ChE 452 Lecture 02
Basic Concepts
1
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
2
3311 ,
Example
2CO + O2 2CO2
CO + ½ O2 CO2
3
Answer
Answer
Reaction Rate
Priestley Total Moles/hr
Van’t Hoff Moles/hr/volume
6
Priestley’s Experiment
7
Changing the amount of powder did not change the rate of weight loss
Van’t Hoff’s Experiments
8
2N2O5 4NO2 + O2
Reaction rate scaled as the volume of the reactor
Special poisoned walls
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
9
A
r
r
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
10
AR
The Rate Of A Reaction
11
r riA
A1
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.
12
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)
13
General Effect Of Concentration
Rate for homogeneous reactions goes up with concentration
14
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
Rate Of Heterogeneous Reactions Often Shows A
Maximum
15
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.
Rate Equations
Rate as a function of the concentration of all of the species in the reactor
16
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
17
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.
18
(2.13)
Notation
k1, k2 rate constants
K1, K2 equilibrium constants
CA=[A] concentration of species A
19
Discussion Problem 1
What is the order of reaction?
20
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
Discussion Problem 2
What is the order of the reaction?
22
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
More Complex Rate Equations
• Very few real reactions have simple reaction orders over a wide range of conditions:
24
[Sugar])K(1
Sugar E..ColiKkr
2
21ecoli
(2.18)
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
Rate Equation Not Simply Related To Stoichiometry
]Br[[HBr]
K + 1
]Br][H[k = r
2
2
221
HBr
26
HBr2BrH 22
(2.21)
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.
27
Not All Reactions Have Rate Equations
28
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
Summary For Today
Table 2.1 Summary of key definitions
29
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.
Summary For Today
Table 2.4 The key definitions from Section 2.3.
30
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.
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.
31
Class Question
What did you learn new today?
32
Top Related