CHEM 1212 1nd Edition Lecture 6 Outline of Last Lecture I Colligative Properties II Colloids Outline of Current Lecture I Rates of Chemical Reactions II Reaction Conditions and Rate III Effects of Concentration on Reaction Rates IV Rate Equations V The Order of a Reaction VI The Rate Constant k VII Determining a Rate Equation Current Lecture I Rates of Chemical Reactions a Chemical Kinetics a study of the rates of chemical reactions b Reaction Mechanism the detailed pathway taken by atoms and molecules as a reaction proceeds c Rate of reaction Change in concentration Change in time d Average rate of reaction concentration time e Example N2O5 2NO2 O2 i Rate of reaction N2O5 t ii The minus sign is there because the concentration of this compound decreases over time and rate is always expressed as a positive quantity iii Data table for this problem given on pg 670 These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute iv Rate 1 10 mol L 1 22 mol L 0 12 mol L 55 min 40 min 15 min II III IV V VI Rate 0 0080 mol N2O5 consumed L x min f Rates based on changes in concentrations of products will have a positive sign because concentration is increasing g The longer after the reaction has begun the slower the rate will be h We can find the instantaneous rate at a single point in time Reaction Conditions and Rate a Effects of Concentration and Temperature i If the concentration of a reactant is increased the reaction rate will often increase as well ii Chemical reactions occur more rapidly at higher temperatures b Catalysts substances that accelerate chemical reactions but are not themselves consumed c The surface area of a solid reactant can also affect the reaction rate i Only molecules at the surface of a solid can come into contact with other reactants ii The smaller the particles of a solid the more molecules are found on the solids surface Effect of Concentration on Reaction Rate a We determine how concentration affects reaction rate by observing the reaction rate when concentrations are varied b Effect of concentration is different for different reactions Rate Equations a Rate equation or rate law expresses the relationship between reactant concentration and reaction rate b Rate equation k A m B n i The rate equation expresses that the rate of reaction is proportional to the reactant concentration ii Catalysts in reactions are included in the rate equation The Order of a Reaction a Overall reaction order sum of the exponents on all concentration terms b Example 2NO g Cl2 g 2NOCl g i Rate equation k NO 2 Cl2 ii NO second order iii Cl2 first order iv Overall reaction order first second order third order The Rate Constant k a K proportionality constant that relates rate and concentration at a given temperature b Enables you to find the reaction rate for a new set of concentrations c Units of k VII i First order reactions 1 time ii Second order reactions L mol x time iii Zero order reaction mol L x time Determining a Rate Equation a Method of initial rates one way to determine rate equation b Initial rate is the rate of the reaction when t 0