25: the Catalytic Oxidation of Ammonia Using Chromium III Oxide s2

28: Dehydration of sucrose and a version of ‘pharaoh’s serpent’.

Demonstration 1: This can be a spectacular demonstration and is often used to show the very high affinity of concentrated sulfuric acid for water in that it can remove the ‘elements of water’ from a carbohydrate such as sucrose (table sugar).

C12H22O11 - 11H2O 12C

However, it seems that the reaction is not quite that direct. If concentrated sulfuric acid is added to a layer of sugar in a Pyrex beaker the sugar turns brown but usually there is no violent reaction. To get a spectacular column of aerated carbon particles that often reaches beyond the top of the beaker it seems necessary to first add water to the sugar and then pour in the sulfuric acid slowly. It seems that the high temperature generated by the reaction between the acid and the water is sufficient to cause the following reaction to occur:

C12H22O11 11H2O + 12C

This is, of course EXACTLY equivalent to the first equation, but this reaction is itself slightly exothermic, and does not occur readily with dry sugar and concentrated sulfuric acid.

Cover the base of a Pyrex beaker (250cm3) with about 1cm depth of granular table sugar. Pour in sufficient distilled water to just reach the top surface of the sugar granules. Taking great care gently pour in concentrated sulfuric acid until the liquid begins to boil. The boiling usually spreads to cover the whole surface of the sugar and a black foam of finely divided carbon rises in the beaker often reaching well beyond the rim of the beaker.

The reaction is accompanied by the smell of toffee/caramel although tainted with sulfur dioxide. It should be carried out on a fume cupboard. (See final section for safety advice.)

Demonstration 2: A second method of dehydrating sucrose that does not use concentrated sulfuric acid is to use an oxidising agent with a substantial excess of sugar. The oxidation of some of the sucrose produces sufficient heat to raise the temperature such that dehydration of the remaining sucrose will occur. In some ways this reaction which results in an extended mass of very finely divided carbon resembles a classical demonstration of the pharaoh’s serpent. (A ‘serpent-like’ mass of resulting from the burning / decomposition of a pellet – the serpent’s egg – made from mercury II thiocyanate – this reaction uses and produces highly toxic materials and could not be recommended for use nowadays.) There are many recipes to be found on the internet to make a so-called ‘black snake’ or serpent and that given below is based on a suggestion from the University of Minnesota (www.chem.umn.edu/services/lecturedemo/info/sugar_dehydration.html) (accessed June 2011). The heat generated by the oxidation of a small proportion (about 16%) of the sugar is sufficient to provide the activation energy for the dehydration of the rest – as shown by the equation in the previous section.

· Carefully cut the nozzle end from a disposable plastic syringe (Somewhere between 25 and 60mL seems to be a suitable size) to create a suitable mould to form a ‘serpent’s egg’

· Grind separately in a pestle and mortar 15g of table sugar (sucrose) and 4g of potassium chlorate. Carefully mix these two powders intimately – being aware that this is a potentially explosive mixture so mixing with a wooden spatula is preferred to even light grinding in a mortar with a pestle.

· Pull back the plunger of the syringe to about the 20mL mark and add sufficient of the powder mixture to produce a layer about 1cm in depth. Add sufficient ethanol (methylated spirits) to the syringe to thoroughly wet the powder and tap the plunger lightly to remove air bubbles.

· Repeat the addition of powder and methylated spirits until the powder is used.

· Cover the open end of the syringe with a piece of scrap metal or the base of a metal dish and depress the plunger firmly on to the metal to compress the ‘plug’ of powder soaked in methylated spirits.

· Very carefully invert the syringe on to the surface of a metal tray on which the reaction can occur (ensure the surface under the tray is sufficiently protected from the considerable heat generated when the reaction occurs) and slowly push the plug out onto the surface. The smaller the diameter of the syringe the taller will be the ‘egg’ – and the more care needs to be taken to avoid the ‘egg’ breaking up as it is ‘laid’!

· Ensure there is no inflammable material within 1m and ignite the plug with a match or lighted taper. Initially the methylated spirits burn quietly before igniting the reaction mixture. As the black foam emerges it looks vaguely living (from a horror movie) and some spitting may occur.

· Wait until the resulting mass of carbon (with KCl) and metal tray have cooled before attempting to clear up.

Education issues:

Usually done to show the strong affinity between sulfuric and water although as discussed above the connection is perhaps not quite as direct as is often implied.

The second reaction seems to demonstrate pyrotechnic applications rather than direct chemical principles although a creative teacher may find a number of reasons to show the demonstration. It would certainly provide an interesting event at a science fair or a parents’ evening.

Risk assessment: (You MUST make your own)

Demonstration 1: Concentrated sulfuric acid is a dangerous liquid that is a strong oxidising agent (it produces sulfur dioxide when it acts this way) and reacts violently with water to form a strongly acidic solution. Eye protection must be worn and care taken not to get the acid on the skin. (Should this happen, wipe off as much of the acid ac possible with a dry cloth – any piece of clothing – and rinse immediately with large amounts of water. (It is important not to dilute large amounts of the acid whilst in contact with skin since temperatures well over 150oC can be generated and cause additional serious burns. For this reason and substantial amount of concentrated acid in the eye or mouth would be very serious and require medical attention.)

The beaker should be placed inside a larger beaker or on an acid-proof tray to contain any spillage should the beaker break due to the thermal shock. (This should not happen but may do if the beaker has and deep scratches or small cracks.)

The remaining mass of carbon remains very hot for some time – it also contains substantial amounts of sulfuric acid. When cold it can be placed under a running water tap when the diluted acid and much of the carbon powder is flushed down the sink. It may be difficult to remove all the carbon residue from the beaker in which case it may be useful to dry the beaker and reserve it for any repeat runs of this demonstration.

Demonstration 2: The main potential hazard has already been mentioned since vigorous mixing of the mixture can conceivably cause the reaction to begin. It is very unlikely to happen (and I have never managed to start the reaction this way even when I have tried.) It is nevertheless wise to take precautions.

The reaction produces flame and sparks and eye protection and a safety screen should be used. There are no hazardous by-products and the reaction can be carried out in a well ventilated room. Wait until all has cooled sufficiently before attempting to move the tray or clear up.