Half life and Shelf life

reaction rate represents

the decomposition of the drug

Reaction rate refers to the rate at which a drug undergoes a chemical transformation, such as decomposition, hydrolysis, oxidation, or degradation over time.

Zero order linear plot

X axis and Y axis

what is zero order and its half life

Concentration over time

the drug degrades at a constant rate regardless of how much remains. This means the drug is lost in equal amounts per unit time, and the half-life changes depending on the starting amount.

First order linear plot

what is zero order and its half life

Log concentration over time

first-order kinetics represent a reaction where the rate depends on the concentration of the drug. As the concentration decreases, the rate slows down — but the half-life remains constant because the drug always degrades by a fixed percentage per unit time.

Second order linear plot

1/Concentration (1/[A]) over time

Best fit data based on previous order shows what?

best reaction rate law order

In what law does half life is constant?

first law

Does half-life stay the same if the degradation of aspirin in solution is first order?

yes because the rate of degradation depends on the concentration of the drug

Does half-life stay the same if the degradation of aspirin in suspension (= too much drug molecule in the solvent can handle) ?

The half-life changes when a drug is in suspension because the degradation follows zero-order kinetics. In a suspension, the drug is not fully dissolved, and as the dissolved portion degrades, more solid drug continues to dissolve to maintain a constant concentration at the saturation level. This leads to a constant degradation rate, and the half-life depends on the initial drug amount, so it is not constant.

what happens to half life of 2nd law

half life decreases as the initial concentration increases 

half life increases as the concentration decreases

explain aspirin in solution vs suspension

what is suspension?

suspension means the concentration of drug in solution is saturated at the solubility limit

When the concentration of aspirin exceeds the solubility limit of the solvent, a suspension forms. In this state, there is undissolved aspirin present. As the dissolved aspirin degrades (e.g., by hydrolysis), more solid aspirin dissolves to replenish it, maintaining a constant saturated concentration in solution. Because the dissolved concentration stays constant, the reaction rate also appears constant, which mimics zero-order kinetics — the drug degrades at a constant rate regardless of how much remains.

However, once all the excess solid aspirin is gone and the system becomes an unsaturated solution, the concentration of aspirin starts to decrease over time, and the degradation now follows first-order kinetics, where the half-life becomes constant and the rate depends on the drug’s remaining concentration.

shelf life affected by temperature is expressed using

Arrhenius equation

explain variable

A:

Ea:

range

R:

1.973cal/mol = kcal/mol

decrease in 10 degree temperature gives ___

frequency of molecule to achieve activation energy

activation energy = threshold energy to go forward or product side

10,000cal/mol ~ 30,000cal/mol = 10kcal/mol=30kcal

1.973×10^-3

reduce reactivity by 2-3 times