Metal extraction and chemical reactivity

* Metals have similar physical properties

* Sodium: soft; reacts violently with air and water

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* Iron: reacts with air and water but slowly; forms rust

* Gold: unreactive

* Sodium> iron> gold (> = more reactive)

a. Metal reactions

i. With acid

* Metal + HCl(dilute) – H2 + metal chloride

* An order of reactivity, giving the most reactive metal first is called the reactivity series.

* With air/ oxygen

* Metal + O2 – metal oxide

ii. With water / steam

* (More reactive) metal + cold water – metal hydroxide + H2

* (Less reactive) metal + steam – metal oxide + H2

* Iron + copper: used in everyday objects

* Mg: construction

* Na and K: coolants for nuclear reactors

b. Using the reactivity series

i. Competition reactions in the solid state

* A More reactive metal has more tendency to form a metal ion by losing electrons than a less reactive metal.

* More reactive metal + less reactive metal’s oxide – more reactive metal’s oxide + less reactive metal – exothermic and redox reaction

* Fe2O3 + 2Al -Al2O + 2Fe

Thermit reaction

Used to weld damaged railway lines; incendiary bombs

ii. Competition reactions in aqueous solutions

* In displacement reaction, a more reactive metal will displace a less reactive metal from a solution of its salt.

c. Identifying metal ions

* When an alkali dissolves in water, it produces hydroxide ions.

* Most metal hydroxides are insoluble..

* If hydroxide ions from a solution of an alkali are added to a solution of an metal salt, an insoluble often colored metal hydroxide is precipitated from solution.

* In some cases the precipitate dissolves in excess hydroxide, owing to the amphoteric nature of the metal hydroxide.

d. Amphoteric hydroxides

* The hydroxides of metals are basic and they react with acids to form salts.

* Some hydroxides react with strong bases such as sodium hydroxide to form soluble salts. Hydroxides of this type are said to be amphoteric.

* Also the oxides of such metals are amphoteric as they react with both acids and alkalis.

e. Discovery of metals

* An ore is a naturally occurring mineral from which metal can be extracted.

* Metals which are highly reactive are difficult to isolate.

f. Extraction of metals from their ores.

* Ores are: carbonates, oxides or sulphides of the metal mixed with impurities.

* Large lumps of the common ores are first crushed and ground up by very heavy machinery.

* Some ores are already concentrated. But the other ores are concentrated before extraction.

* The method used for extraction depends on the metal’s reactivity.

g. Extraction of reactive metals

* Difficult to extract

* Electrolysis of the molten, purified ore

* Metal is produced at the cathode; non-metal – anode

* Expensive method

* To reduce costs, electrolysis is done where hydroelectric power is available.

Metal

Name of ore

Chemical name of compound in ore

Aluminium

Bauxite

Aluminium oxide

Al2O3.2H2O

Electrolysis of oxide dissolved in molten cryolite

Copper

Copper pyrites

Cooper iron sulphide

CuFes2

The sulphide ore is roasted in air

Iron

Haematite

Iron oxide

Fe2O3

Heat oxide with carbon

Sodium

Rock salt

Sodium chloride

NaCl

Electrolysis of molten sodium chloride

zinc

Zinc blende

Zinc sulphide

ZnS

Sulphide is roasted in air and the oxide produced is heated with carbon

h. Extraction of fairly reactive metals

* Reduce the metal oxide with carbon.

i. Iron

* Extracated from oxides in a blast furnace

* Blast furnace is a steel tower approx. 50 m high lined with heat resistant bricks.

* Loaded with charge i.e. iron ore + coke + limestone

* A blast of hot air is sent in near the bottom of the furnace through holes (tuyeres) which makes the charge glow as the coke burns.(C+O2- CO2)

* CaCO3 decomposes(CaCO3- CaO + CO2)

* This CO2 reacts with more hot coke higher up, producing co – endothermic reaction (CO2 +C – 2CO)

* CO – reducing agent – rises up and reduces iron oxide oxide ore – 700c (Fe2O3 + 3O2 – 2Fe + 3CO2)

* Molten iron trickles to the bottom

* Cao reacts with impurities to form liquid slag which is mainly silicate (CaO + SiO2 – CaSiO3)

* CaSiO3 trickles to the bottom and floats on Fe

* Both these substances are tapped off

* Waste gases escape and are used in a heat exchange process to heat incoming air.

