Chem 130 Chapter 14 notes part 1
Chem 130 Chapter 14 notes part 1 CHEM 130 - 003
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This 6 page Class Notes was uploaded by Izabella Nill Gomez on Friday September 11, 2015. The Class Notes belongs to CHEM 130 - 003 at University of Tennessee - Knoxville taught by Bin Zhao in Summer 2015. Since its upload, it has received 96 views. For similar materials see General Chemistry II in Chemistry at University of Tennessee - Knoxville.
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Date Created: 09/11/15
Chem 130 Chapter 14 notes Reaction rate the speed at which a chemical reaction occurs Reaction mechanism step by step molecularlevel view of the pathway from reactants to products Chemical kinetics area of chemistry concerned with the speedrates of reactions 4 factors that allow us to change the rate at which a reaction occurs 1 Physical state of the reactants The more readily the reactants collide with one another the more rapidly they react Most are homogeneous Heterogeneous conditions limit the area of contact of reactants They tend to proceed faster if the surface area of the solid is increased 2 Reactant concentrations Most chemical reactions proceed faster if the concentration is increased 3 Reaction temperature Reaction rates generally increase as temperature increases As molecules move more rapidly they collide more frequently 4 Presence of a catalyst Catalysts are agents that increase the reaction rates without themselves being used up The speed of an event is de ned as the change that occurs in a given time interval The reaction rate of a chemical reaction is the change in the concentration of reactantsproducts per unit of time Ms changeEconcentration of B MB A B Average rate of appearance B Changee me At changeEconcentration of A AA Average disappearance of A ChangeEtlme T N Rates are always expressed in positive quantities It is typical for rates to decrease as a reaction proceeds because the concentration of reactants decreases Ex C4H9ClaqH20l gtC4H90HaqHClaq f C4H9Cl Measuring concentration 0 various times after t0 The resulting data is used to calculate the average rate of disappearance of C4H9Cl Graphs showing how concentration of a reactant or product changes with time allows us to evaluate the instantaneous rate of a reaction the rate at a particular instant during a reaction determined from the slop of the curve at a particular point in time datataken 017 042M 800 400s AC4H9Cl Instantaneous rate T 63105 l graphl 5 At t 01 instantaneous rate This is the initial rate of the reaction The rate of appearance of a compound equals the rate of disappearance of the other 0411901 6 Rate AZ Z In general for a reaction aAbBCCdD Lowercase letters are coef cients 1AA 1AlBl lmlcl mlDl Rate is given by Rate a b C At At At At Changing the initial concentration of either reactant changes the initial reaction rate Z aQN282H20l Ex NH Z 6 If NH doubled and N0 held constant the rate doubles The same if the rst compound is increased by a factor of 4 the reaction increases by 4 The way reaction concentrations are depended by the rate is expressed Ratek NH N0 Rate lawk A B wherekis the rate constant magnitude changes with temperature and determined how the temperature affects rate Product concentration does not appear in the rate aw rate law is for reactants There is a linear relationship between reaction rate and concentration 1 n Reaction orders m and n in the k reacmml reacmmz Ex Because the Z Z exponent of NH is 1 the rate is rst order in N0 Overall reaction order is the sum of the orders with respect to each reactant represented in the rate law 112 reaction is second order overall Exponents in a rate law can indicated how the rate is affected by each reaction concentration Ex rate depends on how many Z g powers NH is increased The same with NO 2 2 2 If a rate law is second order with respect to reactant A then 2 4 3 9 For any reaction the rate law must be determined experimentally 9 In general k 10 or higher is a fast reaction and k 10 or lower is a slow reaction 2 Units of rate units of rate constantunits of concentration 6 2 umts of concentration o 4 6 units of rate Z Units of rate constant for a reaction of second order overall In most reactions reaction orders mn are 012 If 0 the reactant has no effect on the reaction Rate of a reaction depends on the concentration but not the rate constant Rate constant is affected by temperature or a catalyst First order reaction one whose rate depends on the concentration of a single AlAl reactant raised to the rst power Ex rate M klA this expressed how rate depends on the concentration differential rate law 1nAt kt1nA0 Integrated rate law similar to ymxb Second order reaction one whose rate depends on either reactant concentration raised to the second power or concentrations of 2 reactants each raised to the rst 1 1 kz powequot W W Zero order reaction one in which the rate of disappearance of A is independent of A Ex gas in decomposition on the surface of a solid ALI ktA0 Overall Reaction Order Units for k 1 Zero MS 1 First S M 1 1 Second S I I I M1 0verallorder 1 The unIt of k for a reaction of any overall order IS 5 Halflife time required for the concentration of a reactant to reach half its initial value A 12lAlo Convenient to describe how fast it occurs expecially for rst t12 693 orders Fast reactions have short half lifes t12 k For a rst order rate law t does not depend on initial concentration of any reactant Halflife is constant throughout the reaction In a rst order reaction the concentration of the reactant decreases by 12 in each of a series of regularly space time interval each equivalent to t12 For a second order rate law halflife depends on the initial concentration of the 1 M N NIH reactant the lower the initial concentration the longer the halflife The rates of most chemical reactions increase as temperature rises The faster rate is due to an increase in the rate constant with increasing temperature Rate constant k is temperature dependent Approximately the rate of reaction doubles for each 100 C rise The central idea for the collision model is that molecules must collide to react The greater the frequency the greater the reaction rate Orientation factor molecules must be oriented in a certain way during collision for a reaction to occur The ineffective collision of molecules will not result in a reaction Molecules must possess a certain amount of energy to react and this minimum energy required to activate a chemical reaction is the activation energy Ea This is the energy barrier molecules must overcome for a reaction to occur The lower the activation energy the more molecules that can participate in a reaction During a reaction the chemical compound that is being reacted must twist the bond 1800 at the highest energy point where it is ready to break into its components After the climax chemicals change and form a new bond If it does not pass the energy barrier it returns to its original form A H lt 0 exothermic A H gt0 endothermic EaAE The activation energy for the reverse reaction is The rate depends on the magnitude of Ed generally the lower the value the faster the reaction At a lower temperature molecules have less energy to react At a higher temperature a larger amount of molecules have higher energy As temperature increases the fraction of molecules that can overcome the activation barrier increases The collision frequency also increases As a result reaction rate increases
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