Acid ! part 2

OK many of you will have seen the reaction of sulphuric acid with table sugar, it is a reaction which tends to make lots of steam, acid spray and a big black tar thing. I choose to do a related dehydration reaction recently. I choose to react a cheaper carbohydrate (bread) with sulphuric acid.

This reaction is a bit more gentle than the reaction of sugar, here is the film of it. I will explain in the next post what is happening.

Chemistry sets

I am a chemist who earns a living by doing chemistry but as a boy things were different. I used to do chemical experiments for fun. I was recently in a toy shop getting a toy tool kit for the two year old son of my wife’s best friend. I saw a chemistry set for teenagers, I was so fascinated by it that I purchased it.

While I did not expect it to have some of the “hardcore in your face” chemical experiments which I have done. I was still shocked at the experiments in the set. What shocked me was not the narrow choice of chemicals in the set, but the lack of chemistry in the experiments.

I think that it is quite right and proper to exclude some substances from a teenagers chemistry set. The dirty washing list of things which I have had dealings with as an adult but must be excluded from little Johnny’s chemical lab include

Mercury metal, cadmium salts, phenol, aniline, liquid ammonia, methyl iodide, americium dissolved in nitric acid, potassium fluoride, sodium borohydride, acetylene cylinders, benzene, lead(II) acetate, hydrazine, trimethylsilyl cyanide (a HCN equivalent), silica, hydrogen gas cylinders, mercury(II) chloride dissolved in acetone, hexanes, phosphorus pentasulfide, metal carbonyls, carbon monoxide, litres of sodium dried ether etc etc

While I am sure that many of the professional chemists who are reading this blog will instantly be able to think of plenty of interesting chemical experiments which need at least one of the above, I have to say that these chemicals which the public (and many modern school chemistry teachers) will have nightmares about might be needed to do some interesting experiments

BUT not all interesting experiments require “dirtywashing list” chemicals.

It is important to avoid the trap of the following inductive reasoning…

1. The synthesis of pinacol (2,3-dimethylbutane-2,3-diol) is an interesting chemical experiment

2. Some of the chemicals (benzene, mercury(II) chloride, acetone) required for the traditional synthesis of pinacol are too dangerous for the general public to have access to.

So all interesting chemical experiments need dangerous chemicals which the general public must not be allowed to buy. (Very bad reasoning)

What I would to point out is that plenty of interesting and fun chemical experiments exist which do not require nasty chemicals. For instance I recently extracted a pH indicator from red cabbage in my kitchen (using a microwave, tap water and a vegetable purchased from ICA) which I then used with a whiskey tumbler and a drinking straw to show that my exhaled breath contains a gas (carbon dioxide) which forms a mildly acidic aqueous solution while household vinegar is much more acidic.

But lets get back to the real topic of the lack of chemistry in the booklet which went with the chemistry set. One experiment which made my blood boil was one in which the teenaged chemist was told to heat copper(II) sulphate pentahydrate (blue solid) in a test tube to get a white solid which later in the booklet was combined with water to make a blue solid (hydrated copper sulphate) again.

What bothered me was that the booklet gave no explanation of why anhydrous copper sulphate is white and the hydrated form is blue.

For those of you who love crystallography the unit cell of copper(II) sulphate pentahydrate is 6.141 Å by 10.736 Å by 5.986 Å  (and the angles are 82.27 107.43 102.67) and I got the coordinates (Bacon, G.E. and Titterton, D.H, Zeitschrift fuer Kristallographie, 1975, 141, 330-341), after putting the data through my copy of ORTEP I was able to draw the groups which are bonded to one of the two copper atoms in the asymmetric unit. Each copper has four waters around it arranged in a flat square. The copper water oxygen distance is 1.98 Å. At a greater distance from the copper 2.39 Å is a sulphate oxygen, when the sulphate oxygens are combined with the water oxygens the copper has a distorted octahedral environment. It is this interactions of the oxygen atoms with the atomic orbitals (d orbitals) of the copper which gives copper(II) sulphate its blue colour.

For those of you at home with a copy of ORTEP then here is a xyz file.

23
XYZ file for : CopperSulfateVIPentahydrate in space group P -1
  H        1.05067       -0.06762       -2.39354
  H       -1.53736        0.20986       -2.05372
  H        1.12882        1.38683       -1.76026
  O        1.13438        0.44252       -1.55755
  O        0.97946        3.22395       -1.31232
  O       -1.63160        0.02719       -1.11226
  O       -1.22010        4.16086       -0.84019
  H       -2.21262       -0.72919       -1.00720
  O       -0.60933       -4.50639       -0.69182
  O        0.36532       -2.31153       -0.46428
  S       -0.01186        3.55041       -0.26845
 Cu        0.00000        0.00000        0.00000
  S        0.01186       -3.55041        0.26845
  O       -0.36532        2.31153        0.46428
  O        0.60933        4.50639        0.69182
  O        1.22010       -4.16086        0.84019
  H        2.21262        0.72919        1.00720
  O        1.63160       -0.02719        1.11226
  O       -0.97946       -3.22395        1.31232
  O       -1.13438       -0.44252        1.55755
  H       -1.12882       -1.38683        1.76026
  H        1.53736       -0.20986        2.05372
  H       -1.05067        0.06762        2.39354