* Slag is used by builders and road makers

* The iron obtained by this process – pig iron (4% C)

* Pig iron is called so because it is poured into moulds called pigs. This iron is brittle.

* Uses: gas cylinders since they are unlikely to get deformed.

* Majority of this iron produced in the blast furnace is converted into steel alloys.

* Types of steel – manganese, tungsten, stainless

ii. Zinc extraction

* Zinc ore is first concentrated by froth floatation.

* The crushed ore is fed into tanks of water containing a chemical frothing agent.

* Air is blown through so that the whole mixture froths up.

* The rock particles become soaked with water and sink to the bottom of the tank.

* But the zinc sulphide particles, into which the water cannot soak are carried to the top of the tank by the air bubbles and are skimmed off and dried.

* The zinc sulphide is then heated very strongly in a current of air in a furnace to convert it to oxide.

* The SO2 (co-product) is used to manufacture H2SO4

* The ZnO is mixed with powdered coke in a furnace and is heated very strongly to 1400 c. it thus gets reduced to zinc.

* The mixture of zinc vapor and carbon monoxide passes through an outlet near the top of the furnace and the zinc metal cools and condenses.

* Uses:

* Alloys such as brass

* To galvanize steel

* For electrodes in batteries

i. extraction of unreactive metals

i. copper

* Crushed ore is concentrated by froth floatation

* A chemical known as a collector is added to an ore/water mixture and sticks to the surface of copper pyrites particles, giving them a water repellent coating.

* Detergent is added and air is blown into the mixture to make it froth.

* The CuFeS2 particles are concentrated in the froth and can be removed easily

* They are then roasted in a limited supply air to ensure conversion to Cu2S.

* Si is then added and the mixture is heated in the absence of air.

* The FeO is converted into FeSiO3 and run off

* The Cu2S is then reduced reduced to Cu by heating in a controlled amount of air.

* Cu is then refined by electrolysis.

* Uses:

* Ductile – electrical wires

* Alloys – bronze and brass

* Cooking utensils

* Central heating pipes

* Steam boilers

ii. Silver and gold

* Silver is extracted by treating Ag2S with sodium cyanide.

* Zn is then added to displace Ag from the solution

* Uses – jewellery, electronics (because of high electronic conductivity)

j. Recycling metals

* Main recycled metals – Al and Fe

k. Rusting of iron

* Rust is an orange red powder mainly hydrated Fe2O3 (Fe2O3.xH2O)

* Both H2O and O2 are needed for iron to rust and if one of them is not present, iron does not rust,

* Rusting is encouraged by salt.

l. Rust prevention

* Painting: if paint is scratched, the iron beneath it will start to rust and corrosion can then spread under the paintwork which is still sound.

* Oiling/greasing: oil prevents contact with air or moisture

* Coating with plastic: prevents steel structure from rusting

* Plating: tin coating and chromium plating

* Galvanizing: it involves dipping the object into molten zinc. The thin layer of the more reactive Zn metal coating the steel object slowly corrodes and loses electrons to the iron.

* Sacrificial protection: Zn bars are placed above Fe and since it’s more reactive than Fe it will react first thus preventing Fe from rusting.

* Corrosion: process which takes place when metals and alloys are chemically attacked by o2, water or any other substances found around them.

* More reactive = more corrosion

* Veralgris:cuco3.cu(oh)2

m. Alloys

* Mixtures of two or more metals

* Have more useful properties than the original pure metals

n. Production of steel

* Pig iron has s,si,and p – impurities which make it hard and brittle

* These must be removed by the basic o2 process

* Molten pig iron is poured into the basic o2 furnace

* A water cooled pipe or lance is introduced into the furnace and o2 is blown onto the surface of the molten Fe.

* C is oxidized to co and co2 while s is oxidized to co and co2 while s is oxidized to so2. These gases escape.

* Si and p are oxidized to si2o5 and p2o5(solid oxides)

* Cao is added to remove these as slag which may be skimmed or poured off.

* Samples are taken continuously and when the required c content is reached, the blast of o2 is turned off.