Today Kinetics How fast are reactions? What are the...
Transcript of Today Kinetics How fast are reactions? What are the...
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TodayKinetics
How fast are reactions?What are the rates?
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Diamond Graphite
∆RG° = -3 kJ mol-1
Interconversion of two forms of carbon
If you own diamond jewelry, why should you not worry about this reaction?
A. we know diamond is more stable from ∆RG°
B. the reaction is extremely slow
C. the reaction will only happen if we add energy
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Thermodynamics vs. Kinetics
Diamond Graphite∆RG° = -3 kJ mol-1
Graphite is lower in free energy than DiamondReaction of Diamond to Graphite is spontaneous
THE REACTION IS JUST VERY VERY SLOW
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Thermodynamics
Compares Free energy of reactants and productsThis is the ideal case assuming everything can find
its lowest energy state (time is irrelevant)
KineticsWhat is actually happening
How long does it take convert reactants to products
Diamonds are unstable
Diamonds are "kinetically trapped" in the unstable state
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Kinetics
To understand how fast a reaction proceeds we need to think about what is actually happening during the reaction
Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)
What actually happens?
Presumably somehow the reactants have tophysically interact before there can be any chemistry
(assuming we are not doing this in a electrochemical cell)
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Let’s look at some reactions
Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)
Cu(s) + 2H+(aq) Cu2+(aq) + H2(g)
A. Zn
B. Cu
C. no way to since, since kinetics doesn’t have anything to do with thermodynamics
which will be faster to react?
∆RG° = +65 kJ mol-1
∆RG° = -147 kJ mol-1
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Let’s look at some reactions
Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)
A. Zn dust
B. Zn chunks
C. they will react at the same rate (they’re both Zn)
which will be faster to react?
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Let’s look at some reactions
Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)
A. higher concentration HCl
B. lower concentration HCl
C. they will react at the same rate (they’re both strong acids)
which will be faster to react?
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What can affect the rate of reactions?
1. Nature of the reactants
2. Concentration of the reactants
3. Temperature
4. Presence of a Catalyst
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First you have to get the reactants together
Often we have two species that need to interact (physical contact)
before there can be a chemcial reaction (bond breaking/forming)
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First you have to get the reactants together
How do you speed up a reaction?
More interactions
“Nature” of the reactanct (more surface area for solids)
Higher concentrations (more collisions for species in solution)
Often we have two species that need to interact (physical contact)
before there can be a chemcial reaction (bond breaking/forming)
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What prevents reactions form going"downhill" in energy?
2H2(g) + O2(g)
2H2O(g)
Energy
Thermodynamics deals with the initial and final states
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What prevents reactions form going"downhill" in energy?
2H2(g) + O2(g)
2H2O(g)
Energy
Thermodynamics deals with the initial and final states
Kinetics deals withthe path between them
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Why is there a "barrier"?
You have to break the "old" bonds before you can form the "new" ones
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Why is there a "barrier"?
You have to break the "old" bonds before you can form the "new" ones
How do you speed up a reaction?
Raise the temperature (more molecules over the barrier)
Add a catalyst(lower the barrier)
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How do we know how fast a reaction is?
We look at the rate
Rate is change per timeReaction rate is change in concentration per time
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For this reaction
A. the rate for all the species is constant
B. the rate if largest at the start of the reaction
C. the rate is largest at equilibrium
D. the rate is randomly fluctuating
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Rate is change in concentration per unit time
Rate is the slope of the graph of concentration vs time
Steepest slope at the start
at equilibrium rate = 0 (reaction has “stopped”)
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CO(g) + H2O(g) CO2(g) + H2(g)
If you know the rate of one reactant or product you know them all
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2H2(g) + O2(g) 2H2O(g)
Rate of consumption of H2
=∆[H2]∆t
Change
-
reactant decrease
Rate of formation of H2O
∆[H2O]∆t
+
products increase
=
= d[H2]
dt-
d[H20]
dt+ =
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2H2(g) + O2(g) 2H2O(g)
Rate of consumption
of H2
Rate of formation of H2O
2 x the Rate of consumption
of O2
= =
H2 and H2O have rates that are faster since 2 moles reaction for each 1 mole of O2
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For this reactions
N2(g) + 3H2(g) 2NH3(g)
the rate of production of NH3 is
A. 2 times the rates of consumption of H2
B. 1.5 times the rate of consumption of H2
C. 2/3 times the rate of consumption of H2
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For this reactions
N2(g) + 3H2(g) 2NH3(g)
Rate of Reaction
d[N2]
dt- =
11
d[H2]
dt- =
13
d[NH3]
dt+ =
12
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For this reactions
N2(g) + 3H2(g) 2NH3(g)
Rate of Reaction
d[N2]
dt- =
11
d[H2]
dt- =
13
d[NH3]
dt+ =
12
Generic Reaction
aA + bB cC + dD
Rate ofReaction
d[A]
dt- =
1a
d[B]
dt- =
1b
d[C]
dt+ =
1c
d[D]
dt+ =
1d
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How to get the rate? Do the experiment!
