Chem 102 Module 2: The Rates of Reactions
Chem 102 Module 2: The Rates of Reactions Chem 102
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This 4 page Class Notes was uploaded by Lyna Nguyen on Tuesday February 9, 2016. The Class Notes belongs to Chem 102 at Texas A&M University taught by Dr. Bethel in Spring 2016. Since its upload, it has received 16 views. For similar materials see General Chemistry 2 in Chemistry at Texas A&M University.
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Date Created: 02/09/16
Module 3: The Rates of Reactions (14.1 – 14.6) 14.1: Rates of Chemical Reactions Reaction rate = change in concentration / change in time Calculating a rate: o Concentrations: Measuring the absorbance of light by a solution o Average rate is the change in concentration per unit time o Units: mol / (L * time) o Negative slope, but expressed as positive value o Instantaneous rate: single point in time Slope of tangent line at specific point Relative Rates and Stoichiometry o aA + bB -> cC + dD 14.2: Reaction Conditions and Rate What effects reaction rate? o Reactant concentrations, temperature, and presences of catalyst Also surface area if solid Effects of concentration and temperature o Illustrated by “iodine clock reaction” If concentration of a reactant is increase, the reaction rate will increase Chemical reactions occur more rapidly at higher temperatures Catalysts: substances that accelerate chemical reaction rates but are not consumed by reaction o Catalase: biological catalyst/enzyme o Increase reaction rate Surface area (only solids) o Smaller particles -> more molecules on surface o More molecules, more reaction 14.3: Effect of Concentration on Reaction Rate Rate Equation/ Rate Law o Relationship between reactant concentrations and reaction rate is expressed by an equation Rate equations o Evaluating how the rate is affected when concentrations of the reactants are varied o K[Reactant] The Order of Reaction o Order: exponent of its concentration term in the rate law expression Not stoichiometric coefficients o Overall reaction order: sum of all exponents o Gives insight into how the reaction occurs The Rate Constant o K: relates rate and concentration at a given temperature Knowing k allows calculation at certain reactant concentrations Units: 0 order: mol/L*time st 1 order: 1/time 2ndorder: L/mol*time Wide range of values Reaction dependent o Determining a rate equation “method of initial rate”: the initial rate is the instantaneous reaction rate at the start of the reaction (rate @ time = 0) 14.4: Concentration-Time Relationships: Integrated Rate Laws Integrated Rate Laws o Zero Order [Ro] – [Rt] = kt o First Order Ln([R t/[Ro]) = -kt o Second Order 1/[R t – 1/[Ro] = kt Half Life o Zero Order [Ao] / 2k o First Order Ln2 / k o Second Order 1 / k[Ao] 14.5: A Microscopic View of Reaction Rates Collision theory of reaction rates o 3 requirements must be met reacting molecules must collide with one another reacting molecules must collide with sufficient energy to initiate the process of breaking and forming bonds molecules must collide in an orientation that can lead to rearrangement o the atoms and the formation of products Collision Theory: Concentration and reaction Rate o Dependence of reaction rate on concentration because number of collisions between 2 reactant molecules is directly proportional to the concertation of each reactant Collision Theory: Activation Energy o Activation energy: energy required to cross the carrier If the barrier is low, energy is low, fast reaction If the barrier is high, energy is high, slow reaction o Reaction Coordinate diagram Horizontal axis: reaction progress Module 3 Page 2 Vertical axis: potential energy o Transition state: energy of the system reaches a maximum Also known as activated complex Collision theory: Activation Energy and temperature o As temperature increases, average energy of the molecules in sample increases Collision theory: Effect of Molecular Orientation on Reaction Rate o Must come together in the correct orientation o Lower probability of achieving the proper alignment, smaller value of k, slower reaction o Bond rearrangement: both bond breaking and bond formation occurring The Arrhenius equation o Summarized dependency on energy, frequency of collisions, temperature, and geometry o K = Ae^(-E /Ra) R: 8.314 o Can be used to calculate E fram temperature dependence of the rate constant o Can be used to calculate the rate constant if E , a and A are known o Temperature increase, rate increase Effects of catalyst on reaction rate o Speeds up the rate of a chemical reaction o Not consumed in a reaction o Function: provide a different pathway with a lower activation energy for the reaction o Reaction Intermediates: species formed in one step of the reaction and consumed in a later step 14.6: Reaction Mechanisms Reaction mechanism: sequence of bond making and bond breaking steps that occurs during the conversion of reactants to products o 1 or more steps Elementary step: each step in a multistep reaction sequence o Formation/rupture of chemical bond o Has its own activation energy and rate constant Molecularity of Elementary Steps o Elementary steps: classified by number of reactant molecules that come together Molecularity: number of molecules 1 mole: unimolecular 2 moles: bimolecular 3 moles: termolecular Rate Equations for elementary steps o Rate equation for any elementary step is defined by its stoichiometry Reaction Mechanisms and rate Equations o Products of a reaction can never be produced at a rate faster than the rate of the slowest step o Rate determining: slowest step, determines rate of reaction Module 3 Page 3 Module 3 Page 4
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