dr. mihelcic honors chemistry1 chemical kinetics rates and mechanisms of chemical reactions
TRANSCRIPT
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Dr. Mihelcic Honors Chemistry 1
Chemical KineticsRates and Mechanisms of Chemical
Reactions
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• KINETICS — the study of REACTION RATES and the MECHANISM (steps) the reaction follows.
• We can use thermodynamics to tell if a reaction is spontaneous.
• Only KINETICS will tell us HOW FAST the reaction happens!
• KINETICS — the study of REACTION RATES and the MECHANISM (steps) the reaction follows.
• We can use thermodynamics to tell if a reaction is spontaneous.
• Only KINETICS will tell us HOW FAST the reaction happens!
Chemical Kinetics
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Meaning of Rate of Reaction
rate – the change per interval of time
Ex—speed (distance/ time) is a RATE
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• Types of rates -– initial rate, average rate, and
instantaneous rate
Reaction Rates Reaction Rates
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Units
• [ ] = Molarity = moles/L• Time= hr, min, sec• Overall= M/time or
moles/L.time
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C. MeasurementGeneral reaction rate: calculated by dividing rate expressions by stoichiometric coefficients
aA + bB cC +dD
Disappearance of A = -1 ∆[A] a ∆time
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Decomposition of N2O5
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 20 40 60 80 100
Time (sec)
Mo
lar
Co
nce
ntr
atio
n o
f N
2O5
(mo
l/L
)
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• Instantaneous rate – the slope of the tangent line at a given point
• Average rate – the rate over a given time interval
• Initial rate – rate at “time zero”
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Example 2: Sample Calculation of Average RateRate of decomposition of N2O5(g) at 67oC:Time(min) 0 1 2 3 4
[N2O5]0.160 0.113 0.080 0.056 0.040
What is the average rate of decomposition of dinitrogen pentoxide during the first three
minutes?
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II. Reaction Rate and ConcentrationA. Reactant concentration and
collision theory*Reactions occur when
molecules collide to exchange or rearrange atoms
*Effective collisions occur when molecules have correct energy and orientation
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1. Concentration and physical state of reactants and products
2. Temperature 3. CatalystsRate Law: relates the rate of the
reaction to the concentration of the reactants
Factors Affecting Factors Affecting Rate: Rate:
Factors Affecting Factors Affecting Rate: Rate:
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B. Rate Expression and Rate Constant
The rate law is: R = k[A]m[B]n[C]p
EOS
The exponents in a rate law must be determined by experiment. They are not the stoichiometric coefficients and k is the rate constant.
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B. continued
m, n, and p • are the reaction order• can be 0, 1, 2 or fractions• must be determined by experiment!
Overall Order = Sum of m, n and p
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Rate Constant, k
• Relates rate and concentration at a given temperature
General Formula M(1- overall order) /time
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-For one reactant
aA products
Rate = k [A]m
m= order of reaction with respect to A
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• If m = 0 (zero order) Rate = k [A]0 rate is independent of [A]
• If m = 1 (1st order)Rate = k [A]1
If [A] doubles, then rate doubles• If m = 2 (2nd order)
Rate = k [A]2
If [A] doubles, then rate quadruples
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C. Determining Rates
Use data from experiments 1 and 2
Use a ratio of rate expressions for the 2 chosen points.
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The initial rate of decomposition of acetaldehyde, CH3CHO, was measured at a series of different concentrations and at a constant temperature.
Using the data below, determine the order of the reaction (m) in the equation:
Rate = k[CH3CHO]m
CH3CHO(g) CH4(g) + CO(g)CH3CHO (mol/L) 0.162 0.195 0.273 0.410 0.518
Rate (mol/L*min) 3.15 4.56 8.94 20.2 35.2
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Consider the rate data for the decomposition of CH3CHO given in the above example. Knowing that the reaction is 2nd order, determine the
• a. value of the rate constant (k)
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• b. the rate of the reaction when [CH3CHO] = 0.452mol/L
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Order of a Reaction for more than one reactant
• aA +bB products• Rate = k[A]m[B]n
• Overall order of reaction = m + n
• Use one equation to determine each reactant’s order, where the other reactant concentration is held constant.
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The data below are for the reaction of nitrogen (II) oxide with hydrogen at 800oC. 2NO(g) + 2H2(g) N2(g) + 2H2O(g)
Determine the order of the reaction with respect to both reactants, calculate the value of the rate constant, and determine the rate of formation of product when [NO]=0.0024M and [H2]=0.0042M.
1 0.0010 0.0040 0.12
2 0.0020 0.0040 0.48
3 0.0030 0.0040 1.08
4 0.0040 0.0010 0.48
5 0.0040 0.0020 0.96
6 0.0040 0.0030 1.44
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Ex The initial rate of a reaction A + B C was measured with the results below. State the rate law, the value of the rate constant, and the rate of reaction when [A] = 0.050M and [B] = 0.100M.
1 0.1 0.1 4.0x10-5
2 0.1 0.2 4.0x10-5
3 0.2 0.1 16.0x10-5
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Ex The following data were collected for this reaction at constant temperature:2NO(g) + Br2(g) 2NOBr(g)State the rate law, and determine the rate of this reaction when [NO]=0.15M and [Br2]=0.25M.
1 0.1 0.1 12
2 0.1 0.2 24
3 0.2 0.1 48
4 0.3 0.1 108
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Properties of ReactionsOrder Rate Expression0 (zero) rate = k1 (first) rate = k[A]2 (second) rate = k[A]2
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Activation Energy
the minimum amount of energy required for the reaction to occur, Ea.
activated complex – a temporary-transition state formed from the reactants; can form products or reform reactants
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Effective Collisions
How can the number of effective collisions be increased?
• Increase concentration of reactants,
• Increase temperature of reaction,
• Increase surface area of reactants.
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Potential Energy Diagrams and Catalysts
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Potential Energy Diagrams and Catalysts
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CATALYSISA catalyst provides an alternate reaction pathway, which has a lower activation energy than an uncatalyzed reaction.
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REACTION MECHANISMSREACTION MECHANISMS
mechanism: the simple steps involved in a chemical reaction.
Most DON’T occur in a single step!
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Reaction Mechanisms
B. Rate Determining Step:the slowest step in the mechanism; thus determining the overall rate
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C. Intermediates:A substance that is not an
original reactant or a final product; it is produced in one step and consumed in another step of the mechanism
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D. Catalysts:A substance that speeds up the
reaction but is not permanently changed by the reaction; both an original reactant and a final product
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Sample ProblemH2O2(aq) + I1-
(aq) H2O(l) + IO1-(aq) Slow
H2O2(aq)+ IO1-(aq) H2O(l) + O2(g)+ I1-
(aq) Fast
Identify the following:
•Rate determining step?
•Are there any intermediates or catalysts in the reaction? If yes who?
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Sample Problem
O3(g) + Cl(g) O2(g) + ClO(g)
SlowClO(g) + O(g) Cl(g) + O2(g) Fast
Identify the following:
Rate determining step?
Are there any intermediates
or catalysts in the reaction? If
yes who?