Metals

Electro-refining of impure silver

                 Battery / DC source
                  (+) -------- (-)
                   |            |
                   |            |
        +----------+------------+----------+
        |          |            |          |
        |      ANODE (+)     CATHODE (-)    |
        |   impure silver   pure silver     |
        |        ||             ||          |
        |        ||             ||          |
        |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
        |   Electrolyte: AgNO3 solution     |
        |                                  |
        |      Anode mud (impurities)       |
        +----------------------------------+
              Beaker / electrolytic cell

• Anode (+): impure silver block, connected to the positive terminal.
• Cathode (-): pure silver strip, connected to the negative terminal.
• Electrolyte: silver nitrate (AgNO$_3$) solution.
• Anode mud: impurities that fall below the anode.

Working: On passing current, silver dissolves from the impure anode as $\text{Ag}^+$ ions and pure silver deposits on the cathode. Impurities settle below the anode as anode mud.

Anode: $\text{Ag} \to \text{Ag}^+ + e^-$; Cathode: $\text{Ag}^+ + e^- \to \text{Ag}$.

Example: $2\text{ZnS} + 3\text{O}_2 \to 2\text{ZnO} + 2\text{SO}_2$ (roasting); $\text{ZnCO}_3 \to \text{ZnO} + \text{CO}_2$ (calcination).

Chalcopyrite ($\text{CuFeS}_2$). It is the main sulphide ore of copper; argentite is a silver ore, haematite an iron ore and bauxite an aluminium ore.

Haematite. Haematite ($\text{Fe}_2\text{O}_3$) contains about 70% iron and is the chief ore of iron; argentite is a silver ore, chalcopyrite a copper ore and siderite ($\text{FeCO}_3$) a minor iron ore.

(a) Impure silver is placed at A, because A is connected to the positive terminal and so is the anode, where the impure metal dissolves as $\text{Ag}^+$ ions.

(b) C is the electrolyte (silver nitrate solution). It contains $\text{Ag}^+$ ions and conducts current; silver ions from the anode move through it and are deposited as pure silver on the cathode B.

Electro-refining of impure copper

        Battery / DC source
         (+)-----| |-----(-)
          |             |
          |             |
   +------+-------------+------+
   |   ANODE        CATHODE    |
   | (impure copper)  (pure copper)|
   |     |            |        |
   |     |            |        |
   |  Electrolyte: copper sulphate (CuSO4) solution with a little H2SO4 |
   +-----------------------------+

• Anode (+): thick block of impure copper.
• Cathode (-): thin strip of pure copper.
• Electrolyte: copper sulphate (CuSO$_4$) solution with a little H$_2$SO$_4$.
• Anode and cathode are connected to the positive and negative terminals of the battery.

Working: On passing current, copper dissolves from the anode as ions and the same ions gain electrons and deposit as pure copper on the cathode. Impurities settle as anode mud.

Anode: $\text{Cu} \to \text{Cu}^{2+} + 2e^-$; Cathode: $\text{Cu}^{2+} + 2e^- \to \text{Cu}$.

$2\text{Al} + 6\text{HCl} \to 2\text{AlCl}_3 + 3\text{H}_2\uparrow$

Aluminium reacts with dilute hydrochloric acid to form aluminium chloride and hydrogen gas.

$2\text{Al} + 3\text{H}_2\text{SO}_4 \to \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2\uparrow$

Aluminium reacts with dilute sulphuric acid to form aluminium sulphate and hydrogen gas.

The main ore of silver is argentite (silver glance, $\text{Ag}_2\text{S}$).

Electro-refining of impure gold

        Battery / DC source
         (+)-----| |-----(-)
          |             |
          |             |
   +------+-------------+------+
   |   ANODE        CATHODE    |
   | (impure gold)  (pure gold)|
   |     |            |        |
   |     |            |        |
   |  Electrolyte: gold chloride (AuCl3) solution acidified with HCl |
   +-----------------------------+

• Anode (+): thick block of impure gold.
• Cathode (-): thin strip of pure gold.
• Electrolyte: gold chloride (AuCl$_3$) solution acidified with HCl.
• Anode and cathode are connected to the positive and negative terminals of the battery.

