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Unformatted text preview: 1 Chapter 15 Chemical Kinetics o Thermo Review o Reaction Rates o Rate Laws o Concentration and Time o Reaction Mechanisms o Models for Chemical Kinetics 1 Big Idea: The rates of chemical reactions are described by simple expressions that allow us to predict the composition of a reaction mixture at any time. These expressions also suggest the steps in which the reactions takes place. Chapter 15: Chemical Kinetics Reaction Rates Catalyst: A substance that increases the rate without being consumed in the reaction. Homogeneous Catalyst: A catalyst that is in the same phase as the reactants. Heterogeneous Catalyst: A catalyst that is in a different phase than the reactants. 2 Chapter 15: Chemical Kinetics Reaction Rates Reaction Rates: The change in concentration of one of the reactants or products divided by the time interval over which the change takes place. 3 R P Average Rate of Consumption of R: ?¡¢ = − ∆ ∆¡ Note: Rates are always positive, therefore, since the reactants are consumed, a negative sign must be added to make the rate positive. Average Rate of Production of P: ?¡¢ = ∆? ∆¡ Unique Average Rate (UAR) ?£ + ?¤ ¥¦ £ = − 1 ? ∆ £ ∆¡ = − 1 ? ∆ ¤ ∆¡ = 1 ¥ ∆ ¦ ∆¡ Chapter 15: Chemical Kinetics Reaction Rates Instantaneous Rate of Reaction The best approximation to the rate at a single instant is obtained by drawing a tangent to the plot of the concentration against time. The slope of the tangent is called the instantaneous rate of the reaction. 4 Chapter 15: Chemical Kinetics Rate Laws Rate Law: An equation expressing the instantaneous reaction rate in terms of the concentrations, at any instant, of the substances taking part in the reaction. ?¡¢ = § £ ¨ ¤ © … 5 Note: k is the rate constant and x ,y, … are the orders of reaction. Note: This form of the rate law is called the differential rate law. Note: The units of rate are always ª«¬® ?¯?° × ¬° , therefore, the units of k will differ depending on the overall reaction order. Chapter 15: Chemical Kinetics Rate Laws Things to know about the rate law: Rate laws can contain products, reactants, catalysts but usually only starting material. Rate laws can ONLY be determined experimentally. Orders do NOT correlate with coefficients in balanced equation! Orders can be an integer, 0, a fraction, positive OR negative! Each species has its own individual reaction order. The overall reaction order is the sum of the individual orders found in the reaction. 6 2 Chapter 15: Chemical Kinetics Order th 1 st 2 nd time Initial Concen- tration Rate Law ?¡¢ = £ ?¡¢ = £[¤] ?¡¢ = £[¤] 2 Rate Laws 7 0 10 20 30 0 10 20 30 0 2 4 6 Rate 0 2 4 6 0 2 4 6 0 2 4 6 [A] 0 2 4 6 0 10 20 30 Chapter 15: Chemical Kinetics Rate Laws Determine the rate law: A + B + C 2D 1) General Rate Law: 2) Order with respect to A: 3)...
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This note was uploaded on 01/18/2012 for the course CHEM 1B taught by Professor Watts during the Winter '08 term at UCSB.
- Winter '08