Spectrophotometric Methods For Kinetic Study
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| Spectrophotometric Methods For Kinetic Study |
Spectrophotometry is widely used
for monitoring the reaction. It is used for reactions in which one of the reactant
or product is UV visible (active). UV-visible active means the substances that
absorb radiations in UV (200-400 nm) or visible region (400 800 nm).
If two compounds are active, then
λmax of two compounds must be very different. Now the very basic things we know
about absorbance and transmittance is that
IT/Io = T
(transmittance)
Here “IT” = transmitted radiation
Io = incident radiation
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If all radiations are transmitted, then
no radiation is absorbed. As
-log T = A (Absorbance)
A = -log T
A = log 1/T
A = log Io/IT
Every UV-VIS active compound absorbs
radiation at a certain wavelength. For example, methylene blue absorbs
radiation at 600-700nm. The wavelength at which a compound absorbs maximum
radiation is called lambda max or λmax
Also keep in mind that Absorbance ∝ Concentration and Absorbance ∝ path length
A ∝ Cl
A = εCl
Here ε = molar absorptivity constant
The above equation is of
Beer-Lambert law.
We can monitor 1st and 2nd
order reaction kinetic spectrophotometrically.
Monitoring 1st order by
spectrophotometric method
Consider reaction in which reactant converts to product.
R P
at t=0 b 0
at t b-x x
Absorbance of reactants R = AR
Absorbance of products P = AP
Absorbance due to solvent = α
For reactants, A ∝ C
AR ∝ [R]
AR = β[R]
For products, AP ∝ [P]
AP = Æ”[P]
Alchemist
At start of reaction, when t=0 then Ao is the absorbance of reaction mixture.
Ao = AR + α
Ao = β[R] + α
Ao = βb + α eq. 25.1
Absorbance of reaction mixture at time "t" will be (At)=
At = AR + AP + α
At = β(b-x) + Ɣ(x) + α eq.25.2
At the end of reaction, let the absorbance of reaction mixture be A∞
A∞ = AP + α
A∞ = Æ”[P] + α
A∞ = Æ”(b) + α eq.25.3
Subtracting eq.25.3 from 25.1 we got,
A∞ - Ao = Æ”(b) + α - βb + α
A∞ - Ao = b (Æ”-β) eq.25.4
Subtracting eq.25.3 from 25.2 we got,
A∞ - At = Æ”(b) + α - β(b-x) + Æ”(x) + α
A∞ - At = Æ”b + α - βb + βx - Æ”x - α
A∞ - At = Æ”(b - x) - β (-x + b)
A∞ - At = (Æ”- β) b - x) eq.25.5
According to 1st order rate equation we know that
eq.25.6 Dividing eq.25.4 wit 25.5 we got
From eq.25.6 we can write that
So, the above equation is 1st order rate equation to determine kinetics of reactions by spectrophotometric method.
Monitoring 2nd order by spectrophotometric method
or 
Ao - At = βb + α - (β(b-x) + Ɣ(x) + α )
Ao - At = βb + α - βb + βx - Ɣ(x) - α
Ao - At = x(β- Ɣ) eq.25.7
Now subtracting eq.25.2 from 25.3
At - A∞ = βb- βx + Æ”x + α - (Æ”(b) + α)
At - A∞ = βb- βx + Æ”x + α - Æ”b - α
At - A∞ = - Æ”(-x + b) + β (-x + b)
At - A∞ = (β - Æ”) (-x + b) eq.25.8
Dividing eq.25.7 with 25.8 we got
From 2nd order rate equation we can write that,
eq.25.9Alchemist
So, the equation 25.9 is 2nd order rate equation to determine kinetics of reactions by spectrophotometric method. Graphically,
From graph, K = slope / b
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