Chemical Kinetics_XII (for graphs refer SN)

Question 1

Whats rate of reaction?

Answer 1

Change in conc. or pressure of reactants/products with time.

Question 2

Rate of rxn is written as

Answer 2

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.

Question 3

Write Rate of rxn in terms of conc. (C) , moles(n), pressure(P).

Answer 3

-+ 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

Question 4

Rate law ??

Answer 4

Is an experimental law.

States ROR is directly proportional to active mass of reagents raised to power determined experimentally.

Question 5

Unit of rate constant (k) is

Answer 5

(Mol L)^1-n upon time
Where n is order of rxn.

Its used to determine order of rxn if not given in qs.

Question 6

Expression of rate law is

Answer 6

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

Question 7

Characteristics of order of rxn.??

Answer 7

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.

Question 8

What’s molecularity?

Answer 8

No. Of molecules undergoing simultaneous collision in an elementary step of complex rxn.

Question 9

Characteristics of molecularity??

Answer 9

Theoretical concept.
Can be 1,2 or 3 (can’t exceed 3)
For elementary rxns, molecularity = order of rxn

Question 10

For zero order rxn, t ½ =

Answer 10

t ½ = Ao/2k

Directly proportional to the initial concentration (Ao) of reactants.

Question 11

t ½ for 1st order rxn =

Answer 11

t ½ = 0.693/k

Independent of initial concentration of reactants.

Question 12

For 1st order rxn, k =

Answer 12

K= (2.303/t) log (Ao/Ao-x)

Ao = initial concentration of reactants 
Ao-x = concentration of reactants ‘remaining’ after time ‘t’.

Question 13

For a particular order rxn, t ½ =

Answer 13

t ½ ∝ 1/(Ao)^n-1

Where n is order of reactants

Question 14

Examples of zero order rxn?

Answer 14

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.

Question 15

Examples of 1st order rxn??

Answer 15

Radioactivity 
Hydrolysis of H2O
Inversion of sucrose (sugar)
Decomposition of H2O2 or N2O5;
   2H2O2 ——> 2H2O + O2
   N2O5 ——>2NO2 + ½ O2

Question 16

Pseudo 1st order rxns/unimolecular rxns ?

Answer 16

1 reagent is present in excess with respect to another.

the reagent present in excess doesn’t affect overall order of rxn.

Question 17

Threshold energy?

Answer 17

Minimum energy possessed by colliding molecules for a chemical reaction to occur.

Question 18

For endothermic reaction :
Δ H =
(Ea)f =

Answer 18

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

Question 19

For exothermic rxn:
ΔH =
(Ea)b =

Answer 19

ΔH<0 => Hp-Hr<0 => Hp<hr> (Ea)b > (Ea)f

Hint/suggestion : just remember its graph & this’ll be obvious.

Question 20

Factors affecting ROR: ?

Answer 20

Temperature : for every 10° rise ROR becomes 2-3 times
Catalyst
Surface area : Increase in S.A Increases ROR
Nature of reactant : ROR= solid

Question 21

Whats a catalyst?

Answer 21

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

Question 22

Relation b/w ROR in presence of catalyst / in catalysed rxns?

Answer 22

ln K1/K2 = 1(Ea2 - Ea1)/RT

Question 23

Arrhenius =n

Answer 23

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

Question 24

Results of Arrhenius =n if

a) Ea=0
b) T—> infinity
c) T=0

Answer 24

a) Ea=0 ; K=A
b) T—> infinity ; K=A
c) T=0 ; no rxn.

Question 25

Differential form of Arrhenius =n is

Answer 25

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.