1 chemical kinetics: principles of reactivity. 2 kinetics reaction rates - how fast the reaction...
TRANSCRIPT
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Chemical Kinetics:Principles of Reactivity
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Kinetics
• Reaction rates - How fast the reaction occurs (the change in reactant and product concentration as a function of time)
• Reaction Mechanisms –The detailed pathway taken by atoms and molecules as the reaction proceeds (how the reaction occurs).
• Our goal is to understand chemical reactions at the molecular level.
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The Rate Laws: Factors
Reaction Rate Concentration
Concentration – As concentration increases, the reaction rate increases.
Physical State of Reactants – Reactants must mix and collide in order to react.
Temperature – As temperature increases, the reaction rate increases..
Reaction Rate µ Temperature
Catalyst – A substance that increases the rate of a reaction without being consumed in the process..
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Reaction Rates
Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.
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Rate of Reaction
• Rate of Reaction: positive quantity that expresses how concentration of a reactant or product changes with time.A B
]][ 2 1
12
12
[A
t
A -
t - t
[A] - [A] -
time in Change
[A] Change reaction of Rate
A
12
1
][][ BB
t
t
B
t
[B] - [B]
time in Change
[B] in Change reaction of Rate
2
12
Note:Note: [ A ] mean conc. in mol/L (M) Units? [ A ] mean conc. in mol/L (M) Units? = M/s= M/s
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RATES OF REACTIONS
For General reactions A → B, the graph will be:
Focus on: Average rate- between time intervalsAP Chem (instantaneous rate & initial
rate)
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Reaction Rates
In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
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Reaction Rates: Average Rate
The average rate of the reaction over each interval is the change in concentration divided by the change in time:
Average rate =[C4H9Cl]
t
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
We monitor the appearance of the product
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Reaction Rates: Average Rate
• The average rate decreases as the reaction proceeds.
• Reason: as the reaction goes forward, there are fewer collisions between reactant molecules.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
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Calculating Average Rate
C2H4 (g) + O3 (g) C2H4O (g) + O2 (g)
t
OOH
t
OHC
][
t
]C[
][ -
t
][- Rate 242342
Rate of [O3] Depletion
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
3.00E-05
3.50E-05
0 20 40 60
Time (sec)
[O3]
sLmol
7-
-5-53
s 60-0 10 x .50 s 0 - s 60
mol/L 10 x 3.20 - mol/L 10 x 1.10 -
t
[O Rate 3
]
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Rates of Reactions and Stoichiometry
••For every 1 mole of HFor every 1 mole of H22 or I or I2 2 that react, 2 moles of HI that react, 2 moles of HI is formed.is formed.
So that rate of formation of HI is two times faster So that rate of formation of HI is two times faster than the depletion of Hthan the depletion of H22 or I or I22..
Write the Rate Laws:Write the Rate Laws:
H2 (g) + I2 (g) 2 HI (g)
oorrt
]HI[
t
]I[ 2-
t
]H[2- Rate 22
t
]HI[
2
1
t
]I[ -
t
]H[- Rate 22
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Reaction Rates and Stoichiometry
• General Rule for Every reaction:
aA + bB cC + dD
Rate = −1a
[A]t
= −1b
[B]t
=1c
[C]t
1d
[D]t
=
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Reaction Rates and Stoichiometry
• In this reaction, the ratio of C4H9Cl to C4H9OH is 1:1.
• The rate of disappearance of C4H9Cl is the same as the rate of appearance of C4H9OH.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Rate =-[C4H9Cl]
t=
[C4H9OH]t
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Reaction Rates: Instantaneous speed
• A plot of concentration vs. time.
• The slope of a line tangent to the curve at any point is the instantaneous rate at that time.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
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Reaction Rates: Initial Rate
• All reactions slow down over time.
• Best indicator of the rate of a reaction is the instantaneous rate near the beginning.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
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The Rate Laws
Determining Rate Laws Experimentally• Rate laws
determined by measuring initial reaction rates at varying initial reactant
concentrations.• Done by following:
– color change (spectroscopy); – precipitation formation (turbidity,
absorbance); – gas formation (pressure monitoring); – conductometry; etc.
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Concentration and Rate
• The rate of a reaction changes with changes in concentration.
• Rates of reactions increase as concentrations increase since there are more collisions occurring between reactants.
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Concentration and Rate
Comparing Experiments 1 and 2, when [NH4+]
doubles, the initial rate doubles.
