Kinetics - Chemical Kinetics 1 CHEMICAL KINETICS...

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Unformatted text preview: Chemical Kinetics 1 CHEMICAL KINETICS Objectives: How fast does a chemical reaction occur? How quickly a reaction mixture will reach equilibrium? How does the rate depend on pressure, temperature, or concentration of the species (reactants, products, catalysts, etc.)? Empirical chemical kinetics We follow how a reaction proceeds by monitoring the concentration of a product or a reactant. Reactions stop after a certain time because: a) either a reactant has been depleted, or b) the concentrations of the products and the reactants have reached equilibrium. Chemical Kinetics 2 Methods to determine rates of reactions 1. Pressure changes N 2 O 5 gas 2 NO 2 gas + O 2 gas 1 mole of N 2 O 5 decomposed makes 5 / 2 moles of NO 2 and O 2 . yellow orange green red blue purple 3. Polarimetry Sometimes the optical activity of a mixture changes in the course of a reaction. Hydrolysis of sucrose was the first rate studied (Wilhelmy, 1850). Sucrose + H 2 O Glucose + Fructose 5 . 66 20 D 5 . 52 20 D . 92 20 D 4. Conductivity of a solution. Changes of concentration of ions, for example, changes in pH with time. 5. Mass spectrometry, chromatography, etc. 2. Spectroscopy H 2 gas + Br 2 gas 2 HBr gas orange gas, so it absorbs in the blue monitor the intensity of absorption of visible light by Br 2 Chemical Kinetics 3 Example 22.1 Monitoring the variation in pressure 2 N 2 O 5 gas 4 NO 2 gas + O 2 gas initially: P N2O5 0 0 P T = P N2O5 later: P N2O5- P N2O5 2 P N2O5 P N 2O5 P T = P N2O5 + 3 / 2 P N2O5 = 5 / 2 P N2O5 if reaction goes to completion, = 1: 0 2 P N2O5 P N 2O5 P T = 5 / 2 P N2O5 By measuring the total pressure, one can calculate , the degree of decomposition of N 2 O 5 , at any time. Chemical Kinetics 4 Ways of applying the techniques 1. Real time analysis. The composition is analyzed while the reaction is in progress. 2. Quenching. The reaction is frozen after a certain time and the composition is analyzed. Can be used for slow reactions, where the time it takes to freeze the substances is negligible. 3. Flow method reactant reactant radiation source material transparent to phototube and recorder mixing chamber movable spectrometer Rates are very susceptible to changes in temperature. So it is extremely important to maintain a constant temperature. Chemical Kinetics 5 Rate of reactions Reactant Product dt ] oduct [Pr d dt ] t tan ac [Re d Rate action Re Chemical Kinetics 6 Rate of reactions A + 2 B 3 C + D dt ] B [ d 2 1 dt ] A [ d dt ] C [ d 3 1 dt ] D [ d Rate action Re The rule is general for all reactions. So we define the reaction rate as: and the conversion rate as x Volume (how many moles per second) Illustration 22.1: 2 CH 3 gas C 2 H 6 gas Rate of change of CH 3 = -1.2 M s-1 ....
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This note was uploaded on 05/01/2011 for the course CHEM 345 taught by Professor Cardelino during the Fall '09 term at Spelman.

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Kinetics - Chemical Kinetics 1 CHEMICAL KINETICS...

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