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58 Cards in this Set
- Front
- Back
chemical kinetics
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study of rates of reaction
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6 slow reactions
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rusting
decomposition fermentation yellowing of paper setting of concrete baking |
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3 fast reactions
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combustion
neutralization precipitation |
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In a reaction we can measure (7):
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change temp over time
change pressure over time change mass/moles of rctnts volume product pH colour (spectural photometer) conductivity |
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Rate =
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change [product] divided by time
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concentration of product over time
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starts at bottom fast and gets slower bcs u run out of reactants
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Less reactants means
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less collisions and thus slower reaction
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concentration of reactant over time
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starts at top big and gets smaller. 1st fast then slow
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reactants with a 2,
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used up twice as fast
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6 factors affecting reaction rate
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1 temp. up up
2 agitation up up 3 surface area down down 4 catalysts (faster), inhibitors (slow) 5 [ ] increase of reactants 6 Nature of reaction (some naturally fast, some naturally slow) increase rate |
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Why would u ever want to slow down a reaction?
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ex: to slow down food expiration
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HOW does temp increase reaction rate?
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When heated, reactants gain energy (meaning there are more molecules with sufficient Kinetic Energy). Hence, they move faster, and there is a bigger chance of collision and reaction. Bigger reaction rate.
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HOW does concentration increase reaction rate?
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There are more reactants available, and hence there is a bigger chance they're collide and react.
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HOW do catalysts increase reaction rate?
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Change the pathway of the reaction?
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The rate law relates:
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reactant concentrations and rate quantitatively.
r=k[A]^m[B]^n |
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in rate law, what depends on temp?
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k = proportionality constant/rate constant
NOT rate law exponents |
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RATE LAW: m and n are usually
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0,1,2,3 but can b fractions
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Rate constant:
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indicated speed of reaction.
k large: fast: 10^2 (.1 s) k small: slow: 10^ -3 (2h) |
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k depends on
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not reactant. constant thruout reaction
temp and pressure |
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2nd order rate law units
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L/ mol x s
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1 L how many cm^3
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1000 cm^3 in
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systems consist of
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atoms, ions, molecules, in constant motion with varying KE's.
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temp =
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avg KE = .5mv^2
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Reactions occur when particles collide with:
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a) sufficient energy
b) correct geometry (alignment, position) |
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Reaction rate depends on
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the frequency of collisions
amount of energy |
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homogeneous rxns
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reactants in the same state
ex: N2(g) + 3H2(g) --> 2NH3(g) |
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Heterogeneous rxn
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reactants in diff states
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bimolecular rxn
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2 reactants
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trimolecular rxn
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3 reactants
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unimolecular rxn
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1 reactant
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Rate = x * y
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collision frequency times fraction of effective (sufficient) collision
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3 things that affect collision frequency
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concentration
surface area (only 4 heterogeneous) temperature |
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3 things that affect fraction of effective (sufficient) collision
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temp
catalysts nature of reactant |
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the higher the temp, the more
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molecules with higher KE, the temp
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in "# molecules w/particular KE" vs "KE" graph (y,x),
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the graph righter is higher temp
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low rate even when collision frequency high, bcs
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effective collisions can b low
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activation energy
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the min increase in PE of a system for molecules to react (begin bond braking)
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Maxwell-Boltzmann distribution
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KE vs # molecules (x,y)
With lower activation energy, more molecules react |
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Lower activation energy when (2)
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weak bonds
simple geometry thrfr faster rate |
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# molecules with min energy required increases with
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increasing temp (increase #molecules . . .)
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when asked: y high temp=more collisions?
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say:there are more molecules w/min energy
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catalysts provide a
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lower energy pathway for the rxn to occur. Diff mechanism w/intermediates.
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where y is PE,
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you point out activation energy and delta H.
x is reaction progress |
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2 other word 4 activation energy
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energy hill
activated complex |
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During climb up energy hill (2)
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repulsive forces increase causing molecules to slow down
and PE increases as KE decreases |
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Products that have higher PE but lower KE. Molecules they come in contact with,
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slow down. Hence, lower KE means feels cooler
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After energy hill there is a
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transition state. Unstable molecules
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A half life (t .5) is
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the time for .5 of the nuclei in a radioactive sample to decay OR in a 1st order rxn the time 4 .5 the amount of a reactant 2 b used up.
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In living organisms there are 15.3
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disintegrations of C-14 per minute per gram.
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When organism dies, rate of disintegration
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decreases w/a t.5 = 5730 years
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Radiodating, they
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look at ration of c-14 to c-12 of organism and compare to recently decayed organism
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relative half-life to rate constant
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kt.5=ln2=.693
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half life to rate constant come from
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simple integration of rate law expression as it relates to t.5
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spontaneity of a reaction
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inherent tendency not speed
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to find reaction rate, we must choose conditions in which
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reverse reaction neglected
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differential rate law
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rate divided by concentration
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integral rate law
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rate divided by time
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half life (txtbk)
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ln([a]intl divided by [a]) = kt
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