Lecture 7
1
Moustafa M
Dr.Hager By
2
? Questions first
What unit ok K
3
We first consider consecutive unimolecular type first- order reactions such as
The rate equations for the three components are Irreversible Reactions in Series.
[44]
[46]
[45]
Start with a concentration CA0of A
To find the changing concentration of R, substitute the concentration of A from Eq. 47 into the differential equation governing the rate of change of R, Eq. 45;
thus
[47]
[48]
• which is a first-order linear differential equation of the form
By multiplying through with the integrating factor the solution is
Applying this general procedure to the integration of Eq.
48, we find that the integrating factor is ek2t. The constant of integration is found to be –k1CAo/(k, - k,) from the initial conditions CR0= 0 at t = 0, and the final expression for the changing concentration of R is
[49]
Noting that there is no change in total number of moles, the stoichiometry relates the concentrations of reacting components by
• which with Eqs. 47 and 49 gives
[50]
• Now if k2is much larger than k1, Eq. 50 reduces to
Values of k1, and k2also govern the location and maximum concentration of R. This may be found by differentiating Eq. 49 and setting dCR/dt = 0. The time at which the maximum concentration of R occurs is thus
• Now if k1is much larger than k2, Eq. 50 reduces to
[51]
The maximum concentration of R is found by combining Eqs. 49 and 51 to give
[52]
Starting with a concentration ratio M=CR0/CA0 the rate equation is
First-Order Reversible Reactions
[53a]
[53b]
Now at equilibrium dCA/dt = 0. Hence from Eq. 53 we find the fractional conversion of A at equilibrium conditions to be
and the equilibrium constant to be
• Combining the above three equations we obtain, in terms of the equilibrium conversion,
With conversions measured in terms of XAe, this may be looked on as a pseudo first-order irreversible reaction which on integration gives
[54]
• A plot of -In(1 - XA/XAe) vs. t, as shown in Fig. 3.13, gives a straight line.
• The similarity between equations for the first-order irreversible and reversible reactions can be seen by comparing Eq. 12 with Eq. 54 or by comparing Fig. 3.1 with Fig. 3.13. Thus, the irreversible reaction is simply the special case of the reversible reaction in which
For the bimolecular-type second-order reactions
Second-Order Reversible Reactions
[55a]
[55b]
[55c]
[55d]
with the restrictions that CA0= CB0 and CR0= Cso= 0, the integrated rate equations for A and B are all identical, as follows
[56]
A plot as shown in Fig. 3.14 can then be used to test the adequacy of these kinetics
If Eq. 54 or 56 is not able to fit the data, then the search for an adequate rate equation is best done by the differential method.
Reversible Reactions in General
In searching for a kinetic equation it may be found that the data are well fitted by one reaction order at high concentrations but by another order at low concentrations.
Consider the reaction Reactions of Shifting Order
[57]
At high CA the reaction is of zero order with rate constant k1/k2
At low CA the reaction is of first order with
Integrating Eq. 57:
[58a]
[58b]
or
• Two ways to test this rate form are then shown in Fig.
3.16
By similar reasoning to the above we can show that the general rate form
[59]
Another form which can account for this shift is
[60]
Reactant A decomposes in a batch reactor
The composition of A in the reactor is measured at various times with results shown in the following columns 1 and 2.
Find a rate equation to represent the data.
Guess First-Order Kinetics.
SOLUTION
Guess Second-Order Kinetics.
Guess n
th-Order Kinetics.
Let's plan to use the fractional life method with F = 80%.
Then Eq. 33b becomes
Next take logarithms
Plot the CAvs. t data, and then by eye carefully draw a smooth curve to represent the data. This curve most likely will not pass through all the experimental points.
Determine the slope of this curve at suitably selected concentration values. These slopes dCA/dt = rAare the rates of reaction at these compositions.
Now search for a rate expression to represent this rAvs. CA
data, either by
(a) picking and testing a particular rate form, -rA= kf (CA), see Fig. 17, or
(b) testing an nth-order form :by taking Differential Method of Analysis of Data
The procedure is as follows.
With certain simpler rate equations, however, mathematical manipulation may be able to yield an expression suitable for graphical testing. As an example, consider a set of C, vs.
t data to which we want to fit the M-M equation
[57]
• By the differential method we can obtain -rAvs. CA. [61]
Alternatively, a different manipulation (multiply Eq.
61 by yields another form, also suitable for testing, thus
[62]
Lecture 8
35
Try to fit an nth-order rate equation to the concentration vs. time data of Example 3.1.
take logarithms of both sides (see columns 3 and 4), or
Varying-Volume batch reactormuch more complex than the simple constant-volume batch reactor.
Their main use would be in the microprocessing field where a capillary tube with a movable bead would represent the reactor (see Fig. 3.20).
Varying-Volume Batch Reactor
[63a]
[63b]
where eAis the fractional change in volume of the system between no conversion and complete conversion of reactant A. Thus
[64]
Noting that
[65]
we have, on combining with Eq. 63,
Thus
[66]
The rate of reaction (disappearance of component A), is, in general
Replacing V from Eq. 63aand NA from Eq. 65we end up with the rate in terms of the conversion
or in terms of volume, from Eqs. 63
[67]
The procedure for differential analysis of isothermal varying volume data is the same as for the constant- volume situation except that we replace
Differential Method of Analysis
[68]
This means plot In V vs. t and take slopes.
So far we have examined the effect of concentration of reactants and products on the rate of reaction, all at a given temperature level.
To obtain the complete rate equation, we also need to know the role of temperature on reaction rate. Now in a typical rate equation we have
Temperature and Reaction Rate
Chemical theory predicts that the rate constant should be temperature-dependent in the following manner:
However, since the exponential term is much more temperature-sensitive than the power term, we can reasonably consider the rate constants to vary approximately as e-E/RT.
[74]
Experimental studies of a specific decomposition of A in a batch reactor using pressure units show exactly the same rate at two different temperatures:
(a) Evaluate the activation using these units (b) Transform the rate expressions into concentration
units and then evaluate the activation energy.
The pressure is not excessive, so the ideal gas law can be used.
