REACTION KINETICS
rate of reaction: change of molar conc of reactant left or product formed per unit time [mol/(dm³time)]
rate eqn: rate = k[A]^m[B]ⁿ, k = rate const, [] = molar conc of reactant left, m/m = order of reaction wrt A/B, m + n = overall order
rate of reaction = -gradient of [reactant]-time graph
rate of reaction = -gradient of [product]-time graph
rate equation: expression that relates rate of reaction to (1)rate constant, (2)orders of reaction of reactants & (3)molar conc of reacts left at particular time
-rate equation determine experimentally (also based on stoichiometry of slow step / rate-determining step mechanism in a reaction, or from eqn if single-stepped reaction; slow step- A₂ + 2B₂ > 2AB₂ > rate = k[A][B]²[C]⁰)
order of reaction: order of reaction wrt reactant = power of molar conc of reactant in rate eqn {[A]⁰ : rate of reaction does not depend on A}
overall order of reaction: sum of powers of reactants in reaction eqn
rate const: a const that molar conc of reactants are multiplied to obtain rate of reaction [rate const: for particular reaction at const temp, varies w/ temp for exo/endothermic reactions, k: units vary depending on overall order]

Determination of order of reaction
(1)Initial rate method
rate = k[A]^m[B]ⁿ
conc of A kept const, vary conc of B, rate of reaction measured
R₁ = k[A₁]^m[B₁]ⁿ
R₂ = k[A₁]^m[B₂]ⁿ
R₁/R₂ = {[B₁]/[B₂]}ⁿ
> conc of B const, vary conc of A
(2)Conc of reactant-time graph
zero-order reaction: straight line graph of constant -ve gradient (shows rate of reaction is independent of conc of reactant) [rate of reaction = grad of conc-time graph]

non-zero order reaction: curve

(3)Rate of reaction-reactant conc graph
zero order: straight line, zero grad

1st order:st line through origin, grad = constant

higher order: curve

for n^th order: st line through origin obtained if graph of rate reaction- (mol/dm³)ⁿ {rate of reaction of n^th order ∝ [reactant]ⁿ}


(4)Half-life of reaction
half-life: time taken for conc of reactant to fall to half original value
if half-life = constant > 1st order
higher orders: not constant

Measuring rate of reaction (based on property of reactant / product)
(1)Polarimetric method (hydrolysis of sucrose) (for optically active subs)


conc of sucrose ∝ angle of rotation of plane polarized light
use polarimeter to measure angle
conc of sucrose decreases > angle decreases
plot angle-time graph
plot gradient-angle graph > st line through origin > rate = k[sucrose]
(2)Colorimeter method (for subs w/ colour)


colour intensity ∝ conc of subs
colorimeter detects change in intensity (decreasing- reactants, increasing- products)
plot intensity-time graph > plot gradient-intensity graph
(3)Titrametric method (catalytic decomposition of H₂O₂)


pipette H₂O₂ at diff times > titrate w/ KMnO₄ > obtain [H₂O₂]
[H₂O₂] ∝ V(MnO₄^-) used
plot V(MnO₄^-)-time graph
plot gradient-V(MnO₄^-) graph > st line through origin > rate = k[H₂O₂]
(4)Volumetric method (catalytic decomposition of H₂O₂)


V_max ∝ initial conc of H₂O₂
V_t ∝ conc of H₂O₂ used at time t
(V_max - V_t) ∝ conc of H₂O₂ left at time t
plot (V_max - V_t)-time graph
plot gradient-(V_max - V_t) graph > st line through origin > rate = k[H₂O₂]

Collision theory of reaction
fro chem. reaction to occur, reactant particles must collide
not every collision causes a reaction
reaction occurs when energy of collision greater or equal to activation energy / energy barrier of reaction (activation energy, E_A = min energy required to start a reaction by breaking bonds in reactant particles)
factors affecting reaction rate
conc of reactants, pressure (gaseous reactants), temperature, surface area (of solid), intensity of light (photochemical reactions), presence of catalyst
Concentration (rate constant not affected)
reactant conc increased > reaction rate increased
higher conc > collision freq increased > more collisions w/ energy getgetgetget E_A > increase reaction rate
Pressure (rate constant not affected)
pV = nRT => p = nRT/V => p ∝ n/V (conc) [const temp] => pressure ∝ molar conc of gaseous products
higher pressure > higher conc > collision freq increased > more collisions w/ energy getgetgetget E_A > increase reaction rate
Temperature (rate constant k changes w/ temp, rate const ∝ temp)
temp ∝ reaction rate
usually: small temp increase > large reaction rate increase
small temp rise: tremendous increase of no of molecules w/ energy equal or greater than E_A according to Botlzmann distribution of molecular energy
Botlzmann distribution of molecular energy: shows distribution of molecular energies in a gas at const temp

no of molecules w/ energy E at temp T = Ae^-(E/RT)        [R = rate const, A = const ]
area under distribution curve = total no of gas molecules
curve only meets energy axis at infinity energy
only molecules w/ energy getgetgetget E_A react on collision
temp increase > Boltzmann distribution curve shifts to right (higher energy) w/ lower peak

small temp increase > large increase in no of molecules w/ energy getgetgetget E_A > more collisions per second w/ energy getgetgetget E_A > increase reaction rate
Surface area
finer/smaller physical size of solid reactant > greater exposed area to fluid reactant > more effective collisions w/ energy getgetgetget E_A > increase reaction rate
Intensity of light (for photochemical reactions)
intensity ∝ reaction rate
Catalyst
catalyst: subs that alters rate of reaction w/o being chemically changed at end of reaction
+ve catalyst: increase reaction rate by using different reaction mechanism in which E_A lowered to E_A`

collision freq & more molecules w/ energy getgetgetget E_A` increases > reaction rate increased
-ve inhibitor: decrease reaction rate
catalysed reactions
Haber process: 3H₂ + N₂ <(Fe)> 2NH₃
Contact process: 2SO₂ + O₂ <(V₂O₅)> 2SO₃
manufacture of margarine: vegetable oil + H₂ >(Ni)> margarine

enzymes: biological catalysts- hydrolysis of starch, replicatoin of DNA


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