Contents: 8 sections
Cambridge IGCSE Chemistry 0620 · Core and Extended
Syllabus points
- Describe the transition elements as metals with high densities and high melting points.
- Describe the transition elements as forming coloured compounds.
- Describe the transition elements and their compounds as often acting as catalysts.
- Compare the properties of the transition elements with those of the Group I metals.
- Extended only: describe transition elements as having ions with variable oxidation numbers.
Where they are
The transition elements are the block of metals between Group II and Group III, starting in period 4. The ones that appear in questions are iron, copper, zinc, chromium, manganese, nickel, cobalt and silver.
Compared with the Group I metals
| Property | Group I metals | Transition elements |
|---|---|---|
| Density | Low, 0.53 to 0.97 g per cm<sup>3</sup> | High, iron 7.9 and copper 8.9 g per cm<sup>3</sup> |
| Melting point | Low, sodium melts at 98 °C | High, copper melts at 1085 °C and iron at 1538 °C |
| Hardness | Soft, cut with a knife | Hard and strong |
| Reactivity | Very reactive, react with cold water | Much less reactive, do not react with cold water |
| Compounds | White solids, colourless solutions | Coloured |
| Oxidation number | Always +1 | Often variable |
| Catalytic behaviour | No | Often used as catalysts |
Worked example. Take a 10 cm<sup>3</sup> block of each metal.
mass of iron = 10 × 7.9 = 79 g
mass of sodium = 10 × 0.97 = 9.7 g
The iron block is eight times as heavy as the sodium one of the same size, which is what "high density" means in practice and why sodium floats on water while iron sinks.
Coloured compounds
This is the property most often used to identify a transition element, and the colours are recall marks.
| Ion | Colour of its solution | Precipitate with aqueous sodium hydroxide |
|---|---|---|
| Copper(II), Cu<sup>2+</sup> | Blue | Light blue, insoluble in excess |
| Iron(II), Fe<sup>2+</sup> | Pale green | Green, turning brown at the surface on standing |
| Iron(III), Fe<sup>3+</sup> | Yellow-brown | Red-brown |
| Chromium(III), Cr<sup>3+</sup> | Green | Green, dissolving in excess to a green solution |
| Zinc, Zn<sup>2+</sup> | Colourless | White, dissolving in excess to a colourless solution |
Solid compounds carry colours too: copper(II) oxide is black, copper(II) carbonate is green, and potassium manganate(VII) is purple.
With aqueous ammonia instead of sodium hydroxide, copper(II) gives a light blue precipitate that dissolves in excess to a deep blue solution, which is the standard test for copper(II) ions.
Zinc is the exception to the colour rule. Its compounds are white as solids and colourless in solution, so a white precipitate is not evidence against a transition element.
For contrast, two ions that are not transition elements also give white precipitates with sodium hydroxide. Aluminium gives a white precipitate that dissolves in excess, and calcium gives a white precipitate that does not. Zinc, aluminium and calcium are told apart by what happens when more alkali is added, not by the first observation.
Catalysts
Transition elements and their compounds are frequently used as catalysts, and four should be known by name and by process:
- Iron in the Haber process, making ammonia.
- Vanadium(V) oxide in the Contact process, making sulfur trioxide.
- Nickel in the hydrogenation of alkenes to alkanes.
- Manganese(IV) oxide in the decomposition of hydrogen peroxide to water and oxygen.
Notice that two of these are the element itself and two are compounds of it, which is why the syllabus says the elements and their compounds act as catalysts.
Extended only: variable oxidation numbers
A Group I metal forms one ion and only one, always +1. A transition element commonly forms more than one ion, with different oxidation numbers:
- Iron forms Fe<sup>2+</sup>, iron(II), and Fe<sup>3+</sup>, iron(III).
- Copper forms Cu<sup>+</sup>, copper(I), and Cu<sup>2+</sup>, copper(II).
- Manganese appears at +2, at +4 in manganese(IV) oxide and at +7 in potassium manganate(VII).
This is why the names carry Roman numerals. "Iron chloride" is ambiguous, because it could be FeCl<sub>2</sub> or FeCl<sub>3</sub>. Naming it iron(II) chloride or iron(III) chloride fixes the formula.
The two ions have different colours and different chemistry, so a change between them is visible. A pale green iron(II) solution turning yellow-brown means Fe<sup>2+</sup> has been oxidised to Fe<sup>3+</sup>, losing an electron:
Fe<sup>2+</sup> → Fe<sup>3+</sup> + e<sup>−</sup>
Going the other way, yellow-brown to pale green, is reduction. This is the reason the sodium hydroxide test distinguishes them: green precipitate for iron(II), red-brown for iron(III). An iron(II) precipitate left standing in air slowly turns brown at the surface because the air oxidises it, which is a favourite observation to be asked to explain.
Uses that follow from the properties
- Iron and steel for construction, because iron is hard, strong and has a high melting point.
- Copper for electrical wiring and for pipes, because it conducts well, is ductile and does not react with water.
- Nickel and chromium in stainless steel and in plating, because they resist corrosion.
Every one of these uses follows from the table at the top: high melting point, hardness and low reactivity are exactly what the soft, reactive Group I metals do not offer.
Common mistakes
- Saying transition elements are more reactive than Group I metals.
- Expecting all transition metal compounds to be coloured, when zinc compounds are white or colourless.
- Giving iron(II) hydroxide as red-brown and iron(III) hydroxide as green, which is the wrong way round.
- Saying copper(II) hydroxide dissolves in excess sodium hydroxide, when it does not.
- Leaving the Roman numeral out of a name where the oxidation number varies.
- Saying transition elements always form +2 ions.
- Naming a catalyst without naming the process it catalyses, which is usually where the mark is.