Discovering chemistry
Discovering chemistry

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Discovering chemistry

3 Catalysts in action

Catalysts are widely used in the industrial manufacture of bulk commodity chemicals.

One example is in the preparation of nitric acid, a chemical which has many uses, but most is combined with ammonia to produce ammonium nitrate a widely used fertilizer.

Nitric acid, HNO3, is made on a huge industrial scale by the Ostwald process in which gaseous ammonia is converted first to NO and then to NO2 over a catalyst made from platinum metal. This reaction, where combination with oxygen is occurring is known as oxidation, and is demonstrated on the laboratory scale in the following video.

Download this video clip.Video player: The oxidation of ammonia.
The oxidation of ammonia.
Interactive feature not available in single page view (see it in standard view).
  • In the video, what evidence was there that a catalytic reaction (involving the spiral of platinum metal) was occurring?

  • The platinum wire was initially heated in a flame until red-hot. On lowering it into the flask of ammonia (through which oxygen was bubbling) it continued to glow red (accompanied by the occasional explosion!)

The NO2 is then dissolved in water to give a concentrated aqueous solution of the acid, and the NO produced in this step is recycled back into earlier stages.

Now attempt the following question.

  • The combination of sulfur dioxide with oxygen, and the decomposition of steam into hydrogen and oxygen are both reactions of great potential value. These reactions and their equilibrium constants at 427oC (700K) are as follows.

    multiline equation row 1 equation left hand side multiline equation line 1 two times normal cap s times normal cap o sub two times open normal g close plus normal cap o sub two of normal g equals right hand side two times normal cap s times normal cap o sub three postfix times open normal g close cap k equals 10 super six postfix times mol super negative one postfix times postfix times litre row 2 equation left hand side two times normal cap h sub two times normal cap o of normal g equals right hand side two times normal cap h sub two of normal g plus normal cap o sub two of normal g cap k equals 10 super negative 33 postfix times mol postfix times prefix times of litre super negative one
    • i.Write expressions for the equilibrium constants of the two reactions.
    • ii.When the two reactions are attempted at 700K, neither seems to occur. Which of the two might be ‘persuaded’ to proceed at this temperature, and what form might your persuasion take?
  • The equilibrium constant of the first reaction, K1, is given by

    equation left hand side cap k sub one equals right hand side open normal cap s times normal cap o sub three times open normal g close close squared divided by open normal cap s times normal cap o sub two times open normal g close close squared times open normal cap o sub two of normal g close

    That of the second,

    equation left hand side cap k sub two equals right hand side open normal cap h sub two of normal g close squared times open normal cap o sub two of normal g close divided by open normal cap h sub two times normal cap o of normal g close squared

    The data show that K2 is tiny: at equilibrium, the concentrations of the hydrogen and oxygen in the numerator (the top line of the fraction) are minute in comparison with the concentration of steam in the denominator (the bottom line of the fraction). So in a closed system at 700 K, significant amounts of hydrogen and oxygen will never be formed from steam.

    By contrast, K1 is large, so the equilibrium position at 700 K lies well over to the right of the equation, and conversion of sulfur dioxide and oxygen to sulfur trioxide is favourable. The fact that the reaction does not occur must be due to a slow rate of reaction. We may therefore be able to obtain sulfur trioxide in this way if we can find a suitable catalyst to speed up the reaction. A suitable catalyst is vanadium pentoxide, V2O5, and at 700 K, this reaction is the key step in the manufacture of sulfuric acid from sulfur, oxygen and water.

DC_1

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