UEMK 3153 Chemical Reaction Engineering IIDate: 14 & 21 Jun 2013Tutor-in-charge: Yap Yeow Hong

Tutorial No.1

Lesson 1: Non-elementary reactions

1. What does it mean by elementary reaction and what are the differences between elementary and non-elementary reactions?

Chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state

2. What do you think is the reason that the rate law derived based on the observation from experiment is different from the rate law derived based on Law of Mass Action?

Because the rate law derived based on observation from the experiment comprises a series of elementary steps. That is why Law of Mass Action based on elementary reaction is not applicable in this case.

3. What are the characteristics of active intermediates and why Pseudo-Steady-State Hypothesis is used in the search for reaction mechanisms for non-elementary reaction?

Active intermediates results from collision or interaction between molecules and serve as an intermediate between initial reactants to products. They are extremely short-lived (because of its high reactivity) and only present in low concentration.

Pseudo-Steady-State Hypothesis states that active intermediates are short-lived and present in low concentration and overall generation rate can be assumed to be zero. By assuming PSSH, the concentration of active intermediates derived from elementary steps can be replaced to obtain a rate law based on concentration that can be observed and compared against the experimental observation.

4. Based on Fogler Q 7-5: The rate law for formation of phosgene, COCl2 from chlorine, Cl2, and carbon monoxide, CO, has the rate law

Suggest a mechanism for this reaction that is consistent with this rate law. [Hint: Cl formed from the dissociation of Cl2 is one of the two active intermediates]Postulated mechanism:

Net rate of COCl2 produced (assuming elementary reaction)------------------------------------------------------------(1)

However, we find that the above rate equation depends on the concentration of active intermediates (COCl) which we cannot measure. Therefore we need to replace this.

COCl is an active intermediate, so we can derive a concentration of COClNet rate of COCl formation:----------------(2)

Here we obtained the concentration of COCl active intermediate.

Substitute (2) into (1):----------------------------(3)

Here we find that the equation is dependent on another active intermediate (Cl) and therefore we need to replace this active intermediate which we cannot measure.

Cl is also an active intermediate, we can derive a concentration of ClNet rate of Cl formation:

There are simply too many terms, so we combine the above equation with rCOCl from (2)----------------------------------------(4)

Here we obtained the concentration of Cl active intermediate

Substitute (4) into (3) to replace Cl active intermediate:

--------------(5)

Therefore we have postulated a reaction mechanism that is consistent with the experimental observation.

5. Based on Fogler Q7-7: One of the major reasons for engine oil degradation is the oxidation of the motor oil. To retard the degradation process, most oils contain an antioxidant. Without an inhibitor to oxidation process, the suggested mechanism at low temperatures is

where I2 is an initiator and RH is the hydrocarbon in the oil

When an antioxidant is added to retard the degradation at low temperatures, the following additional termination steps occur:

a) Derive a rate law for the degradation of the motor oil in the absence of an antioxidant at low temperaturesb) Derive a rate law for the degradation of the motor oil in the presence of an antioxidant for low temperatures.c) How would you answer to part (a) change if the radicals I were produced at a constant rate in the engine and then found their way into the oil?

a) Low temperature with absence of anti-oxidant

Pay attention that every step here is not reversible.

Rate law for degradation (consumed) of motor oil, i.e. RH in the red box. We can see that it is dependent on several active intermediates.

There are three active intermediates here: I, RO2, R

Compute for net rate of formation for the active intermediates:

b) Low temperature with presence of anti-oxidant

c) If the radicals are formed at a constant rate, then the differential equation for the concentration of radicals becomes:

It is at constant rate, and there is no reactant

Lesson 2: Heterogeneous Reactions

6. Based on Fogler 10-8: In 1981 the U.S. government put forth the following plan for automobile manufacturers to reduce emissions from automobiles over the next few years.

ComponentYear

198119932004

Hydrocarbons0.410.250.125

CO3.43.41.7

NO1.00.40.2

All values are in grams per mile. An automobile emitting 3.74 lb of CO and 0.37 lb of NO on a journey of 1000 miles would meet the current government requirements.

To remove oxides of nitrogen (assumed to be NO) from automobile exhaust, a scheme has been proposed that uses unburned carbon monoxide (CO) in the exhaust to reduce the NO over a solid catalyst, according to the reaction:

Experimental data for a particular solid catalyst indicate that the reaction rate can be well represented over a large range of temperatures by

where PN = gas-phase partial pressure of NOPC = gas-phase partial pressure of COk, K1, K2 = coefficients dependent only on temperature

a) Based on your experience with such systems, you are asked to propose an adsorption-surface reaction-desorption mechanism that will explain the experimentally observed kinetics.b) A certain engineer thinks that it would be desirable to operate with a very large stoichiometric excess of CO to minimize catalytic reactor volume. Do you agree or disagree? Explain.c) When this reaction is carried out over a supported Rh catalyst, the reaction mechanism is believed to be

When the ratio of PCO/PNO is small, the rate law that is consistent with the experimental data is

What are the conditions for which the rate law and mechanism are consistent?

a) Proposal of mechanism

This is an example of LHHW

b)

c) Since PCO appears in the denominator, the CO.S surface reaction should be rate limiting. PCO/PNO must be small. The mechanism must be a dual site mechanism (which it is).

7. Based on Fogler Q 10-5: Figure 1 illustrates the experimental data obtained from the dehydration of n-butyl alcohol (butanol) over an alumina-silica catalyst at 750 F in a modified differential reactor. The feed consisted of pure butanol.

a) Suggest a mechanism and rate-controlling step that is consistent with the experimental datab) Evaluate the rate law parametersc) At the point where the initial rate is a maximum, what is the fraction of vacant sites ? What is the fraction of occupied sites by both A and B?d) What generalization can you make from studying this problem?

Figure 1