1. Kinetics: order of reactions
King Saud University
College of Pharmacy
Department of Pharmaceutics
Lab 6
Kinetics: order of reactions

2. Rates and orders of reactions
Rate: the rate of a chemical reaction of a process is the velocity with which the reaction occurs. Consider the following chemical reaction: drug A → drug B
Rate constant: the order of a reaction refers to the way in which the concentration of drug influences the rate of a chemical reaction or process.

3. Zero-Order Reactions
When the reaction rate is independent of the concentration of the reacting substance.
A = A₀ - k₀t
Where: A₀: amount of drug at t=0. A: amount of drug at time t. k₀: zero-order rate constant, (mass/time).

4. Zero-Order Reactions A A₀ Time Slope = ∆Y = y₂-y₁ ∆X x₂ - x₁
Graph of zero-order equation A = A₀ - k₀t
slope = -k₀
Slope = ∆Y = y₂-y₁
∆X x₂ - x₁

5. First-Order Reactions
When the reaction rate depends on the concentration of the reactant.
A = A₀ e
Or log A = -kt/ log A₀
Where: A₀: amount of drug at t=0. A: amount of drug at time t. k: first-order rate constant, (time⁻¹).
-kt

6. First-Order Reactions
log A
log A ₀
Time
Graph of first-order equation log A = -kt/ log A₀
slope = -k/2.303

7. Half-Life
Half-Life (t½): is the time required for the amount or concentration of a drug to decrease by one half.
Zero-order half-life: t½ = 0.5 A₀/ k₀
First-order half-life: t½ = 0.693/ k

8. Shelf life
Shelf life (t½): is the time required for the amount or concentration of a drug to decrease to 90% of the initial concentration.
Zero-order shelf life: t90 = 0.1 A₀/ k₀
First-order shelf life : t90 = 0.105/ k

9. Practice Problems Find the log of 35. Find the log of 0.028.
Evaluate e – 1.3
Find the value of k in the following expression: = 50e –4k
1.5441
0.2725
K= 0.173

10. Practice Problems
Plot the following data on both semilog graph paper and standard rectangular coordinates.
Does the decrease in the amount of drug A appear to be a zero- order or a first-order process?
Does the amount of drug A extrapolate to zero on the x axis?
What is the equation for the line produced on the graph?
What is the rate constant k?
What is the half-life t 1/2 t90?
Time (min)
Drug A (mg) 10
96.0 20
89.0 40
73.0 60
57.0 90
34.0
120
10.0
130
2.5

11. Home Work Answer questions a, b, c, d, and e as stated in Q5.
Plot the following data on both semilog graph paper and standard rectangular coordinates.
Answer questions a, b, c, d, and e as stated in Q5.
Time (min)
Drug A (mg) 4
70.0 10
58.0 20
42.0 30
31.0 60
12.0 90
4.5
120
1.7