reaction kinetics Flashcards
Why is chemical kinetics important in pharmacy + pharmacology?
1) determination of shelf-life of drugs
2) determination of amount of drug in body after a particular time period
What is chemical kinetics?
study of rate of reaction
reaction rate
concentration of reactants that disappear + products that form per unit time
What is the unit for reaction rate?
mol L⁻¹s⁻¹
instantaneous rate
rate at a particular time
collision theory
rate of a reaction is proportional to the number of effective collisions
effective collisions
collision that gives product molecules
How does concentration influence the number of effective collisions per second?
as the concentration of reactants increase, the number of effective collisions increases
Why do only a small number of collisions lead to a net change?
1) molecular orientation
2) activation energy
3) temperature
How does molecular orientation influence the number of effective collisions per second?
when two reactant molecules collide they must be oriented correctly for a reaction to occur
activation energy
minimum energy required for a reaction to occur
How does activation energy influence the number of effective collisions per second?
at a given temperature, only a certain fraction of collisions possess enough energy to be effective + form products
How does temperature influence the number of effective collisions per second?
as temperature increases, the rate of a reaction increases
Arrhenius equation
k = Ae⁻ᴱₐ/ᴿᵀ
k =
1) rate constant
2) Ae⁻ᴱₐ/ᴿᵀ
Eₐ
activation energy
R
universal gas constant, 8.314 J mol⁻¹ K⁻¹
T
temperature, K
A
pre-exponential/frequency factor
ln k₂/k₁ =
-Eₐ/R (1/T₂ - 1/T₁)
temperature coefficient, Q₁₀
1) measure of how the rate of reaction changes for every 10°C change in temperature
2) 2-3
t₁₀%
shelf-life
shelf-life
time taken for 10% of a drug to decompose at 25°C
t₁₀% (T₂) =
t₁₀% (T₁) / Q₁₀Δᵀ/¹⁰
rate determining step
slowest step
How do most active pharmaceutical ingredients (APIs) decompose?
1) hydrolysis
2) oxidation
3) photochemical decomposition
4) polymerisation
5) isomerisation
What is hydrolysis?
reaction of a chemical with water
How is hydrolysis catalysed?
presence of acid/base
How can hydrolysis be reduced?
1) reducing storage temperature
2) processing API in solid state
3) store in dry conditions
4) controlling pH
What is oxidation a major cause of?
instability in dosage forms
How can oxidation be initiated?
presence of free radicals formed from exposure to light/heat
Is oxidation fast or slow?
slow
How can oxidation be reduced?
1) storage in amber bottles
2) storing heat-sensitives at low temperatures
3) removing trace metals from tablet pressing
4) antioxidants
What is photochemical decomposition?
decomposition initiated by light
How does photochemical decomposition proceed?
via a free-radical mechanism
How can photochemical decomposition be reduced?
1) keep away from light
1) coating tablet with suitable polymer
3) storage in dark/amber bottles
What is isomerisation?
process where an API may be converted into its different isomers
How can isomerisation be reduced?
controlling pH