Working: On passing current, gold dissolves from the anode as ions and the same ions gain electrons and deposit as pure gold on the cathode. Impurities settle as anode mud.

Anode: $\text{Au} \to \text{Au}^{3+} + 3e^-$; Cathode: $\text{Au}^{3+} + 3e^- \to \text{Au}$.

He connected the metals the wrong way round. Impure silver must be the anode (positive terminal) so that it dissolves, and pure silver must be the cathode (negative terminal) so that pure silver deposits on it. (AgNO$_3$ as electrolyte is correct.)

Argentite is the ore of silver, so the errors are:

1. The electrolyte is wrong: CuSO$_4$ is used; it should be a silver salt solution such as AgNO$_3$.
2. The electrodes are wrongly placed: the anode (+) is pure metal and the cathode (-) is impure metal; impure silver should be the anode and pure silver the cathode.

Roasting: the concentrated sulphide ore is heated strongly in a furnace in an excess supply of air (below its melting point). The sulphide is oxidised to the metal oxide, and sulphur dioxide and other volatile impurities escape.

$2\text{ZnS} + 3\text{O}_2 \to 2\text{ZnO} + 2\text{SO}_2$

(ii) Siderite. Siderite ($\text{FeCO}_3$) is a carbonate ore, which on strong heating in limited air gives the oxide: $\text{FeCO}_3 \to \text{FeO} + \text{CO}_2$. Haematite and cuprite are already oxides.

It is called calcination: strong heating of the ore (carbonate or hydrated) in a furnace in the absence or limited supply of air to form the metal oxide.

(i) To change the ore into metallic oxide: roasting (for sulphide ores, in excess air) or calcination (for carbonate ores, in limited air).

(ii) To separate hydrophilic (gangue) and hydrophobic (ore) particles: froth floatation process.

(c) Reduction. In smelting the metal oxide is mixed with a reducing agent such as carbon (coke) and heated above its melting point, so the oxide is reduced to the metal.

(d) Concentration, Oxidation, Reduction, Refining. The ore is first concentrated, then converted to oxide (roasting or calcination), then reduced to the crude metal, and finally refined to pure metal.

Froth floatation concentrates sulphide ores (e.g. zinc blende, galena, copper pyrite), because their particles are wetted by oil and float in the froth while the gangue is wetted by water and sinks.

Magnetic separation, because iron ore is magnetic while the gangue is non-magnetic.

It is used to concentrate ores when either the ore or the gangue is magnetic and the other is non-magnetic, e.g. iron ore (magnetic) is separated from non-magnetic gangue.

Magnetic separation. Iron dust is attracted by a magnet while zinc sulphide is non-magnetic, so a magnet pulls out the iron dust.

Mineral: a naturally occurring, inorganic substance in the earth's crust, with a definite chemical composition, from which a metal may or may not be extracted profitably.

Ore: a mineral from which a metal can be extracted easily and profitably.

Example: clay is a mineral of aluminium but not an ore; bauxite is an ore of aluminium.

An ore is a mineral from which a metal can be extracted easily and profitably. Example: haematite for iron.

(iv) Silver. Argentite is silver sulphide ($\text{Ag}_2\text{S}$), the main ore of silver.

Gold is very unreactive (low in the reactivity series), so it does not react with oxygen, moisture or other substances in air and soil, and is found in the native (free) state.

Gold is a very unreactive metal. It does not react with oxygen, water, carbon dioxide or acids in the soil, so it does not rust, corrode or dissolve. Therefore the ring's weight did not change after two months.

Aluminium < Magnesium < Sodium. Sodium (group 1) is most reactive, then magnesium, and aluminium is least reactive of the three.

(i) Be < Mg < Ca. Reactivity of metals increases down a group as atoms lose electrons more easily. In (ii) and (iv) the order of Li, Na, K is wrong, and reactivity decreases from Mg to Si across a period.

Atomic number 12 is magnesium and 8 is oxygen, so the compound is magnesium oxide (MgO).

$\text{MgO} + \text{H}_2\text{O} \to \text{Mg(OH)}_2$

Magnesium hydroxide is a weak base. It can be taken as medicine (milk of magnesia) as an antacid when there is hyperacidity (excess acid in the stomach), because it neutralises the extra acid.

