CHEM-C 117: CH. 13 CHEMICAL KINETICS
59 Cards in this Set
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Chemical kinetics
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study of speeds of reactions and nanoscale pathways/ rearrangements by which atoms and molecules are transformed from reactants to products
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homogeneous reaction
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reactants and products are all in same phase
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Factors that affect speed of reaction
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1properties of reactants and products 2concentrations of reactants and sometimes products 3temperature 4presence of catalyst and its concentration
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heterogeneous reactions
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take place at surface; interface between two different phases; rate depends on area and nature of surface at which reaction occurs
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rate
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change in some measurable quantity per unit of time; speed of process
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reaction rate
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change in concentration of a reactant or product per unit time; change in concentration of one reactant divided by change in time
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Reaction rates and stoichiometry
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rate of change in concentration of any reactant or product multiplied by reciprocal of stoichiometric coefficient to find rate of reaction
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Average reaction rate
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reaction rate calculated from change in concentration divided by change in time; smaller as concentration of one or more reactants decreases
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Instantaneous reaction rate
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rate at particular time after reaction has begun; slope of line tangent to concentration-time curve at point corresponding to specified concentration and time
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Rate law
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mathematical equation that summarizes relationship between reactant concentration and reaction rate
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Rate constant, k
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proportionality constant of rate law equation; independent of concentration but has different values at different temperatures (usually becomes larger as temperature increases)
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initial rate
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instantaneous rate determined at very beginning of reaction; good approximation is to calculate -delta[reactant]/delta t after no more than 2% of limiting reactant has been consumed
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rate law of most homogeneous reactions
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rate=k[A]m[B]n
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Integrated rate law for first-order reaction
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rate=(-delta[A]/deltat)=k[A]
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Linearized integrated rate law of first-order reaction
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Ln[A]t=(-kt)+ln[A]0
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Graph first-order reaction
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ln[A] on y-axis and t on x-axis; ln[A]0=y-intercept, -k=slope
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Integrated rate law for second-order reaction
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Rate=k[A]2
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Linearized integrated rate law for second-order reaction
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1/[A]t=kt+(1/[A]0)
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Graph second-order reaction
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1/[A]t vs. t; slope=(-k), y-intercept=1/[A]0
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Integrated rate law for zeroth-order reaction
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Rate=(-delta[A]/deltat)=k[A]0=k
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Linearized integrated rate law zeroth-order rate law
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[A]t=kt+[A]0
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Graph of zeroth-order reaction
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At vs. t; slope=(-k), y-intercept=[A]0
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Rate=
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(-delta[A]/deltat)=k[A]m
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Half-life
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time required for concentration of reactant A to fall to one half its initial value
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T1/2=
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(-ln2/-k)=.693/k
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Unimolecular reactions
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structure of a single particle rearranges to produce different particle or particles; has a molecularity of one
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Bimolecular reactions
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two particles collide and rearrange bonds to form products; has a molecularity of two
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Elementary reactions
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simplest nanoscale reactions
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Transition state/ activated complex
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structure at the top of an energy diagram
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Activation energy
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minimum energy required to surmount energy barrier
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delta Eo for reaction
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difference between Ea for forward and reverse reactions
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Steric factor
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depends on 3d shapes of reacting molecules; only a collision from particular side is effective
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Reaction rates ___ (increase/ decrease) with temperature
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increase
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Arrhenius equation
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k=Ae-Ea/RT
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R for Arrhenius equation
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8.314 j/molK
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E
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2.718
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Linearized Arrhenius equation
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ln(k)=ln(A)+(-Ea/RT)
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Graph of linearized Arrhenius equation
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ln(k) on y axis and 1/t on x-axis; slope is -Ea/R and y-intercept is ln(A)
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Reaction mechanism
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set of elementary reaction equations, each with its own Ea and k
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Rate-limiting step
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slowest step of the sequence; limits rate at which other steps/ overall reaction can occur
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Rate-limiting step
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atoms, molecules, or ions that are produced in one step and consumed in a later step
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Catalyst
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participates in reaction mechanism to increase reaction rate; rate-limiting step has lower Ea
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Homogeneous catalyst
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catalyst is present in the same phase as reacting substance
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Enzyme
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highly efficient catalyst for one or more chemical reactions in a living system
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Globular proteins
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polymers of aas in which one or more long chains of aas fold into a nearly spherical shape
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Cofactors
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inorganic or organic molecules or ions that work with enzymes to perform catalysis
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Substrate
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molecule whose reaction is catalyzed by an enzyme
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Active site
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part of enzyme molecule that interacts with substrate via same kinds of noncovalent attractions that hold enzyme in its globular structure
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Induced fit
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change in shape of either the enzyme, substrate, or both when they bind; can lower Ea; can stretch bonds and make them easier to break
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Noncovalent attractions that hold enzyme in globular structure
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Hydrogen bonds, attractions of opposite ionic charges, dipole-dipole and ion-dipole forces
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Reasons enzymes are extremely effective
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1bring substrates into close proximity and hold while rxn occurs 2hold substrates in most effective shape 3act as acids and bases 4PE of bond distorted by induced fit is already partway up activation energy hill 5sometimes contain metal ions
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Maximum rate
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increasing concentration of substrate does not increase rate of reaction because enzyme is limiting
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Denaturation
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Increased motion as temperature increases; can disrupt structures of enzymes and other proteins; globular protein loses coiled structure, properties change; active site no longer available
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Inhibitor
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molecule or ion that can fit in active site but do not react; decreases effective concentration and rate of reaction
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Heterogeneous catalysis
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present in different phase from that of reactants being catalyzed; usually catalyst is a solid; used in industry because they are more easily separated from products and leftover reactants
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Reaction Rate=
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-delta[R]/deltat
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Reaction rate units
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(moles/L)/sec
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Half life=
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1/2 [A]0
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____(equation) gives fraction of all reactant molecules that have sufficient energy to surmount activation energy barrier
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e-Ea/RT
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CHEM-C 117: CH. 13 CHEMICAL KINETICS