Chapter 24: Transition Elements Reactions
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What are ligand substitution reactions?
• Reactions where one ligand in a complex ion is replaced with another
What happens when copper sulfate is dissolved in water?
• The complex ion [Cu(H2O)6]2+ is formed
What does [Cu(H2O)6]2+ look like?
• Pale blue solution
What happens when ammonia is added to [Cu(H2O)6]2+ ? What are these types of reactions called?
• First as ammonia is added dropwise, a blue precipitate, Cu(OH)2 is formed • Precipitation • After an excess of ammonia is added, [Cu(NH3)4(H2O)2]2+, a dark blue solution, is formed • Ligand substitution
What happens when concentrated HCl is added to [Cu(H2O)6]2+ ?
• [Cu(Cl)4]2- is formed, a yellow solution • Before the reaction is complete, the solution is green due to both the blue and yellow solutions being present
Give the equation for the formation of [Cu(Cl)4]2- from [Cu(H2O)6]2+ . Add an extra detail. Why does [Cu(Cl)4]2- have 4 ligands? What shape is it?
• [Cu(H2O)6]2+ + 4Cl- <-> [Cu(Cl)4]2- + 6H2O • Since the reaction is reversible, adding water would create a blue solution again • Chloride ligands are bigger than other ligands, such as water and ammonia, so copper’s coordination number changes • Tetrahedral
What happens when [Cr(H2O)6]3+ is reacted with excess ammonia?
• [Cr(H2O)6]3+ is a violet solution • As ammonia is added dropwise, a grey-green precipitate, Cr(OH)3 is formed • After an excess of ammonia is added, [Cr(NH3)6]3+, a purple solution, is formed
How does carbon monoxide poisoning work?
• Oxygen binds to the Fe2+ ion in haemoglobin • It is then released where it is needed in the body, and replaced by water or carbon dioxide • The problem with is that carbon monoxide’s bond with the Fe2+ ion is stronger than oxygen’s • Carbon monoxide therefore replaces oxygen
What colour are iron (II) ions in solution, and after: adding aqueous sodium hydroxide and ammonia, and excess sodium hydroxide and ammonia?
• Pale green solution • After adding aqueous sodium hydroxide/ ammonia: green precipitate of iron (II) hydroxide, but turns brown if left in air as iron (II) is oxidised to form iron (III) hydroxide • In excess sodium hydroxide/ ammonia: no change; the precipitate is insoluble
What colour are iron (III) ions in solution, and after: adding aqueous sodium hydroxide and ammonia, and excess sodium hydroxide and ammonia?
• Pale yellow solution • After adding aqueous sodium hydroxide/ ammonia: orange-brown precipitate of iron (III) hydroxide • In excess sodium hydroxide/ ammonia: no change, the precipitate is insoluble
What colour are manganese (II) ions in solution, and after: adding aqueous sodium hydroxide and ammonia, and excess sodium hydroxide and ammonia?
• Pale pink solution • After adding aqueous sodium hydroxide/ ammonia: light brown precipitate of manganese (II) hydroxide, but turns dark brown if left in air • In excess sodium hydroxide/ ammonia: no change, the precipitate is insoluble
What happens to [Cu(H2O)6]2+ when aqueous sodium hydroxide is added?
• At first, Cu(OH)2 is formed • However, since the precipitate is insoluble in excess NaOH, it remains a blue precipitate
What happens to [Cr(H2O)6]3+ when aqueous sodium hydroxide is added?
• At first, Cr(OH)3 is formed • Then [Cr(OH)6]3- is formed, which is a dark green solution
What can oxidise iron (II) to form iron (III)? What is the colour change when this happens, and what causes it?
• Manganate ions in acidic conditions (MnO4-, H+) • Purple to colourless • Manganate ions are purple, but manganese (II) ions are colourless
What can reduce iron (III) to form iron (II)? What is the colour change when this happens, and what causes it?
• Iodide ions • Orange-brown to brown • Iron (III) ions are orange-brown, and iron (II) ions are pale green, but their colour is not visible as iodine is formed, which is brown
What can be used to reduce dichromate ions? Construct the equation. What is the colour change?
• Zinc • Dichromate ions can be reduced to chromium (III) ions • Zinc is oxidised to form zinc (II) ions • Orange to green
Give an example of a reaction where chromium (III) is oxidised to chromium (VI). What is the colour change?
• Chromium (III) ions are oxidised by alkaline hydrogen peroxide to form chromium (VI) in CrO4^2-, turning the solution from green to yellow • (Because the reaction takes place in alkaline conditions, balance each half equation using the alkaline method)
What can copper (II) ions be reduced by? What happens in this reaction? What observations can be made?
• Iodide ions • Copper (I) iodide and iodine are formed • CuI is a white precipitate, and iodine is brown • So, there is an overall change from pale blue to brown, with the precipitate of CuI visible at the bottom
What is the ionic equation for the reaction of copper ions with iodide ions?
• 2Cu 2+ (aq) + 4I- (aq) -> 2CuI (s) + I2 (aq)
When does copper disproportionate? Give the equation. What colour changes are there?
• When copper (I) oxide is reacted with hot dilute sulfuric acid • Cu2O + H2SO4 -> Cu + CuSO4 + H2O • A brown precipitate of copper and a blue solution of copper sulfate is formed