CHM 104 1st Edition Lecture 5 Outline of Last Lecture I Integrated Rate Law Equations II Half life Equations Outline of Current Lecture III Arrhenius Equation Current Lecture Rate of reactions are highly dependent on temperature A 10 degree increase will double or triple reaction rates Only change is temperature there is no change in reactant How do we explain this Reactant Product Rate k reactant n0 temperature dependence is contained in k k is only constant at constant temperature The relatively between k and T was quantified by Swedish chemist Svank Arrhenius Arrhenius Equation K A x e Ea Rt R gas constant 8 314 J mol k unit conversion factor T temperature units of kelvin A constant frequency factor or pre exponential factor deals with the number of molecule collisions and orientation Ea activation energy activation barrier J mol Activation Barrier is an energy barrier that must be passed climbed over in order for reactant to transform into products 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 Describes the energy needed to make the activated complex The more stable the transition state the lower the activation energy the faster the reaction Frequency Factor Anytime the N C part of the molecule wiggles and approaches the activation barrier A describes how often that happens most attempts are not successful k Ae Ea RT Range from 0 to 1 If Ea RT then e Ea RT 0 If Ea RT then e Ea RT 1 Represents the fraction of molecules that have enough energy to get over the barrier but doesn t mean they have the correct orientation As T increases the number of molecules having enough thermal energy RT to make it over the barrier increases Increasing temperature doesn t move the energy barrier it changes the fraction of reactant molecules that have sufficient energy to make it over past the barrier Slow reactions are characterized by Small A reactants don t approach activation barrier k will be small Large Ea energy barrier is high and not many reactant molecules will make it over past k will be small Low T not many reactants will have enough RT k will be small Fast reactions characterized by Large A reactants approach barrier often k will be large Small Ea easy for reactants to pass barrier Large T higher the temperature the more thermal energy more energetic molecules How do we determine Ea and A for a chemical reaction By measuring how the rate constant k varies with temperature K Ae Ea RT Ea and A can be determined by measuring k at 2 different temperatures
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