Lecture 3 Professor Hicks Inorganic Chemistry (CHE152) 1 Many - - PDF document

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Lecture 3

Professor Hicks Inorganic Chemistry (CHE152)

Many reactions happen in steps

A + B  C C + 2B  D D+ B + E  F

A + 4B + E  F

• verall reaction

Rate law Rate = k[A][B] Rate = k[C][B]2 Rate = k[D][B][E] 1) rate law for each Elementary Step can be predicted

• order for each species is its stoichiometric number

2) rate law cannot be predicted from the overall reaction know these facts 3) substances that do not appear in the overall reaction are called intermediates An Elementary Step is one where collisions actually occur the way the step is written

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Mechanism of reaction

• series of elementary steps that convert

reactants  products

• steps of mechanism add up to overall reaction
• each step has its own rate law, rate constant
• order observed for overall reaction determined

by slowest step

• Slowest Step is called the Rate Limiting or

Rate-Determining Step

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Molecularity

• for an elementary step molecularity (order) is

the number of molecules that actually collide

2 molecules bimolecular or molecularity = 2 a second order reaction third order (termolecular) reactions are not common because collision

• f three molecules is not common

Overall Reactions

• Many reactions must happen in series of steps

C6H12O6 + 6O2  6CO2 + 6H2O

• To occur in a single step 1 C6H12O6 and 6O2 would

have to simultaneously collide – very unlikely

• Observed rates for the overall reactions with many

steps can be very complicated/unpredictable

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rate determining step

A + B  C C + 2B  D D+ B + E  F

A + 4B + E  F Rate law Rate = k[A][B] Rate = k[C][B]2 Rate = k[D][B][E] 1) if one step is much slower than all the others then it is known as the rate determining step 2) The rate observed for the overall reaction is determined by the rate-determining step (fast) (slow) (fast) Rate = k[C][B]2

• verall reaction

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rate-determining step

narrow part funnel like rate limiting step

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by the rate limiting step

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Rate of reverse reaction

say a reaction happens in one elementary step A + B  C + D as C and D form the Collision Model says they can react to reform A+B the rate laws are: rate forward reaction = kf[A][B] rate reverse reaction = kr[C][D]

Approaching equilibrium

rate forward = kf[A][B] rate reverse = kr[C][D] initially if you only add reactants [C]=0 and [D]=0 so backward rate =0 but as a C and D build up reverse reaction speeds up and A and B are declining so as forward reaction slows eventually they will become equal  concentrations stop changing!!!

Equilibrium Constant

• when rates become equal

rate forward = rate reverse kf[A][B] = kr[C][D]

kf [C][D] kr [A][B] = [C][D] [A][B] Keq=

Keq is called the equilibrium constant

• When the reactant and product calculate to the value
• f the equilibrium constant the reaction appears to

have stopped!

• substantial amounts of reactants may remain

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Equilibrium is Dynamic

• Equilibrium is the condition where the rates of the forward and

reverse reactions are equal - it is dynamic

• Many reactions reach equilibrium with substantial amounts of

reactants present  Limiting reagent is a theoretical maximum not always achieved

• once the reaction reaches equilibrium, the concentrations

remain constant because reactants and products are consumed and made at the same rate

• rates of forward and backward reaction are NOT zero

Dance Floor Bar

1) People constantly are leaving both bar and dance floor 2) Number of people at both is constant because rates are equal 3) # people bar  # people dance floor

dynamic equilibrium

equilibrium  equal

• but that does not mean the concentrations of reactants

and products are equal

• some reactions reach equilibrium only after almost all

the reactant molecules are consumed – we say the position of equilibrium favors the products

• other reactions reach equilibrium when only a small

percentage of the reactant molecules are consumed – we say the position of equilibrium favors the reactants