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LAB 6 Kinetics: order of reactions King Saud University College of Pharmacy Department of Pharmaceutics

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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.

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

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Zero-Order Reactions A Time Graph of zero-order equation A = A - kt Slope = Y = y-y X x - x slope = -k

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First-Order Reactions When the reaction rate depends on the concentration of the reactant. A = A e Or log A = -kt/2.303 + log A Where: A: amount of drug at t=0. A: amount of drug at time t. k: first-order rate constant, (time¹). -kt

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First-Order Reactions log A Time Graph of first-order equation log A = -kt/2.303 + log A slope = -k/2.303

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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 A/ k First-order half-life: t½ = 0.693/ k

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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: t 90 = 0.1 A/ k First-order shelf life : t 90 = 0.105/ k

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Practice Problems 1. Find the log of 35. 2. Find the log of 0.028. 3. Evaluate e – 1.3 4. Find the value of k in the following expression: 25 = 50e –4k 1.5441 - 1.553 0.2725 K= 0.173

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Practice Problems 5. Plot the following data on both semilog graph paper and standard rectangular coordinates. a. Does the decrease in the amount of drug A appear to be a zero- order or a first-order process? b. Does the amount of drug A extrapolate to zero on the x axis? c. What is the equation for the line produced on the graph? d. What is the rate constant k? e. What is the half-life t 1/2 t 90 ? Time (min)Drug A (mg) 1096.0 2089.0 4073.0 6057.0 9034.0 12010.0 1302.5

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Home Work 6. 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) 470.0 1058.0 2042.0 3031.0 6012.0 904.5 1201.7

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