Chemistry · Chemistry of the elements · Grade 9

Copper, silver, gold and zinc

Why is copper sulfate blue, why is silver the best conductor, and how is gold melted?

The situation

At the Almalyk Mining and Metallurgical Combine, pure copper for wires and cables is obtained from ore, and gold is mined at the Muruntau deposit in the Navoi region. Gardeners protect grapevines from fungal diseases with Bordeaux mixture made from copper sulfate and lime, jewellers make rings of 585 fineness, and the back of a mirror is coated with silver. Why is copper sulfate blue, and why does copper hydroxide turn black when heated? Why does silver conduct current best, and why does gold not dissolve in acids but does dissolve in cyanide solution or aqua regia?

Topic

Copper, silver, gold and zinc. Almalyk copper and Muruntau gold: copper sulfate, a silver mirror and dissolving gold with cyanide; the amphoteric nature of zinc hydroxide

Key ideas

Copper(II) hydroxide and the ammine complex
Silver and gold
d-elements

Formulas

CuSO_{4} + 2 NaOH \overset{}{\to} Cu (OH)_{2} ↓ + Na_{2} SO_{4}

Blue precipitate: 2 mol of OH⁻ per 1 mol of Cu²⁺ (lab work 12).

Cu (OH)_{2} \overset{\overset{}{t}}{\to} CuO + H_{2} O

On heating, the blue hydroxide turns into black copper(II) oxide.

Cu (OH)_{2} + 4 NH_{3} \overset{}{\to} [Cu (NH_{3})_{4}] (OH)_{2}

It dissolves in excess ammonia solution: the dark blue tetraamminecopper(II) ion.

4 Au + 8 NaCN + O_{2} + 2 H_{2} O \overset{}{\to} 4 Na [Au (CN)_{2}] + 4 NaOH

Cyanidation (the Elsner equation): gold dissolves in the presence of oxygen from the air.

Zn (OH)_{2} + 2 HCl \overset{}{\to} ZnCl_{2} + 2 H_{2} O Zn (OH)_{2} + 2 NaOH \overset{}{\to} Na_{2} [Zn (OH)_{4}]

Zinc hydroxide is amphoteric (lab work 13).

Theory

Group IB subgroup: copper, silver, gold (§ 28)

Copper, silver and gold are the d-block elements of the IB subgroup. Like the alkali metals, they have one s electron in the outer shell: Cu [Ar] 3d¹⁰ 4s¹, Ag [Kr] 4d¹⁰ 5s¹, Au [Xe] 4f¹⁴ 5d¹⁰ 6s¹; but the d-subshell beneath it is filled with 10 electrons. That is why they are completely different from the alkali metals: their reactivity is low, they stand after hydrogen in the reactivity series, they do not react with water or hydrochloric acid, and they also occur native in nature. Oxidation states: Cu +1, +2; Ag +1; Au +1, +3. Copper is reddish and soft, ρ = 8.96 g/cm³, and melts at 1085 °C; in electrical conductivity it is second only to silver — wires, cables and heat exchangers are made from it. Its alloys: bronze (Cu + Sn), brass (Cu + Zn), cupronickel (Cu + Ni). In Uzbekistan, copper at the Almalyk Mining and Metallurgical Combine is obtained mainly from chalcopyrite ore, CuFeS₂.

Copper and its compounds (lab work 12)

Copper heated in air turns black: $2 Cu + O_{2} \overset{}{\to} 2 CuO$ ; in damp air it becomes coated with a green patina — a basic carbonate: $2 Cu + O_{2} + CO_{2} + H_{2} O \overset{}{\to} (CuOH)_{2} CO_{3}$ . Copper does not dissolve in hydrochloric acid or dilute sulfuric acid, but it dissolves in oxidising acids: $3 Cu + 8 HNO_{3} \overset{}{\to} 3 Cu (NO_{3})_{2} + 2 NO + 4 H_{2} O$ (dilute), $Cu + 2 H_{2} SO_{4} \overset{}{\to} CuSO_{4} + SO_{2} + 2 H_{2} O$ (concentrated). Copper(II) sulfate pentahydrate CuSO₄·5H₂O (blue vitriol) is a blue hydrated crystal; when heated it loses its water (36.1 % of its mass) and becomes white anhydrous CuSO₄, which turns blue again if water is dropped on it — a test for water. Lab work: if alkali is added to a CuSO₄ solution, blue Cu(OH)₂ precipitates ( $Cu^{2 +} + 2 OH^{-} \overset{}{\to} Cu (OH)_{2}$ ); on heating it turns into black CuO ( $Cu (OH)_{2} \overset{}{\to} CuO + H_{2} O$ ); it dissolves in acid ( $Cu (OH)_{2} + 2 HCl \overset{}{\to} CuCl_{2} + 2 H_{2} O$ ); in excess ammonia solution it dissolves, forming the dark blue complex [Cu(NH₃)₄]²⁺ — this solution (Schweizer’s reagent) even dissolves cellulose. Bordeaux mixture is made from copper sulfate and lime — it protects plants from fungal diseases.

