 + C6H12O6
R = K[C12H22O12] [H2O]^0
INTEGRATED RATE EQN OF ZERO ORDER
K= [R] - [R]0/ t
EXAMPLES OF ZERO ORDER
ITHOTINHIERE
- SOME enzyme catalysed reaction and reactions which occur on metal surface
- decomposition of GASEOUS AMMONIA ON HOT PLATINUM SURFACE AT HIGH TEMP
2NH3__1130K platinum_________> N2 + 3H2
rate = k[NH3]^0
= K
MOLECULARITY = 2
- THERmal decomposition of HI ON GOLD SURFACE
INTEGRATED RATE EQN OF FIRST ORDER
- ln [R] = -kt +ln [R]0
y = mx + c - k = 2.303/t log [R]o/[R]
for graph: log[R]o/[R] = kt/2.303
y = mx + c
examples of FIRST ORDER RNX
- hydrogention of ETHENE
C2H4 + H2 = C2H6
RATE = K [C2H4] - ALL NATURAL AND RADIOACTIVE DECAY OF UNSTABLE NUCLEI
RATE = K [RA] - decomposition of N2O5 AND N2O`
HALF LIFE OF 0 ORDER
t1/2 = [R]0/ k
* t1/2 directly proportional to initial conc and inversely prop to k
HALF LIFE OF 1ST ORDER
lo2 = 0.301
t1/2 = 0.693/k
* t1/2 is independent of initial conc
Arrhenius EQUATION
TEMP DEPENDENCE OF THE RATE OF RNX
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)
WHat happens to rate when the temperature increases to 10degree
RATE IS DOUBLED
ACTIVATION ENERGY
THE energy required to form intermediate called activated complex
* joules/ mol
plot graph from ln k = - Ea/RT +ln A
ln k = - Ea/RT +ln A
y = lnk
x = 1/T
m= -Ea/R
c= ln A
Activation Energy when 2 temperature and 2 rate constant is given
log k2/k2= -Ea/2.303 R [T2-T1/T1T2]
EFFECT OF CATALYST
- REDUCING** Ea** BTWN REACTANTS AND PRODUCTS and hence lower the potential energy barrier
INHIBITOR
the substance when added reduce the rate of rnx
hOW does catalyst affect GIBBS ENERGY
SIKE
it DOES NOT alter gibbs energy
* it catalyses spontaneous rnx and not non spontaneous rnx
hOW does catalyst affect EQUILIBRIUM CONSTANT
SIKE AGAIN
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
COLLISON FREQUENCY (Z)
dontforgetV
the no. of collision per sec per unit volume of rnx mixture
effective collision
why proper orientation needed?
- molecules should collide with sufficient kinetic energy (threshold energy)
- 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
consider steric factor p and collision frequency z then RATE will be?
Rate = PZab e-Ea/RT
drawback of COLLISION theory
- consider atoms and molecules as hard sphere
- ignores their structural aspect
