CHEM 1120 1st Edition Lecture 9 Outline of Last Lecture I Method of Initial Rates A Deriving the Rate Law II Concentration and Time A Quick Logarithm Review B Reaction Orders III Half Life Outline of Current Lecture I How to Control the Rate of a Chemical Reaction A Collision Model II The Arrhenius Equation III Reaction Mechanisms IV Rate Laws for Multistep Mechanisms V Catalysis Current Lecture I To answer the question of how to control the rate of a chemical reaction we must first understand what factors modify the speed of a chemical reaction Four main factors can control the reaction 1 Concentration molecules must collide to react 2 Physical State molecules must mix to collide 3 Temperature molecules must collide with enough energy to react rates generally increase with increasing temperature 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 4 The use of a catalyst A Collision Model i e Collision theory reaction rates depend on collisions which in turn will likely depend on at least 3 factors 1 Collision Frequency number of collisions per second per liter higher concentrations more frequent collisions higher temperatures more frequent collisions 2 Collision Energy fraction of the collisions that are sufficiently forceful powerful collisions reaction gentle collisions no reaction 3 Collision Orientation fraction of the collisions with correctly oriented molecules correct alignment reaction incorrect alignment no reaction II Activation Energy Ea minimum collision energy required for molecules to react Example A BC AB C The lower the activation energy the faster the reaction Typically only a fraction of the molecules in a sample possess the right amount of energy to react The higher the temperature the higher this fraction The Arrhenius Equation explains that if the reaction rate varies with temperature so must the rate constant k rate constant A frequency factor related to collision frequency and collision orientation Ea activation energy R gas constant 8 314 J mol K T Temperature in Kelvin Higher T larger k increased rate Example 2N2O5 4NO2 O2 g measure k at different temperatures plot k vs temperature then using the Arrhenius equation plot ln k vs 1 T if it yields a straight line then we know the slope Ea R III reaction mechanism the sequence of elementary steps in an overall chemical reaction elementary step a simple reaction having no intermediates and only having one transition state A K A elementary reaction or elementary process Example Consider the reaction 2NO2 g F2 g 2NO2F g For which the accepted mechanism is You may be wondering what an intermediate would be If step 1 were to proceed as Then NO2F2would be an intermediate and steps 1a and 1b would be elementary steps rather than the original step 1 The temporary existence of an intermediate implies that there are two transition states in step 1 In order for a mechanism to be judged acceptable it must satisfy 2 requirements 1 The sum of the elementary steps must result in the overall reaction 2 The mechanism must be consistent with the experimentally determine rate law Since elementary steps are simple collision processes their rate laws are determined by their molecularity IV Rate determining step a mechanism s slowest step The rate determining step determines the overall reaction rate analagous to an assembly line s rate determining step The rate overall is equal to the rate of the rate determining step or the slowest step Example V Catalyst a substance that increases the rate of a chemical reaction without undergoing permanent chemical change Catalysts make the activation energy smaller or easier to surmount It is much easier to surmount two energy barriers humps on the energy graph than to surmount one giant energy barrier Homogeneous catalysis catalyst and reactants in the same phase Heterogeneous catalysis catalyst and reactants in different phases Catalysts are extremely important in industry in the production of NH3 gasoline plastics etc Enzymes are biological catalysts that have a region where the reactants attach active site The reactants are referred to as substrates Most enzymes are large proteins with molar masses from 10 4 to 10 6 g mol