Chemical Kinetics_XII (for graphs refer SN) Flashcards
Whats rate of reaction?
Change in conc. or pressure of reactants/products with time.
Rate of rxn is written as
For rxn; aA + bB ———> cC
ROR = dx/dt = 1(-dA/dt)/a = 1(-dB/dt)/b = 1(dC/dt)/c
dA is change in conc. Of A …
Note: -ve sign for reactants & +ve sign for products.
Write Rate of rxn in terms of conc. (C) , moles(n), pressure(P).
-+ dC/dt = +- 1 (dn/dt)/v = +- 1(dP/dt)/RT
Here R = 0.0821 L atm/K mol or 8.314 J/K mol
T = temp. In kelvin.
V = volume
Rate law ??
Is an experimental law.
States ROR is directly proportional to active mass of reagents raised to power determined experimentally.
Unit of rate constant (k) is
(Mol L)^1-n upon time
Where n is order of rxn.
Its used to determine order of rxn if not given in qs.
Expression of rate law is
dx/dt = k.[A]^x.[B]^y
Where k = rate constant; x = order with respect to A; y = order with respect to B
Note: x may/may not be = to stoichiometric coeff. of A & y may/may not be = to stoichiometric coeff. of B
Characteristics of order of rxn.??
1) experimental qty.
2) can be +ve,-ve,zero or fractional.
3) for an elementary rxn, order=stoic.coeff. Of reactants except if the reactant is an intermediate.
What’s molecularity?
No. Of molecules undergoing simultaneous collision in an elementary step of complex rxn.
Characteristics of molecularity??
Theoretical concept.
Can be 1,2 or 3 (can’t exceed 3)
For elementary rxns, molecularity = order of rxn
For zero order rxn, t ½ =
t ½ = Ao/2k
Directly proportional to the initial concentration (Ao) of reactants.
t ½ for 1st order rxn =
t ½ = 0.693/k
Independent of initial concentration of reactants.
For 1st order rxn, k =
K= (2.303/t) log (Ao/Ao-x)
Ao = initial concentration of reactants Ao-x = concentration of reactants ‘remaining’ after time ‘t’.
For a particular order rxn, t ½ =
t ½ ∝ 1/(Ao)^n-1
Where n is order of reactants
Examples of zero order rxn?
1) Photochemical rxns like H2 + Cl2 ———>2HCl
2) decomposition of gases on metal surface
like 2NH3 ——>N2+3H2 & 2HI ——> H2 + I2
3) Adsorption of gases on metal surface.
Examples of 1st order rxn??
Radioactivity Hydrolysis of H2O Inversion of sucrose (sugar) Decomposition of H2O2 or N2O5; 2H2O2 ——> 2H2O + O2 N2O5 ——>2NO2 + ½ O2
Pseudo 1st order rxns/unimolecular rxns ?
1 reagent is present in excess with respect to another.
the reagent present in excess doesn’t affect overall order of rxn.
Threshold energy?
Minimum energy possessed by colliding molecules for a chemical reaction to occur.
For endothermic reaction :
Δ H =
(Ea)f =
Δ H >0 => Hp-Hr >0 =>Hp>Hr
(Ea)f = (Ea)b + ΔH => (Ea)f > (Ea)b
Hint: just remember the graph and all these expressions’ll be obvious.. @check short notes
For exothermic rxn:
ΔH =
(Ea)b =
ΔH<0 => Hp-Hr<0 => Hp<hr> (Ea)b > (Ea)f
Hint/suggestion : just remember its graph & this’ll be obvious.
Factors affecting ROR: ?
Temperature : for every 10° rise ROR becomes 2-3 times
Catalyst
Surface area : Increase in S.A Increases ROR
Nature of reactant : ROR= solid
Whats a catalyst?
Substance that provides alternate path to rxn by altering activation energy Ea( not necessarily lowering) & doesn’t quantitatively change during a reaction.
-ve catalyst increase Ea; +ve catalyst decrease Ea
Relation b/w ROR in presence of catalyst / in catalysed rxns?
ln K1/K2 = 1(Ea2 - Ea1)/RT
Arrhenius =n
K= A e^-Ea/RT
Where A is a constant k/a frequency constant.
Ea is activation energy
R is gas constant = 8.314 J/K mol ≈ 25/3
T is temperature
Results of Arrhenius =n if
a) Ea=0
b) T—> infinity
c) T=0
a) Ea=0 ; K=A
b) T—> infinity ; K=A
c) T=0 ; no rxn.
Differential form of Arrhenius =n is
d ln k /dT = Ea/RT²
Log form : ln K = ln A - Ea/RT
T= temp. k= rate constant Ea = activation energy
Only expression is asked in qs.
