The Decomposition of Acetaldehyde

The main reaction occurring when acetaldehyde in the gas phase1 is heated at 300-800 °C is given in Eq. (8-1). Trace quantities of C2H6 and H2 are also detected. To explain the -order rate law, consider this scheme  

In this scheme, CHO appears irrelevant we return to it later. The rate law can be derived by making the steady-state approximation for each of the chain-carrying radical intermediates  

We return briefly to the formyl radical of Eq. (8-5), a by-product of the initiation reaction. The following sequence is believed to constitute a chain process that couples with the other sequence [Eqs. (8-5)—(8-8)]  

(8-17), M represents a so-called third body. In gas phase reactions of atoms, M plays an essential role in conserving energy. The bulk molecules (reactant, products, added inert gases) play this role. (No third body need be involved in solution reactions, however, owing to the presence of the solvent.) 

In this reaction, H2 is a minor product, which is accounted for by Eq. (8-18). It amounts to less than 1 percent of CH4. This observation, which is implied by the stoichiometry of Eq. (8-1), is consistent with the chain length. 

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Once the order of the reaction is known, the rate constant is readily calculated. Consider, for example, the decomposition of acetaldehyde, where we have shown that the rate expression is... 

Having established the value of k and the reaction order, the rate is readily calculated at any concentration. Again, using the decomposition of acetaldehyde as an example, we have established that... 

The decomposition of acetaldehyde has Eq. (8-6) as the rate-controlling step, this being the one (aside from initiation and termination) whose rate constant appears in the rate law. In the sequence of reactions (8-20)—(8-23), the same reasoning leads us to conclude that the reaction between ROO and RM, Eq. (8-22), is rate-controlling. Interestingly, when Cu2+ is added as an inhibitor, rate control switches to the other propagating reaction, that between R and O2, in Eq. (8-21). The reason, of course, is that Cu2+ greatly lowers [R ] by virtue of the new termination step of reaction (8-30). 

Circle diagrams. Draw circle diagrams for (a) the decomposition of acetaldehyde and (b) the reaction of the organometal RM with 02. 

This agrees with experimental findings on the decomposition of acetaldehyde. The appearance of the three-halves power is a wondrous result of the quasisteady hypothesis. Half-integer kinetics are typical of free-radical systems. Example 2.6 describes a free-radical reaction with an apparent order of one-half, one, or three-halves depending on the termination mechanism. 

From the mechanism given in problem 7-8 for the decomposition of acetaldehyde, derive a rate law or set of independent rate laws, as appropriate, if H2 and C2Hs are major products (in addition to CH4 and CO). 

The decomposition of acetaldehyde was studied at 518°C with an initial pressure tt0 = 363 Torr (Hinshelwood Hutchinson, Proc Roy Soc 111A 380, 1926). The data are of time in sec and change in total pressure, An Torr. Verify that the reaction is second order. 

The decomposition of acetaldehyde is found to be overall first-order with respect to the acetaldehyde and to have an overall activation energy of 60 kcal/mol. Assume the following hypothetical sequence to be the chain decomposition mechanism of acetaldehyde ... 

In words, we describe the process as initiated by the decomposition of acetaldehyde to form the methyl radical CH3 and the formyl radical CHO. Then methyl attacks the parent molecule acetaldehyde and abstracts an H atom to form methane and leave the acetyl radical CH3CO, which dissociates to form another methyl radical and CO. Finally, two methyl radicals combine to form the stable molecule ethane. 

Our first example of a chain reaction, the decomposition of acetaldehyde to methane and CO, is endothermic so the reactor tends to cool as reaction proceeds. However, the oxidation of H2 is exothermic by 57 kcal/molc of H2, and the oxidation of CH4 to CO2 and H2O is exothermic by 192 kcal/mole of CH4. Thus, as these reactions proceed, heat is released and the temperature tends to increase (strongly ). Thus thermal ignition is very important in most combustion processes. 

The decomposition of acetaldehyde and the union of hydrogen and iodine fit equally well into their places in this table. The decomposition of ozone, however, appears to have a greater value of E than would be expected from its velocity, and will be considered further in a subsequent section. 

These considerations do not, however, apply to examples such as the decomposition of acetaldehyde 2 CH3CHO = 2 CH4 + 2 CO, where the unimolecular change is under no disadvantage on purely thermochemical grounds. 

It may also be mentioned here that in specific molecular actions a particularly marked influence of like molecules upon one another is often to be observed. This is encountered in various ways in spectroscopy, in the extinction of the polarization of mercury resonance radiation with increasing vapour pressure, in the damping of fluorescence in concentrated solutions, and in various chemical reactions. As an example of the latter the decomposition of acetaldehyde (p. 70) may be quoted, where collisions between two molecules of the aldehyde are much more effective than collisions of aldehyde molecules with those of other gases. 

For the decomposition of acetaldehyde at 518 C, CH3CH0 => CH4 + CO, data of half and three-quarter times were obtained for several initial pressures P0, torr, as tabulated. Find the order of the reaction. 

