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17 Cards in this Set
- Front
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Spontaneous vs Nonspontaneous reaction what measures spontaneity? if a rxn is spontaneous, will it run fast or slow? hint: is rate determined by spontaneity? |
Spontaneity measured by ∆ G : Gibbs free energy **spontaneity is not equal to a fast reaction **catalysts, such as enzymes, enhance reactions by up to factors of 10^14 |
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Reaction Mechanisms A mechanism is where the reaction proceeds by more.... what is useful about knowing the mechanism of a reaction? |
Mechanism: sum of series of steps which makes an overall reaction ***knowing the mechanism allows: -ascertain reaction rate - position of equilibrium' - thermodynamic characistics |
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what's the generic reaction Alternative: Slow and Fast step if two steps
Which step is the Rate Determining Step |
The general rate reaction:A(2) + 2B --> 2AB@glance: 2 molecules B and 1 A make 2 molecules of AB What if two step rxn? net reaction: Step 1: A(2) + B --> A(2)B (intermediate) SLOW Step 2: A(2)B + B --> 2AB (fast step) |
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Collision Theory of Chemical Kinetics -what has to happen for a reaction to occur kinetically? what two variables affect an affective collision? What is the minimum energy of collision needed for a rxn to take place? |
theory proposed to explain events at atomic level through the process of a reaction Assumed: collision must occur for rxn to happen Collision Theory of chem. Kin: -rate of rxn is proportional to # collisions resulted in a chem. rxn *affective collision: formation of products - correct orientation - sufficient energy to break/form bonds Activation energy (Ea): min. energy of collisions for rxn to occur |
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If only a fraction of collisions are affective, then what is the formula for rate? units? What is the more quantitative equation to measure the rate constant of a reaction? FOCUS on Arrhenius Equation for measuring rate of rxns ---the relationship between variables |
Rate= Z x f or Rate= # collisions x fraction(effective) collisions. K = A x e ^ (-Ea/ RT) k= rate constant A= frequency factor Ea= activation energy rxn ^how often collisions@rxn R=ideal gas constant T=temp. (K)kelvin FOCUS: Frequency factor(A) and rate(k) -direct relationship: A ^ then k^ Rate(k) will ^ if temperature(K)^ - if T^ to infinity, exponent value would be <1 - rate will not decrease b/c the (-) makes the valuable approach zero from a more negative value ** so exponent becomes less (-) and more (+) so the rate constant actually increases with increasing temperature Frequency Factor ^ w/ inc^ the # molecules - more opportunities for collisions@ higher [C] |
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Transition State Theory - how does it relate to the bonds of molecules coming into collision w/ each other Do the transitions have higher or lower energy than both the reactants and products? -Hint: transition also called the activated complex What part of the rate graph shows the transition intermediate? |
Transition theory states: - Molecules w/ energy = or > Ea(acti. Energy) will form transition state *old bonds weaken & new bonds form transition: has highest energy (theoretical) *located at peak of a diagram |
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Energy Absorbed is a (+ or -) ∆ G-free energy A negative ∆ in free energy is an ____ rxn - this means there was a + or - ∆G? What do their rxn diagrams look like? - formation of HCl from H- + Cl- |
Exergonic Rxn: -∆G energy given Off - negative free energy ∆ - on a rxn diagram: the starting free energy should be more than the ending free energy
Endergonic Rxn: +∆G energy absorbed -Positive *remember kinetics and thermodynamics should be considered separately |
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Factors affect Reaction Rate How does temperature affect rxn rate? what reactions are/ are not Temperature-dependent? How does Reaction concentrations affect the rate? - what can you add, that will affect frequency fact. |
Reaction Rate Factors: 1. Reaction Concentration - Inc^[reactants], then inc's^#collisions/time *this leads to ^ in frequency factor (A) if k=Ae^(-Ea/RT), then rate will Inc^ -gases partial pressure ~> [gas] exception: except zero order rxns 2.Temperature -Inc^ Temp, then Inc^ rate kinetic energy molecules will inc^ ALL rxns are temp-dependent - most enzymes have a optimum activity temp. ex: generic human enzyme @ 35-40*C |
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What types of Mediums affect rxn Rate? - which solvent is preferred b/c dipole movements? What does a Catalyst do to a rxn? is it consumed?
