Presentation on theme: "Kinetics: order of reactions"— Presentation transcript:
1 Kinetics: order of reactions King Saud UniversityCollege of PharmacyDepartment of PharmaceuticsLab 6Kinetics: 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 BRate 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 ReactionsWhen the reaction rate is independent of the concentration of the reacting substance.A = A₀ - k₀tWhere: 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 - k₀tslope = -k₀Slope = ∆Y = y₂-y₁∆X x₂ - x₁
5 First-Order Reactions When the reaction rate depends on the concentration of the reactant.A = A₀ eOr log A = -kt/ log A₀Where: 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 Alog A ₀TimeGraph of first-order equation log A = -kt/ log A₀slope = -k/2.303
7 Half-LifeHalf-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/ k₀First-order half-life: t½ = 0.693/ k
8 Shelf lifeShelf 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/ k₀First-order shelf life : t90 = 0.105/ k
9 Practice Problems Find the log of 35. Find the log of 0.028. Evaluate e – 1.3Find the value of k in the following expression: = 50e –4k1.54410.2725K= 0.173
10 Practice ProblemsPlot 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)1096.02089.04073.06057.09034.012010.01302.5
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)470.01058.02042.03031.06012.0904.51201.7