Substituting Equation 1-154 into the predicted rate law allows the temperature dependence of the overall reaction to be predicted. In the decomposition of acetaldehyde, it follows that... 

The decomposition of acetaldehyde (CH3CHO) to methane and carbon monoxide is an example of a free radical chain reaction. The overall reaction is believed to occur through the following sequence of steps ... 

Free radical species are indicated by the symbol. In this case, the free radical CHO that is formed in the first reaction is kinetically insignificant. Derive a valid rate expression for the decomposition of acetaldehyde. State all of the assumptions that you use in your solution. 

We said that if n = 1, the reaction was first-order with respect to A if n = 2, the reaction was second-order with inspect to A and so on. However, a large number of homogeneous reactions involve the formation and subsequent reaction of an intermediate species. When this is the case it is not uncommon to find a reaction order that is not an integer. For example, the rate law for the decomposition of acetaldehyde,... 

The decomposition of acetaldehyde, sensitized by biacetyl, was studied at 499 °C by Rice and Walters , and between 410 and 490 °C by Boyer et al. They found the initial rate to be proportional to the square root of the biacetyl concentration and to the first power of the aldehyde concentration. The chains are initiated by the radicals originating from the decomposition of the biacetyl molecule. The decomposition of acetaldehyde can be induced also by di-r-butyl peroxide (at 150-210 °C, about 10-50 molecules decompose per peroxide molecule added), as well as by ethylene oxide (around 450 °C each added ethylene oxide molecule brings about the decomposition of up to 300 acetaldehyde molecules). For the influence of added diethylether, vinyl ethyl ether, ethyl bromide, and ethyl iodide etc., see Steacie °. 

Iodine accelerates the decomposition of acetaldehyde In the steady-state range, the order is approximately 1.0 and 0.5 in aldehyde and iodine, respectively. The experimental results of Rollefson and Faull have been reinterpreted and added to by O Neal and Benson . The iodine-catalysed reaction is a free radical chain process initiated by the attack of an iodine atom on the acetaldehyde molecule. The proposed mechanism fits the experimental data very well. The thermal decomposition of acetaldehyde is catalysed also by other halogens and halogen compounds . 

Setser carried out rrkm quantum statistical calculations for the decomposition of acetaldehyde molecule into free radicals. He examined three models out of which two were of the loose type ones, fitting essentially the Gorin model , while the third one was a tighter complex. The calculations definitely show that the decomposition is in the pressure-dependent region even around 100 torr. 

The order of a reaction does not have to be an integer fractional powers are sometimes found. At 450 K, the decomposition of acetaldehyde (CH3CHO) is described by the rate expression... 

The free energy changes for the reforming of acetaldehyde, ethylene, and CH4, which could be formed as intermediates during the SRE reactions as discussed above, are shown in Figure 2.25. As can be seen, the decomposition of acetaldehyde to CH4 and CO (line 2 Eq. 2.87 is favorable at room temperature, while a temperature of above 250 °C is required for the steam reforming of acetaldehyde (lines 3 and 5 Eqs. 2.88 and 2.89) and steam reforming of ethylene (line 4 Eq. 2.90). On... 

As an example of the use of concentration measures other than that of moles per unit volume consider the gas reaction A —> B + C, say the decomposition of acetaldehyde vapor into methane and carbon monoxide. If the reaction proceeds at constant volume and temperature, it may be followed by the increase in pressure. Suppose TVo moles of A are introduced into a volume V at temperature T so that the initial pressure is Pq = MRTyr. When the gross extent of reaction is X there will be (M + X) total moles present and the pressure will be P = (TVo + X)Rr/ V, Hence y = (P — Pq) V/ RP. If the reaction is second order and so proportional to the square of the concentration (Aq — X)/ K, we have... 

Example 4-4 A kinetic study is made of the decomposition of acetaldehyde at 518°C and 1 atm pressure in a flow apparatus. The reaction is... 

The dehydrogenation of ethanol over copper catalysts is not complete at 300° C. when moderate times of contact are used but if the temperature is raised to 350° C. or higher, secondary reactions become more and more evident. At temperatures above 350° C., copper catalysts begin to activate the decomposition of acetaldehyde to methane and carbon monoxide, to induce polymerization of the aldehyde, to cause dehydration processes to set in, to cause hydrogenation of the ethylene, and, in general, to promote secondary decompositions and condensations which complicate the product and destroy the activity of the catalyst. Hence, for the production of aldehydes and ketones it is desirable to use moderate temperatures of about 300° C. and to obtain maximum yields from the decomposition rather than maximum decomposition of alcohol per pass over the catalyst. 

Figure 2 shows the effect of W/F on the composition of products at 623 K. Ethane and acetaldehyde were produced at the lowest W/F. In the higher W/F range, methane and carbon monoxide contents were increased with increasing W/F These facts suggest that ethane and acetaldehyde are the initial products, while methane and carbon monoxide are the secondary products. Methane and carbon monoxide may be formed by the decomposition of acetaldehyde on the catalyst surface. 

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