effect of temp and gggggggggggpH 2025.pptx

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1. Kinetics Effect of pH and temperature on rate of reaction

2. Solutions of a number of drugs undergo accelerated decomposition upon the addition of acids or bases. If the solution is buffered, the decomposition may not be accompanied by an appreciable change in acid or base.

3. Hydrolysis of Aspirin

4. Effect of Acidic pH (Specific acid catalysis) By studying the hydrolysis of aspirin in solutions of different pH, we can obtain a rate-pH profile for the reaction. Log K observed = Log K 1 – pH K observed: experimentally determined first-order rate constant at a definite pH. K 1: the rate constant for hydrogen ion catalysis.

5. Effect of Acidic pH (Specific acid catalysis) The plot of K observed against the pH of the solution will give a line of slope = -1 and intercept equal to Log K 1 . Increase pH-------- decrease rate of hydrolysis Slope = - 1 pH Intercept = Log K 1 Log K observed

6. Effect of Acidic pH (Specific acid catalysis) Log K observed can be obtained from first order curve. log C = log C – Kt / 2.303 Slope = - K ∕ 2.303 t Intercept = log C o logC

7. Hypothetical data for decomposition reaction at different acidic pH levels

8. Interpretation pH 2 : The reaction is quite rapid, showing a significant decrease in concentration within the first few minutes. pH 2.5 : The decomposition is slower compared to pH 2, but still notable, with a moderate decrease in concentration. pH 3 : The reaction proceeds the slowest, with the highest concentration remaining at each time point. Conclusion This data indicates that as the pH becomes less acidic (increasing pH value), the rate of the decomposition reaction decreases.

9. Hypothetical Data for Decomposition Reaction at Different Acidic pH Levels

10. Effect of basic pH (Specific base catalysis) Log K observed = Log K 2 K w + pH K observed :experimentally determined first-order rate constant at a definite pH K 2 : the rate constant for hydroxyl ion catalysis. K w (=10 -14 ): dissociation rate constant of water

11. Effect of basic pH (Specific base catalysis) The plot of K observed against the pH of the solution will give a line of slope = 1 and intercept equal to Log K 2 k w . increase pH------ increase rate of hydrolysis Slope = 1 pH K observed Intercept = Log K 2 k w

12. Effect of basic pH (Specific acid catalysis) Log K observed can be obtained from first order curve. log C = log C – Kt / 2.303 Slope = - K ∕ 2.303 t Intercept = log C o logC

13. Hypothetical data for decomposition reaction at different basic pH levels

14. Interpretation pH 7 : The reaction proceeds slowly, with a gradual decrease in concentration. pH 8 : The reaction rate increases slightly, showing a faster decrease in concentration compared to pH 7. pH 9 : The decomposition reaction is the fastest among the three, with a significant drop in concentration over time. Conclusion This data illustrates how increasing basicity (higher pH) accelerates the decomposition reaction.

15. Hypothetical data for decomposition reaction at different basic pH levels

16. Final graph Log K observed pH Effect of H + Effect of OH - Between 3.5 and 6.5 called solvent catalysis

17. Kinetics Effect of Temperature on the rate of Aspirin Hydrolysis

18. The speed of many reactions increases about 2 or 3 times with each 10°C rise in temperature. The effect of temperature on reaction rate is given by Arrhenius equation: K = A e – Ea / RT Or log K = log A –  

19. log K = log A – In which: K : The specific reaction rate. A : Constant called frequency factor. E a : activation energy. R : Gas constant (1.987 calories /degree. mole). T : absolute temperature.  

20. Effect of temperature Log K observed can be obtained from first order curve. log C = log C – Kt / 2.303 Slope = - K ∕ 2.303 t Intercept = log C o logC

21. Effect of temperature Determination of (K) at several temperatures. Slope = - Ea /2.303R Intercept = Log A Slope = - Ea / 2.303 R 1/ T Intercept = log A Log K

22. Hypothetical Data for Decomposition Reaction at Different Absolute Temperatures

23. The relationship of the reaction rate constants at two different temperatures =