Silver and gold (§ 29)

Silver is the best conductor of electricity and heat (ρ = 1.59·10⁻⁸ Ω·m) and the metal that reflects light best. It is applied to the back of a mirror by the silver mirror reaction: the diamminesilver(I) complex is reduced to the metal by glucose (an aldehyde): $C_{6} H_{12} O_{6} + 2 [Ag (NH_{3})_{2}] OH \overset{}{\to} C_{6} H_{11} O_{7} NH_{4} + 2 Ag + 3 NH_{3} + H_{2} O$ . It tarnishes black with H₂S in the air: $4 Ag + 2 H_{2} S + O_{2} \overset{}{\to} 2 Ag_{2} S + 2 H_{2} O$ . Gold is one of the least reactive metals: it is unchanged by oxygen and ordinary acids; it is dissolved by aqua regia (1 volume of HNO₃ + 3 volumes of HCl: $Au + HNO_{3} + 4 HCl \overset{}{\to} H [AuCl_{4}] + NO + 2 H_{2} O$ ) and by cyanide solution in the presence of oxygen from the air. Jewellery is made from an alloy: the fineness is the number of parts of gold per 1000 parts by mass (585 fineness = 58.5 % Au, the rest silver and copper). At the Navoi Mining and Metallurgical Combine, Muruntau ore is crushed and the gold is brought into cyanide solution ( $4 Au + 8 NaCN + O_{2} + 2 H_{2} O \overset{}{\to} 4 Na [Au (CN)_{2}] + 4 NaOH$ ), then adsorbed onto a sorbent or displaced from the solution by zinc: $2 Na [Au (CN)_{2}] + Zn \overset{}{\to} Na_{2} [Zn (CN)_{4}] + 2 Au$ .

Group IIB subgroup: zinc, cadmium, mercury (§ 30, lab work 13)

Zinc, cadmium and mercury form the IIB subgroup: $(n - 1) d^{10} n s^{2}$ , with +2 in compounds (mercury also +1). The d-subshell is full and only the s electrons are given up, so Zn²⁺ and Cd²⁺ compounds are usually colourless. Zinc is bluish-grey and melts at 419.5 °C; iron is coated with it to protect against corrosion (galvanised sheet steel), and it is used in brass and in galvanic cells; it displaces hydrogen from acid: $Zn + 2 HCl \overset{}{\to} ZnCl_{2} + H_{2}$ . Zinc hydroxide is amphoteric: if alkali is added to ZnSO₄, a white precipitate forms, which dissolves both in acid ( $Zn (OH)_{2} + 2 HCl \overset{}{\to} ZnCl_{2} + 2 H_{2} O$ ) and in excess alkali ( $Zn (OH)_{2} + 2 OH^{-} \overset{}{\to} [Zn (OH)_{4}]^{2 -}$ ); unlike Al(OH)₃, it also dissolves in ammonia solution — [Zn(NH₃)₄]²⁺. Cadmium is poisonous and is used in Ni–Cd batteries. Mercury is the only metal that is liquid at room temperature (it freezes at −38.8 °C), and its vapour is highly poisonous: if a thermometer breaks, the droplets are collected, sulfur powder is sprinkled over the spot ( $Hg + S \overset{}{\to} HgS$ ) and the room is ventilated.

d-block elements: electronic structure

In the subgroups the inner d-subshell is filled: 3d from Sc to Zn in period 4, 4d in period 5, 5d in period 6 (with 4f before it). By the Madelung rule copper would be expected to have $3 d^{9} 4 s^{2}$ , but a filled d-subshell is energetically more favourable — one electron moves from 4s to 3d: ** $3 d^{10} 4 s^{1}$ ** (the “electron jump”; the same happens in Ag and Au, while in chromium a half-filled $3 d^{5} 4 s^{1}$ results). On ionisation the s electrons leave first: Cu⁺ is $3 d^{10}$ (its compounds are usually colourless), Cu²⁺ is $3 d^{9}$ : the incompletely filled d-subshell absorbs the red part of visible light, which is why copper(II) solutions are blue. In Zn²⁺ ( $3 d^{10}$ ) there is no such transition — zinc salts are colourless. Most d-block elements have variable oxidation states and form complex compounds.

Safety and the model

Safety: when working with alkali, ammonia and acids, wear safety goggles; do not sniff ammonia — work in a fume cupboard. When heating a test tube, hold it with a holder, first warm the whole tube evenly, and do not point its mouth at yourself or others. Copper salts are poisonous — wash your hands after the work. Cyanides are extremely poisonous — they are not used in school, and this part is only a model. Mercury and cadmium compounds are poisonous. Model: the amounts of precipitate and complexes follow stoichiometry (the final state depends on the moles added; the reactions are taken to be instantaneous). The solution colours are approximate, based on the Beer–Lambert law (absorption in three colour zones for each ion). The heating, silver mirror and cyanidation times have been shortened. The resistance of the wires is calculated from the resistivities at 20 °C: $R = ρ L / S$ .

Did you know?

Muruntau (Navoi region) is one of the largest open-pit gold mines in the world: the pit is ≈ 3.5 km long and ≈ 2.5 km wide, and more than 500 m deep. In 2024 the Navoi Mining and Metallurgical Combine produced more than 3 million troy ounces (about 93 t) of gold.
A mixture of copper sulfate and slaked lime — “Bordeaux mixture” — began to be used against fungal disease in the vineyards around the French city of Bordeaux at the end of the 19th century, and it is still used in horticulture today.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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