NH4+(aq) + NO2
−(aq) N2(g) + 2 H2O(l)
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Concentration and Rate
Likewise, comparing Experiments 5 and 6, when
[NO2−] doubles, the initial rate doubles.
NH4+(aq) + NO2
−(aq) N2(g) + 2 H2O(l)
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Concentration and Rate
• This meansRate [NH4
+]
Rate [NO2−]
Rate [NH+] [NO2−]
or
Rate = k [NH4+] [NO2
−]• This equation is called the rate law,
and k is the rate constant.• Units of k?
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Rate Laws
• A rate law shows the relationship between the reaction rate and the concentrations of reactants.
• The exponents tell the order of the reaction with respect to each reactant.
• This reaction isFirst-order in [NH4
+]
First-order in [NO2−]
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Concentration and Rate
• The rate law: examines the dependence of reaction rate on the concentration of reactants and is given as the rate expression.
Determined EXPERIMENTALLY • The mathematical expression for the Rate Law:
Rate = k [A]m [B]n
– [A] & [B] represent the reactants.– The exponents m and n are called “reaction
orders”. – The proportionality constant k is called the rate
constant.– The overall reaction order is the sum of the
reaction orders. – The overall “order of reaction” is therefore…m + n +
p
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Units of k
Rate expressionUnits of kChange in [A]Change in [B]Total change in rateOrder of reaction
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Rate Law Determination
• Determined only by doing an experiment
• Isolation method: change the concentration of one reactant at a time, while keeping the concentration of all other reactants constant.
• There is no relationship between the order of the reaction and stoichiometry.
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First Order (Gases as Reactants)
Consider the process in which methyl isonitrile is converted to acetonitrile.
CH3NC CH3CN
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First Order (Gases as Reactants)
This data was collected for this reaction at 198.9°C.
CH3NC CH3CN
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First Order (Gases as Reactants)
• When ln P is plotted as a function of time, a straight line results.
• Therefore,– The process is first-order.– k is the negative slope: - 5.1 10-5 s−1.
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Second Order
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Second-Order Processes
The decomposition of NO2 at 300°C is described by the equation
NO2 (g) NO (g) + 1/2 O2 (g)
and yields data comparable to this:
Time (s) [NO2], M
0.0 0.01000
50.0 0.00787
100.0 0.00649
200.0 0.00481
300.0 0.00380
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Second-Order Processes
• Graphing ln [NO2] vs. t yields:
Time (s)
[NO2], M ln [NO2]
0.0 0.01000 −4.610
50.0 0.00787 −4.845
100.0 0.00649 −5.038
200.0 0.00481 −5.337
300.0 0.00380 −5.573
• The plot is not a straight line, so the process is not first-order in [A].
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Second-Order Processes
• Graphing ln 1/[NO2] vs. t, however, gives this plot.
Time (s) [NO2], M 1/[NO2]
0.0 0.01000 100
50.0 0.00787 127
100.0 0.00649 154
200.0 0.00481 208
300.0 0.00380 263
• Because this is a straight line, the process is second-order in [A].
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Half-Life
• Half-life is defined as the time required for one-half of a reactant to react.
• Because [A] at t1/2 is one-half of the original [A],
[A]t = 0.5 [A]0.
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First Order Half-Life
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Half-Life
For a first-order process, this becomes
0.5 [A]0
[A]0
ln = −kt1/2
ln 0.5 = −kt1/2
−0.693 = −kt1/2
= t1/2
0.693k
NOTE: For a first-order process, the half-life does not depend on [A]0.
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Second-Order Half Life
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Temperature and Rate
• Generally, as temperature increases, so does the reaction rate.
• This is because k is temperature dependent.
In a chemical reaction, bonds are broken and new bonds are formed.Molecules can only react if they collide with each other.
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Collision Model
• The collision model assumes that in order for molecules to react they must collide.
• The greater the number of collisions the faster the rate.
• The more molecules present, the greater the probability of collisions and the faster the rate.
• The higher the temperature, the more energy available to the molecules and the faster the rate.
• Complication: not all collisions lead to products. In fact, only a small fraction of collisions lead to product.
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The Collision Model
Molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation.For the reaction: Cl + NOCl NO + Cl2
There are two possible ways that Cl atoms and NOCl molecules can collide; one is effective and one is not.
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Activation Energy
• Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.
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Activation Energy
• Consider the rearrangement of methyl isonitrile:
– In H3C-NC, the C-NC bond bends until the C-N bond breaks and the NC portion is perpendicular to the H3C portion. This structure is called the activated complex or transition state.