Does a catalyst affect the spontaneity? How? - can it make a rxn become spontaneous if not previously? How does a catalyst affect the free energy of the products and reactants? |
3. Medium -Aqueous vs Nonaqueous -solid, liquid, or gas - polar solvents usually preferred w/ dipoles 4. Catalysts - Lowers activation energy (Ea) - not consumed in process - Homogenous catalyst: same phase as reactant - solid, liquid, gas - Heterogeneous catalyst: Different phase than reactants - Inc^ spontaneity: rxn will reach equilibrium faster because the rate will increase. - has No Affect on free energy(thermodynamics) it only affects the activation energy of fwd. and reverse rxn. |
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Definition of Rate to write rate law, should look for .... What is the equation? What order reactions does MCAT focus on? what is the common trap w/ rate law and stoichiometric coefficients? |
Disappearance/Consumption of reactants over time or appearance/formation of products over time or rate = -∆[A]/ a∆t + -∆[B]/∆t = +∆C/∆t Rate units: mol Liter/s or M/s *need data to determine rate law aA + bB = cC+ dD rate law equation: rate= k [A]^x[B]^y k= rate constant, rxn rate coefficient x and y= orders -x is order of reactant A -y is order of reactant B Only time stoichiometric coeff will be match the orders of rxn: 1. rxn mechanism is single step and balanced 2. complete mechanism is given and the rate determined step are equal to the orders of rxn -- stoich coeff on reactant side of rate-det. step equal the orders of the reaction MCAT: 1st, 2nd, and 3rd order |
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Equilibrium Equation vs Rate Equation which equation includes only the reactants? which equation is specific for chemical kinetics? which equation will indicate where the equilibrium position lies? which equation tells you how quickly the rxn will get there? |
equilibrium expression: concentrations of all species in the reaction -products and reactants Rate law expression: includes ONLY Reactants Rate expression -reactants only -specific to chemical kinetics Equilibrium constant, Keq - includes All products n reactants -indicates Keq position |
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Value for rate law= k[A]^x[B]^y is k constant? how is it constant but has many values for rxns? |
k: not actually constant -depends on Ea and temp. of rxn -@specific temp and rxn, k is constant
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How to experimentally determine rate law
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*identify trials which concentration of reactants where one is changed and one is kept constant. - rate of formation: b/c the ∆[C] of a reactant ex: w/ two trials trial 1: [A]=1M and [B]=1M rate = 2 trial 2: [A]=1M and [B]= 2M rate= 8.1 if rate= [B]^x -when [B] x2, the rate inc^ x4 so 4m/s = [2]^x so x=2 trial 2: [A]=1M and [B] = 2M rate(m/s)= 8m/s trial 3: [A]=2M and [B] = 2M rate= 16m/s rate=[A]^x 2 = [2M]^x so x=1 rate law expression=k[A]^1[B]^2, so 2+1=3, third order substitute one of the trials in to get k, rate law |
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Zero Order Reaction raise the concentrations to 0, what do you get? |
Product independent of reactants rate= k[A]^0[B]^0 so [A] and [B]= 1 rate=k(1)(1)= k so rate is independent of ∆[A] or ∆[B] k (M/s) *same k in k=Ae^(-Ae/ RT) so inc^ T or catalyst will dec. Ea. slope is linear and negative |
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First- Order RXN think C-14 |
FIRST: one! Rate is proportional to only one reactant * inc^ the [A] --> inc^ rate of decrease. rate= k[A]^1 or k[B]^1 |
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Second-Order Rxn
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concentration of [A] and [B] rate=k[A]^1 [B] ^1 or k[A]^2 and k[B]^2 **suggests physical collisions, |
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Higher Order Rxns- 3rd Order
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few processes- less likely to collide w/ correct orientation
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