The ammonia replaces water as a ligand to give hexaamminechromium(III) ions. That precipitate dissolves to some extent if you add an excess of ammonia (especially if it is concentrated). With a small amount of ammonia, hydrogen ions are pulled off the hexaaqua ion exactly as in the hydroxide ion case to give the same neutral complex. The ammonia acts as both a base and a ligand. Reactions of hexaaquachromium(III) ions with ammonia solution Note: You will find the reactions between hexaaqua ions and hydroxide ions discussed in detail if you follow this link. In the test-tube, the colour changes are: The precipitate redissolves because these ions are soluble in water. More hydrogen ions are removed to give ions like - and 3. The oxygens which were originally attached to the chromium are still attached in the neutral complex.īut the process doesn't stop there. Note: The colour coding is to show that this isn't a ligand exchange reaction. This is insoluble in water and a precipitate is formed. Once a hydrogen ion has been removed from three of the water molecules, you are left with a complex with no charge - a neutral complex. Hydroxide ions (from, say, sodium hydroxide solution) remove hydrogen ions from the water ligands attached to the chromium ion. Reactions of hexaaquachromium(III) ions with hydroxide ions Note: You will find an extensive discussion of ligand exchange reactions if you follow this link. Once again, notice that replacing water molecules by chloride ions changes the charge on the ion. This happens when two of the water molecules are replaced by chloride ions to give the tetraaquadichlorochromium(III) ion - +. In the presence of chloride ions (for example with chromium(III) chloride), the most commonly observed colour is green. Replacement of the water by chloride ions Two of the positive charges are cancelled by the presence of the two negative charges on the sulphate ion. Notice the change in the charge on the ion. One of the water molecules is replaced by a sulphate ion. You can do this simply by warming some chromium(III) sulphate solution. Replacement of the water by sulphate ions What happens is that one or more of the ligand water molecules get replaced by a negative ion in the solution - typically sulphate or chloride. We nearly always describe the green ion as being Cr 3+ (aq) - implying the hexaaquachromium(III) ion. However, when it is produced during a reaction in a test tube, it is often green. The hexaaquachromium(III) ion is a "difficult to describe" violet-blue-grey colour. Ligand exchange reactions involving chloride or sulphate ions What is said applies equally to the chromium-containing ion. You only need to read the beginning of that page which concentrates on explaining the acidity of the hexaaquairon(III) ion. Note: You will find the full reasons for the acidity of hexaaqua ions if you follow this link. Whenever you write "H + (aq)" what you really mean is a hydroxonium ion, H 3O +. It is being pulled off by a water molecule in the solution. However, if you write it like this, remember that the hydrogen ion isn't just falling off the complex ion. The water is, of course, acting as a base by accepting the hydrogen ion.īecause of the confusing presence of water from two different sources (the ligands and the solution), it is easier to simplify this: The complex ion is acting as an acid by donating a hydrogen ion to water molecules in the solution. A hydrogen ion is lost from one of the ligand water molecules: The ion reacts with water molecules in the solution. In common with the other 3+ ions, the hexaaquachromium(III) ion is fairly acidic - with a pH for typical solutions in the 2 - 3 range. Use the BACK button on your browser to return to this page. Note: If you aren't happy about complex ions (including the way they are bonded and named), it would pay you to follow this link and explore the first couple of pages in the complex ions menu before you go on. The simplest ion that chromium forms in solution is the hexaaquachromium(III) ion - 3+. Reactions of chromium(III) ions in solution Check your syllabus and past papers to find out exactly what you need to know. You are very unlikely to need everything on this page. You will find links to other pages where these reactions are discussed in more detail. The first part of this page is a summary of the reactions of chromium(III) ions in solution. It includes: reactions of chromium(III) ions in solution (summarised from elsewhere on the site) the interconversion of the various oxidation states of chromium the chromate(VI)-dichromate(VI) equilibrium and the use of dichromate(VI) ions as an oxidising agent (including titrations). This page looks at some aspects of chromium chemistry required for UK A level (and its equivalents).
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