SELECTED PRINCIPLES: EQUILIBRIA (2) In economic terms, it is essential to achieve - in a safe environment - the maximum yield in any given industrial process in the minimum time. Not surprisingly, therefore, the variables which affect the yield and the rate of a reversible reaction are important considerations in plant design; and, for example, certainly would have been in the optimization of one industrial method of manufacturing ethanoic acid, which involves the catalyzed, gas-phase reaction between methanol and carbon monoxide.
[Scene. A tea-party in the plush boardroom of Hydragyrum Chapelier Vinaigrette (a small company which, most curiously, manufactures hats and ethanoic acid); the chairman's name is Monsieur Oliver Scrooge.] O. Scrooge: We cannot rest on our laurels of the past. So, I want more, and I want it faster! [The board members ignore this party pooper.] Otherwise, ... each of you will be collecting your Christmas present from the Government. [With this veiled threat, Les Femmes are all attention.] La Vitesse: Well, ... a catalyst would increase the rate. However, it is a compound of a precious metal: ... so there would be a major capital cost. Nevertheless, we can re-use the catalyst ... providing, of course, we minimize the introduction of inhibitors into the reaction vessels. O. Scrooge: Will it increase the yield? [His tone is hopeful.] La Vitesse: Certainly not! [S. looks glum.] No catalyst changes the position of equilibrium. On the other hand, without a catalyst, we would definitely need to use much higher temperatures: ... so, indirectly, it would reduce costs. La Chaleur: True, though a high temperature would increase the rate. O. Scrooge: Yes, ... but will a high temperature increase the yield? La Chaleur: Certainly not! Do pay attention! The reaction is exothermic. [S. looks chastened and even more glum.] La Pression: A high pressure would also increase the rate ... though, again, with increased operating costs. O. Scrooge: Yes, yes, ... [His tone is weary.] ... but will a high pressure increase the yield? La Pression: Certainly! [S. perks up.] To echo La Chaleur, do pay attention! Note that there are fewer moles of gas on the product side of the equation ... well, none in fact. O. Scrooge: How absolutely splendid! I suggest that we should be miserly with the catalyst, use a lowish temperature, and boost the pressure massively. [His tone is bullish.] La Securité: No, ... not necessarily. I must urge caution. Reaction vessels strong enough to withstand very high pressures are exceedingly expensive. The safety of our workers and the general public is paramount! O. Scrooge: Oh dear, are all females of the species this sensible? [His tone is ambiguous, perhaps even patronizing.] La Securité: Certainly: ... well, most of the time. However, we will suspend judgement on your future, ... [Les Femmes glance pointedly at the sharp stiletto heels on their shoes.] ... until after we have received our Christmas bonuses. O. Scrooge: [He picks up a telephone.] Bob, is that you? Please come back ... |
1. One (no longer used) method of obtaining hydrogen chloride involved the copper(II)-catalyzed reduction of chlorine;
(a) Complete the Table below using these bond energies (in kJ mol-¹): 463 (O-H), 243 (Cl-Cl), 432 (H-Cl), and 497 (O=O).
Bonds broken |
Energy absorbed / kJ mol-¹ |
Bonds formed |
Energy released / kJ mol-¹ |
4 O-H |
1852 |
4 H-Cl |
1728 |
2 Cl-Cl |
486 |
1 O=O |
497 |
Total = 2338 |
Total = 2225 |
[3] Calculate the heat energy change (DH) for the above reaction. Energy absorbed - energy released = 2338 - 2225 = DH +113 kJ mol-¹ [2] (b) Explain the purpose of the broken brick. It increases the surface area of the Cu(II)-catalyst, so allowing more particles to collide. [2] (c) Complete and label this energy level diagram for the reaction.
[5] (d) Explain the effects, on the rate of this reaction and the yield of hydrogen chloride, of using: A high pressure The rate would be faster, because there would be more collisions between the particles, but the yield would be lower, because the equilibrium position would move to the left to oppose the increase in pressure. [4] A high temperature The rate would be faster, because more particles would have the required activation energy for successful collisions, and the yield would be higher, because the equilibrium position would move to oppose the temperature decrease of this endothermic reaction. [4] 2. In industry, hydrogen chloride is usually obtained as a co-product of the manufacture of chlorinated hydrocarbons; e.g.,
(a) Suggest the rôle of light in this (substitution) reaction. To provide the particles with the required activation energy. [1] (b) State briefly why the use of a high pressure in this reaction would not increase the yield of products. There is no change in the number of moles of gases. (... so no effect on the position of equilibrium) [1] (c) Suggest one advantage that this industrial method has over that described in 1. Lower energy requirements / No wasted by-products [1]
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