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Lab 6

King Saud University College of Pharmacy Department of Pharmaceutics. Lab 6. Kinetics: order of reactions. Rates and orders of reactions.

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Lab 6

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  1. 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 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/2.303+ 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/2.303+ 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: 25 = 50e–4k 1.5441 - 1.553 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 t1/2 t90?

  11. Home Work • 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.

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