Chem 104 1st Edition Lecture 20 Outline of Last Lecture I. Introduction to chemical kineticsa. Defining ratesi. Reactionii. Averageiii. InstantaneousOutline of Current LectureI. Continuation of lecture on rates.a. Measuring reaction rateb. Continuous monitoringc. Sampling reaction mixture at specific timesd. Factors affecting reaction ratei. Nature of reactantsii. Temperatureiii. Catalystsiv. Reactant concentrationCurrent LectureI. Remember: the chemical rate is how long a reaction takes place. This can be written out in an equation where a lowercase letter represents the coefficient and the upper case letter represents the atoms. Timing is important along with the concentration ofreactants and products at any time.a. Example: If 2.4*102g of NOBr decomposes in a 2.0*102 ml flask in 5.0 minutes, find the average rate of Br2 production.Given: 2 NOBr (g) → 2NO (g) + Br2 (g), 240.0g NOBr2, 200.0ml, 5.0 min, 1 mol=109.91g, 1 ml= 0.001 LFind: ∆[Br2]/∆t, M/sConvert g of NOBr to mol Br, m to M, and 5.0 min to s:240 g NOBr∗(1 mol109.91 g)∗(1 Br2 mol NOBr)=1.09 mol Br1.09 mol Br0.2000 L=.2000 L5.0 min∗(60 s1min)=3.0∗102sSolve for rate:∆[Br]∆ t=5.45 M300 s=0.018MsII. To measure the reaction rate you need to be able monitor constantly. In the lab this is less than an hour so you don’t miss any details. Measurements include: polarimetry, spectrophotometry, and total pressure. Polarimetry is the measurement of the change in degree of rotation of plane-polarized light causes by one of the components over time. Spectrophotometry is the measurement of the amount of light of a particular wavelength absorbed by one component over time. The component absorbs its complementary color. Total pressure is the measurementof total pressure of a gas mixture is stoichiometrically related to partial pressures of the gases in the reaction.III. At specific times during the reaction, you can sample the reaction mixture by drawing off aliquots, samples of reaction mixture, and by doing quantitative analysis.This includes titrations and gravimetric analysis. Gas chromatography is also done bymeasuring the concentrations of various components in a mixture.IV. Factors affecting reaction rate include:a. Nature of the reactants:i. Nature of the reactants means what kind of reactant molecules and what physical condition they are in. Smaller molecules are faster and gases are faster than liquids.b. Temperature:i. Increasing temperature increases the reaction rate. For each 10°C rise = speed of reaction doubles.c. Catalysts:i. Substances that affect the speed of a reaction without being consumed. Most catalyst is used to speed up a reaction.d. Reaction concentration:i. Generally, the larger the concentration of reactant molecules, the faster the reaction. Concentrations also depend on the solute-to-solution ratio aka
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