– The energy required for the above twist and break is the activation energy, Ea.
– Once the C-N bond is broken, the NC portion can continue to rotate forming a C-CN bond.
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Reaction Coordinate Diagram
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Reaction Coordinate Diagrams
• The diagram: shows the energy of the reactants and products (and, therefore, E).
• Transition state: The high point on the diagram is the
• Activated complex: the species present at the transition state is called the.
• The energy gap between the reactants and the activated complex is the activation energy barrier.
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Reaction Coordinate Diagram
Exothermic reaction
ΔH reactants
ΔH products
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Maxwell–Boltzmann Distributions
• Temperature is defined as a measure of the average kinetic energy of the molecules in a sample.
• At any temperature there is a wide distribution of kinetic energies.
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Reaction Mechanism
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Reaction Mechanisms
Reaction mechanism: the sequence of events (path of the reaction) that describes the actual process by which reactants become products is called the.
• Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction.
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Multistep Mechanisms
• In a multistep process, one of the steps will be slower than all others.
• The overall reaction cannot occur faster than this slowest, rate-determining step.
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Reaction Mechanisms
• Case #1: • The first step is a slow step, the
following steps are fast steps
• Case #2: • The first step is a fast step and the
following step is a slow step.
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Road Map for Reaction Mechanisms
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Catalysis
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Catalysis
• A catalyst is a substance that changes the rate of a chemical reaction without itself undergoing a permanent chemical change in the process.
• There are two types of catalyst: Heterogeneous--one that is present in a different phase as the reacting molecules. Homogeneous-- one that is present in the same phase as the reacting molecules.
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The gases exhausted from an automobile engine pass through a catalytic converter where air pollutants such as unburned hydrocarbons (CxHy), CO, and NO are converted to CO2, H2O, N2, and O2. The photo shows a cutaway view of a catalytic converter. The beads are impregnated with the heterogeneous catalyst.
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Catalysis: Homogenous Catalyst
Example: Hydrogen peroxide decomposes very slowly in the absence of a catalyst:
2H2O2(aq) 2H2O(l) + O2(g)
In the presence of bromide ions, the decomposition occurs rapidly in an acidic environment:
2Br–(aq) + H2O2(aq) + 2H+
(aq) Br2(aq) + 2H2O(l)
Br2(aq) + H2O2(aq) 2Br–(aq) + 2H+
(aq) + O2(g)
Br– is a homogeneous catalyst because it is regenerated at the end of the reaction.
The net reaction is still…2H2O2(aq) 2H2O(l) + O2(g)
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Catalysts and Reaction Rates
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Catalysts and Reaction Rates
Catalysts operate by lowering the overall activation energy, Ea, for a reaction…(It lowers the “hill”.)
• However, catalysts can operate by increasing the number of effective collisions.
• A catalyst usually provides a completely different mechanism for the reaction.
• Intermediates are being generated (see decomposition of H2O2, Br2 is the intermediate.
• When a catalyst adds an intermediate, the activation energies for both steps must be lower than the activation energy for the uncatalyzed reaction.
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Heterogeneous Catalysts
• Solid catalyst is a heterogeneous catalyst (catalytic converter in cars)•Many industrial catalysts are heterogeneous.Mechanism of operation: • The first step is adsorption (the binding of reactant molecules to the catalyst surface).• Adsorption occurs due to the high reactivity of atoms or ions on the surface of the solid. • Molecules are adsorbed onto active sites on the catalyst surface.• The number of active sites on a given amount of catalyst depends on several factors such as:
- The nature of the catalyst.- How the catalyst was prepared.- How the catalyst was treated prior to use.
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Heterogeneous Catalysis
One way a catalyst can speed up a reaction is by holding the reactants together (adsorption has to occur) and helping bonds to break.Less energy is required to break the bonds.
Example: C2H4(g) + H2(g) C2H6(g)
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Enzyme Catalysts
• Enzymes are biological catalysts. There may be as many as 30,000 enzymes in the human body. (Ex: Lactase)
• Most enzymes are protein molecules with large molecular masses (10,000 to 106 amu).
• Enzymes have very specific shapes.
• Most enzymes catalyze very specific reactions.
•The substances that undergo reaction at the active site on enzymes are called substrates.
• A substrate locks into an enzyme and a fast reaction occurs. The products then move away from the enzyme.
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Enzymes
• Enzymes are catalysts in biological systems.
• The substrate fits into the active site of the enzyme much like a key fits into a lock.