The metal is magnesium: burning in a jar of CO$_2$ it forms white MgO and black carbon: $2\text{Mg} + \text{CO}_2 \to 2\text{MgO} + \text{C}$.

(i) $\text{MgO} + \text{H}_2\text{O} \to \text{Mg(OH)}_2$

(ii) The solution is magnesium hydroxide, a mild base (milk of magnesia). It neutralises the excess hydrochloric acid in the stomach, giving salt and water, and so relieves hyperacidity: $\text{Mg(OH)}_2 + 2\text{HCl} \to \text{MgCl}_2 + 2\text{H}_2\text{O}$.

The main cause is corrosion/tarnishing of the metals by air pollution: oxygen, moisture, carbon dioxide and polluting gases such as SO$_2$ react with the metal surface and form a dull layer of oxide, carbonate or sulphide, so the lustre fades.

Example (copper): $2\text{Cu} + \text{O}_2 \to 2\text{CuO}$ (and with CO$_2$ and moisture, green basic copper carbonate forms).

Bauxite ($Al_2O_3\cdot 2H_2O$).

Copper (silver and gold are also coinage metals).

He should choose aluminium. It has a low density, so the toy is light, and it forms a thin protective layer of oxide that stops further corrosion, so it does not rust like iron or corrode like copper.

(a) Gold and silver are lustrous, malleable and ductile, so they can be shaped into ornaments. They are also unreactive and do not tarnish or corrode.

(b) Acidic substances (such as pickles or curd) react with copper and iron to form harmful salts and spoil the food. The utensils are also corroded.

(a) Silver (Ag) is the densest of these metals ($10.5\ g/cm^3$ compared with 8.96 for Cu, 7.87 for Fe and 2.7 for Al), so it is chosen when a heavy object is needed. (Copper is cheaper in practice.)

(b) Aluminium, because it is light (low density) and does not rust, as a protective oxide layer forms on its surface.

1. Iron is a good conductor of heat, so food cooks quickly.
2. It has a high melting point, is strong and cheap, and does not melt or break at cooking temperatures.

(a) Metals are good conductors of heat, so food cooks quickly. They also have high melting points and are strong.

(b) Wood and plastic are poor conductors of heat, so the handle stays cool enough to hold.

(c) Copper is expensive and reacts with acidic food to form harmful compounds, whereas iron is cheap. (Alternatively, copper utensils can be used, but they are costly.)

(a) $ZnSO_4$ solution cannot be stored in the Al vessel. Aluminium is above zinc in the series, so it displaces zinc from the solution and the vessel gets corroded: $2Al + 3ZnSO_4 \to Al_2(SO_4)_3 + 3Zn$ Copper is below zinc, so it does not react.

(b) No. Copper lies below hydrogen in the series, so it cannot displace hydrogen from HCl.

Y is magnesium and X is carbon dioxide. Magnesium burns in $CO_2$: $2Mg + CO_2 \to 2MgO + C$, giving white MgO and black carbon (Z).

(a) The white powder is magnesium oxide, $MgO$.

(b) The gas in the jar is carbon dioxide ($CO_2$).

The connections are reversed. Impure copper should be connected to the positive terminal (anode) and pure copper to the negative terminal (cathode). Then copper dissolves from the impure anode as $\text{Cu}^{2+}$ and deposits as pure copper on the cathode. (CuSO$_4$ electrolyte is correct.)

Active metals (Na, K, Ca, Mg, Al) have a very strong affinity for oxygen, so their oxides cannot be reduced by carbon or ordinary reducing agents. Therefore they are extracted by electrolysis of their molten compounds (electrolytic reduction), in which electrons from the electric current reduce the metal ions at the cathode. Electrorefining is not used for extracting active metals from their compounds; it is used only to purify impure metals. Hence the question should say “electrolysis/electrolytic reduction,” not “electrorefining.”

Haematite ($\text{Fe}_2\text{O}_3$) contains the highest percentage of iron (about 70%) and is abundant and easy to reduce, so iron is extracted from it economically.