mixture. Other methods, such as staining and flashing, involve coating the previously prepared glass and then refiring it.
b Cobalt oxide, manganese oxide, iron oxide, copper(II) oxide and silver nitrate can be used to add colour to glass.
3 In the mediaeval painting of about 1365, St John the Baptist with St John the Evangelist (?) and St James, by Nardo di Cione, the picture is on wood and the ground is gesso.
This gesso is a mixture of gypsum, CaSO4.2H2O, and animal glue. The figures were drawn onto the ground and then painted using an egg tempera medium. The paints involved in this painting include the following:
Pigment Chemical composition Method of preparation Red ochre Fe2O3 Grinding of the natural ore.
Minium (red lead) PbO2.2PbO Oxidation of molten lead.
Ultramarine 3Na2O.3Al2O3.6SiO2.2Na2S Grinding, kneading in lye solution with a special dough.
Insoluble colour in suspension is then allowed to settle, is washed and dried.
Crimson lake
(cochineal) C11H12O17 Colour from solution is fixed onto an insoluble white powder such as gypsum, which is then extracted and dried.
White lead 2PbCO3.Pb(OH)2 Stack process—lead in clay pots with vinegar and manure and left to ferment.
Lead tin yellow PbSnO4 Roasting of lead and tin mixture.
Vermilion HgS Heating mercury and sulfur together and HgS condenses from the vapours.
Green earth Various Fe, Mg, Al and K compounds
Grinding of the natural ores.
Gilding Au Found in native condition.
This painting was not varnished; the egg tempera of the paint layer provided a self-protective finish.
4 a An emission spectrum is produced when the visible light emitted from excited atoms of an element is passed through a narrow slit and then a prism. An
absorption spectrum is the result of light first passing through a substance in the vapour phase and then producing a pattern of dark lines across a continuous spectrum. A reflectance spectrum results when white light is shone onto a surface and the reflected light is analysed.
b A reflectance spectrum can be obtained from a painting by shining white light on to the surface of the chosen pigment and analysing the resulting light reflected from the surface of the pigment. The reflectance spectrum can then be compared to those of known compounds and an identification made of the pigment. A 5%
tint of cobalt blue shows over 80% reflectance of light with a wavelength of 480–
490 nm (blue at the green end) and 90% reflectance of light with a wavelength of 700 nm (deep red). A 5% tint of Prussian blue has over 60% reflectance of light in the 440–490 nm range (violet blue).
Laser microspectral analysis can use laser reflectance to identify pigments very accurately.
5 The Bohr model of the atom described the orbit of electrons around the atom as being at fixed radii and energy; the electrons could absorb energy in fixed quanta and move to a higher energy level and then emit these same quanta of energy when they fell back to a lower energy level. This model was very useful in explaining the line spectrum of hydrogen, and it supplied the notion of quanta of energy able to be absorbed and emitted, but it was not as useful for explaining the atomic spectra of atoms other than hydrogen. Nor could it explain the different intensities of spectral lines, the fact that some spectral lines actually consist of a number of very fine close lines, or the splitting of such spectral lines in magnetic fields.
6 A piece of platinum wire was thoroughly cleaned by immersion in hydrochloric acid and heating in a non-luminous Bunsen burner flame. Then the wire was dipped in a small sample of sodium chloride and placed in the non-luminous Bunsen flame and the colour of the flame observed. Sodium ions produced a strong yellow flame. This colour is produced because the electrons of the sodium absorb quanta of energy and move to
higher energy levels. This absorbed energy is emitted as the excited electrons jump back to their ground states. The emitted energy is the yellow range of visible light.
7 a
b The 27 electrons of the cobalt atom fill in the shell order 1s, 2s, 2p, 3s, 3p, 4s, then 3d. In filling the orbitals, two restrictions apply: the Pauli exclusion principle states that an orbital can only hold a maximum of two electrons (and that these must have opposite spins); and Hund’s rule states that if two or more orbitals are available (as in the 3d orbitals for cobalt), then one electron goes into each orbital until all the orbitals are half-full, and then a second electron can go into the orbitals.
8 The transition metals have valence electrons in both the 3d and 4s sub-shells. Because there is very little difference in energy between these sub-shells and their orbitals, the transition metals can lose these electrons with almost equal ease, resulting in a number of oxidation states for most of these metals. The unfilled orbitals in the sub-shells of some of the transition metals also have good electron acceptor properties in forming coordination compounds and so make good oxidising agents because they are themselves so easily reduced. Metals from the s and p blocks do not have the same d-orbital configurations and so have little or no variation in their oxidation states.
9 Colour in aqueous ions of transition metals results from the ability of these metals to absorb quanta of energy and promote electrons to unfilled d-orbitals. The absorbed energy is in the range of visible light and the compounds appear the complementary colour of the energy absorbed: the [Cr(H2O)6]3+ ions appear violet because they have absorbed energy in the yellow-green range and reflect or transmit in the violet range.
Aqueous ions of metals other than transition metals do not have unfilled d-orbitals and so have no opportunity to produce colours.
10 a The ligand EDTA forms a coordinate covalent bond with cadmium ions in complex formation.
b The ligand EDTA has six sets of lone pair electrons through which it bonds to the Cd2+ ion. As the EDTA is donating the pairs of electrons, it is acting as a Lewis base and the metal ion is acting as a Lewis acid. Hence, the sequestering of cadmium into an EDTA complex is a Lewis acid–Lewis base interaction.
11 A strong oxidising agent is itself readily reduced. Potassium permanganate solution, KMnO4(aq), is a deep purple colour as a consequence of the +7 oxidation state of manganese in the MnO4–(aq) ion. If this ion is reduced, its colour should quickly change.
Steps:
1 Make up a 0.1 M solution of acidified (with sulfuric acid) potassium permanganate, KMnO4(aq).
2 Add it dropwise to a solution of violet vanadium(II) ions formed from oxidising dioxovanadium(V) ions using granulated or powdered zinc metal added to acidified (with about 9 M sulfuric acid) 0.1M ammonium metavanadate solution.
3 If you don’t agitate the mixture too much you will see the colours as the vanadium(II) is oxidised progressively to the vanadium(III) ion, to the
oxovanadium(IV) ion, to the dioxovanadium(V) ion. The colour goes from violet to green to blue to yellow and the purple permanganate decolourises.
4 As the vanadate ion (converted to dioxovanadium(V) ion by the sulfuric acid) is also a strong oxidising agent, permanganate must be a very strong oxidising agent.
The net ionic equation for the permanganate reduction is:
MnO4–(aq) (purple) + 8H+(aq) + 5e– → Mn2+(aq) + 4H2O(l) (colourless) The oxidation state of the Mn2+ ion is +2.
The ionic equations for the vanadium(II) oxidation are:
V2+(aq) → V3+(aq) + e−
V3+(aq) + H2O(l) → VO2+(aq) + 2H+(aq) + e– VO2+(aq) + H2O(l) → VO2+(aq) + 2H+(aq) + e–
Because acidification of the metavanadate is with concentrated acid, safety goggles, gloves and protective clothing must be worn. Also, the metavanadate is toxic and a possible mutagen; gloves, goggles and protective clothing must be worn.
12 A ligand is a species (acting as a Lewis base) with a lone pair of electrons able to be donated to a metal cation (acting as a Lewis acid) with orbitals available to receive them. Thus, a coordinate covalent bond forms in this Lewis acid–base interaction as both electrons in the bond originate from the one atom. Chelates are ligands able to form at least two coordinate bonds to the one cation.
For example, ethylene diamine is a ligand that has two nitrogen atoms that can each bond to a cation such as nickel(II).