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]

Dr. R. Peters Next Contents' List & Teacher's Notes