ChE 452 Lecture 02
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Transcript of ChE 452 Lecture 02
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ChE 452 Lecture 02
Basic Concepts
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Stoichiometry
Invented by Lavoisier Molecules react in fixed proportions
1 A + 2B 3C+ 4D Stoichiometric Coefficent, n
Number of molecules produced when reaction goes once
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3311 ,
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Example
2CO + O2 2CO2
CO + ½ O2 CO2
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Answer
Answer
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Reaction Rate
Priestley Total Moles/hr
Van’t Hoff Moles/hr/volume
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Priestley’s Experiment
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Changing the amount of powder did not change the rate of weight loss
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Van’t Hoff’s Experiments
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2N2O5 4NO2 + O2
Reaction rate scaled as the volume of the reactor
Special poisoned walls
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Reaction Rate
production rate (Moles/hr) reactor volume (liter)
• rA called the Rate of production of A
• rA has dimensions moles/lit-hr
• rA is positive for a product, negative for a reactant
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A
r
r
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Heterogeneous Reactions Scale As Surface Area
production rate (Moles/hr) surface area (cm2)• RA is also called the Rate of
production of A• RA has dimensions moles/cm2 –hr
• RA is positive for a product, negative for a reactant
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AR
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The Rate Of A Reaction
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r riA
A1
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Heterogeneous vs. Homogeneous Reactions
Homogeneous Reaction
A reaction which happens throughout the reacting phase.
Heterogeneous reaction
A reaction which happens near the boundary of a reacting phase.
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Variations In RateWith Conditions
Rates vary with: Concentrations of all species
(reactants, products, inerts) (factors of 2-100)
Temperature (factors of 100 or more) The presence of solvents (factors of
1012 or more) The presence of catalysts (factors of
1012 or more)
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General Effect Of Concentration
Rate for homogeneous reactions goes up with concentration
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1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+51E-8
1E-6
1E-4
1E-2
1E+0
1E+2
Pressure, Torr
Rat
e, T
orr/
sec
Slope=2
Slope=1
Figure 2.3 The rate of CH3NC isomerization to CH3CN as a function of
the CH3CN pressure
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Rate Of Heterogeneous Reactions Often Shows A
Maximum
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1012
1013
10-6
Ra
te, M
ole
cule
s/cm
-se
c2
1011
10-7
10-8
10-6
CO pressure, torr10
-710
-8
O pressure, torr2
390 K
410 K415 K
450 K450 K
440 K440 K
425 K
Figure 2.15 The influence of the CO pressure on the rate of CO oxidation on Rh(111). Data of Schwartz, Schmidt, and Fisher.
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Rate Equations
Rate as a function of the concentration of all of the species in the reactor
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Typical Rate Laws For Simple A C Reactions:
rA = -k (CA) First order
rA = -k (CA)2 Second order
rA = -k (CA)3 Third order
rA = -k (CA)n nth order
n is the orderk is the rate constant
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Rate Equations ForA + B C
rA = -k (CA)n(CB)m
nth order in A, mth order in Boverall (m+n)th order
rA = -k(CA)(CB)2
First order in A, second order in B, third
order overall.
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(2.13)
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Notation
k1, k2 rate constants
K1, K2 equilibrium constants
CA=[A] concentration of species A
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Discussion Problem 1
What is the order of reaction?
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CA
Moles/Liter
rateMoles/Lit/
Min
CA
Moles/Liter
rateMoles/Lit/
Min
0.25 0.13 1 0.5
0.5 0.25 2 1.0
Answer
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Discussion Problem 2
What is the order of the reaction?
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CA
moles/liter
CB
moles/liter
ratemoles/
liter-min
CA
moles/liter
CB
moles/liter
ratemoles/
liter-min
1 0.25 0.031 0.25 1 0.13
1 0.5 0.13 0.5 1 0.25
1 1 .5 1 1 0.5
1 2 2.0 2 1 1.0
Answer
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More Complex Rate Equations
• Very few real reactions have simple reaction orders over a wide range of conditions:
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[Sugar])K(1
Sugar E..ColiKkr
2
21ecoli
(2.18)
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More Complex Rate Equations
CH3NC CH3CN
No industrially important reaction is first order over a wide range of
conditions. 25
1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+51E-8
1E-6
1E-4
1E-2
1E+0
1E+2
Pressure, Torr
Rat
e, T
orr/
sec
Slope=2
Slope=1
Figure 2.3 The rate of CH3NC isomerization to CH3CN as a function of
the CH3CN pressure
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Rate Equation Not Simply Related To Stoichiometry
]Br[[HBr]
K + 1
]Br][H[k = r
2
2
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HBr
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HBr2BrH 22
(2.21)
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ChBE424 Assumed First Or Second Order Reactions
Why does this work? Usually reactors run under a limited
range of conditions – fitting rate to a line or parabola often good enough
Rate changes with temperature or catalysts much larger than changes with concentration.
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Not All Reactions Have Rate Equations
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500 700 900300
1012
1014
Rat
e, M
olec
ule
s/cm
-se
c2
1011
1013
Temperature, K
Figure 2.22 2CO+O22 CO2
On ruthenium
0 100 200 300 400 500 6000
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time, seconds
Con
cent
ratio
n
Figure 2.23 Belousov-Zhabotinskii Reaction
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Summary For Today
Table 2.1 Summary of key definitions
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Stoichiometric coefficient
The amount of product produced when the reaction goes once. The stoichiometric coefficient is positive for a product and negative for a reactant.
rA1 The net rate of production of a species A. rA is positive for a product and negative for a reactant.
Rate of reaction 1, r1 for any species participating in reaction1.
Homogeneous reaction
A reaction which happens throughout the reacting phase.
Heterogeneous reaction
A reaction which happens near the boundary of a reacting phase.
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Summary For Today
Table 2.4 The key definitions from Section 2.3.
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Rate equation The rate as a function of the concentration of the reactants.
Order The exponent n in the rate equation.
First order reaction A reaction whose rate is proportional to the reactant concentration to the first power (i.e. n = 1 in eqn. (2.11)).
Second order reaction A reaction whose rate is proportional to the reactant concentration to the second order.
Overall order The sum of the orders for each of the reactants.
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Summary For Today
Real reactions rarely follow simple rate laws over wide ranges of conditions. Simple rate laws OK if the range of
conditions small Some reactions do not have a rate
law.
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Class Question
What did you learn new today?
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