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Characterizing rates
We want the slope
d[C]dt
∆[C]∆t
~
NoteRate is changing with
concentration
Change in concentrationdivided by
Change in time
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Rate Laws
How does the rate depend on the concentrations?
Rate is some function of the concentrationof the reactant molecules
What is the function?
To predict the function we need to know the mechanism (the individual step in the reaction)
Or we can do an experiment to directly measurehow the rate varies with concentration
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2NO2(g) 2NO(g) + O2(g)
Rate = k[NO2]n
For the reaction rate we typically are looking at the forward reaction
We write the rate as a function of the Reactant concentrations
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2NO2(g) 2NO(g) + O2(g)
Rate = k[NO2]n
unknown constant
k is the "rate constant"
unknown exponent
n is the "reaction order" with respect to NO2
For the reaction rate we typically are looking at the forward reaction
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The grand overview of all of the kinetics
rate
aA + bB cC + dD
∆[C]c∆t
-∆[A]a∆t
= = = k[A]x[B]y
we are looking only at the rate of the "forward" reactionThis depends only on the concentration of the reactants
measure in experiment figure out from data
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for example I looked at the data for this reaction and found
rate = k[NO2]2
this reaction is 2nd order in NO2
Four Factors that affect a reaction rate
1. Concentration of reactants
This is the "rate" law. It is found by looking at the dataIt depends on the "mechanism"
2NO2 2NO + O2
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Method of initial rates
Since the rate can change as the reaction proceeds,one way to determine the affect of concentration on a rate
is to perform many experiments at different starting concentrations and measure the initial rate
C3H6O + Br2 C3H5OBr + HBr
Experiment [C3H6O]o [Br]o initial rate (M s-1)
1 0.1M 0.1M 1.64 x 10-5
2 0.1M 0.2M 1.65 x 10-5
3 0.2M 0.1M 3.29 x 10-5
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C3H6O + Br2 C3H5OBr + HBr
Experiment [C3H6O]o [Br]o initial rate (M s-1)
1 0.1M 0.1M 1.64 x 10-5
2 0.1M 0.2M 1.65 x 10-5
3 0.2M 0.1M 3.29 x 10-5
A. the rate is independent on Br2 concentration
B. the rate is linear dependent on Br2 concentration
C. the rate depends on the concentration of Br2 squared
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C3H6O + Br2 C3H5OBr + HBr
Experiment [C3H6O]o [Br]o initial rate (M s-1)
1 0.1M 0.1M 1.64 x 10-5
2 0.1M 0.2M 1.65 x 10-5
3 0.2M 0.1M 3.29 x 10-5
A. the rate is independent on acetone concentration
B. the rate is linear dependent on acetone concentration
C. the rate depends on the concentration of acetone squared
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C3H6O + Br2 C3H5OBr + HBr
Experiment [C3H6O]o [Br]o initial rate (M s-1)
1 0.1M 0.1M 1.64 x 10-5
2 0.1M 0.2M 1.65 x 10-5
3 0.2M 0.1M 3.29 x 10-5
A. k = [C3H6O][Br2]
B. k = [C3H6O]2[Br2]
C. k = [C3H6O] D. k = [Br2]
E. k = constant
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C3H6O + Br2 C3H5OBr + HBr
Experiment [C3H6O]o [Br]o initial rate (M s-1)
1 0.1M 0.1M 1.64 x 10-5
2 0.1M 0.2M 1.65 x 10-5
3 0.2M 0.1M 3.29 x 10-5
A. the rate is first order in acetone and first order in Br2
B. the rate is first order in acetone and zeroth order in Br2
C. the rate is second order overall D. A & C