SELECTED PRINCIPLES: RATES OF REACTIONS (2) Until relatively recently, catalysts had acquired the status of being merely 'wizard's potions', with a particular potion increasing the rate of a certain chemical reaction by lowering its activation energy. However, motivated partly by the recognition that 'Nature has designed' enzymes which are more efficient than the catalysts discovered by Man, and partly by industry's demands for more efficient catalysts (so as to reduce energy costs), researchers have begun to unravel the details of catalysis. Most unfortunately, even a superficial understanding of this research requires a fairly advanced knowledge of atomic structure and chemical bonding; on the other hand, even a 'bird's-eye-view' of one reaction can provide a useful point of reference ...
Aqueous dibromine is but one of several substances known to catalyze the decomposition of aqueous hydrogen peroxide; i.e.,
The experimental evidence is consistent with the interpretation that this reaction occurs essentially in two steps. In the first, dibromine oxidizes hydrogen peroxide to aqueous hydrogen ions and dioxygen; i.e.,
Then, in the second, the bromide ions are oxidized back to dibromine by more hydrogen peroxide; i.e.,
Thus, dibromine reacts in the first step and is then regenerated in the second; so, the substance is not used up in the reaction.
Each catalyst increases the rate of a reaction without being chemically changed at the end of a reaction. Accordingly, each reaction involving a true catalyst can always be represented by a closed loop; the various catalytic species form the main body of the loop, and the reactants and products enter and leave the loop at various places.
Shown below is a closed loop for the dibromine-catalyzed decomposition of aqueous hydrogen peroxide.
1. Nitrogen-fixation is the term used to describe the conversion of molecular dinitrogen into compounds of nitrogen. (a) Industrial fixation involves the reduction of dinitrogen to ammonia using an iron catalyst, high temperatures, and high pressures; e.g.,
Complete and label this energy level diagram for the above reaction.
By reference to the Periodic Table, name two elements which might show similar catalytic activity to iron. ___________________________________  One mole of any gas occupies a volume of 24000 cm³ at room temperature (25°C) and pressure (0.1 MPa). Calculate the density of ammonia at r.t.p. ________________________________________________________________ _______________________________________________________________________  (b) Biological fixation involves the Mo-containing enzyme nitrogenase, under ambient conditions, in an ostensibly similar reduction; one of the important reactions can be loosely summarized by this equation:
Suggest and explain the reason why this reaction is considerably more exothermic than that which occurs in industrial fixation. _____________ _______________________________________________________________________ _______________________________________________________________________  (c) Atmospheric fixation results in the formation of various nitrogen oxides. Name two sources of energy involved in this type of nitrogen- fixation. _____________________________________________________________  2. Carbon-fixation is the term used to describe the incorporation of carbon dioxide into organic compounds; as occurs, for example, during the biological process of photosynthesis:
The kingdom Monera consists of three subkingdoms: Eubacteria (formerly Bacteria; e.g., Bacillus anthracis, Clostridium botulinum, Escherichia coli, Treponema pallidum, and Vibrio cholerae); Cyanobacteria (formerly Blue-green Algae; e.g., Nostoc commune); and Archaebacteria (e.g., Methanococcus jannaschii). The most nutritionally independent organisms in the biosphere are species of Cyanobacteria which can carry out both nitrogen-fixation and carbon-fixation (via photosynthesis). State what these organisms require for survival, apart from a suitable temperature and sunlight. _________________________________________________________ 
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