EXTRACTING METALS

Properties of Metals

✦ Metals have high melting and boiling points

✦ Metals have strong metallic bonds which require large amounts of energy to overcome

✦ They are good conductors of heat and electricity

✦ They have a sea of electrons which are delocalised and free to move which allows them to conduct

✦ Electrons are replaced by other electrons in metals during conduction

✦ They are malleable and ductile

✦ The layers of positive ions can slide over each other easily

✦ The metallic bonds are not broken by the sliding so they can be shaped without breaking

Reactions of Metals

✦ Metals produce Metal Hydroxide and Hydrogen on reaction with cold water

✦ Reaction with steam produces Metal Oxide and Hydrogen gas

✦ Metals above Hydrogen in the reactivity series can react with acids

Metal + Water → Metal Hydroxide + Hydrogen Gas

Alloys

✦ Alloys are a mixture of two or metals or a metal and a nonmetal

✦ They have properties different to their components

✦ They have greater strength, hardness, and resistivity to corrosion

✦ They different sized atoms make it difficult for them to slide over each other

✦ Brass is an alloy of copper and zinc

✦ Aluminium alloy is used for airplanes, it is harder with low density

✦ Alloys are not chemically combined, they are mixtures not compounds

Reactivity Series

✦ The reactivity decreases down the group

✦ Metals with higher reactivity can displace lower reactivity metals

✦ Higher reactivity metals lose electrons and form ions more readily

✦ Aluminium metal is not very reactive despite being high in the series because it has a layer of nonreactive Aluminum oxide over it which prevents it from coming into contact with the reactant, making it resistant to corrosion and nonreactive

Extraction of Metals

✦ Metals are found as ores and need to be extracted

✦ The higher the reactivity of metals, the harder the extraction

✦ Highest reactivity metals are extracted by electrolysis

✦ Medium reactivity metals are extracted in blast furnace

✦ Low reactive metals are extracted by heating

✦ Nonreactive metals are found uncombined as they do not react with other materials

Ores:

• Metals occur naturally in the Earth's crust in the form of ores, which are compounds of the metal usually combined with oxygen, haematite, or carbon. Common examples include hematite (Fe₂O₃) for iron and bauxite (Al₂O₃·2H₂O) for aluminum.

• Ores can be high-grade (with a high concentration of the metal) or low-grade (with a lower concentration).

Extraction of Iron

✦ Iron has to be extracted from its ore haematite

✦ It is extracted in the blast furnace

✦ Iron ore (Fe₂O₃), coke (carbon) and limestone are added at the top of the blast furnace as raw materials

✦ Limited supply of air or oxygen is supplied

✦ The oxygen reacts with the coke (carbon) and oxidises it to carbon dioxide

✦ Excess coke reacts with the carbon dioxide to produce carbon monoxide

✦ Carbon monoxide acts as a reducing agent and reacts with Fe₂O₃, reducing it to Iron

✦ The molten iron contains impurities such as sand (SiO₂) for which limestone is added

✦ CaCO₃ first decomposes in the heat to formed Calcium Oxide and Carbon dioxide

✦ Calcium oxide reactions with sand to produce calcium silicate

✦ The calcium silicate melts and floats over the molten iron as slag

C (g) + O_2 (g) ——> CO_2 (g)

CO_2 (g) + C (s) ——> 2CO (g)

Fe_2O_3 + 3CO (g) ——> 2Fe (III) +3CO_2 (g)

CaCO_3 (s) ——> CaO + CO_2 (g)

CaO + SiO_2 ——> CaSiO_3

✦ Molten iron settles at the bottom of the furnace from where it is tapped off

‏‏Steel

✦ Molten Iron is brittle so it is converted into steel to strengthen it

✦ Molten Iron contains impurities such as carbon, phosphorus and silicon

✦ The molten iron is transferred to a tilting furnace

✦ Oxygen and powdered calcium oxide is added

✦ The oxygen reduces the impurities to their oxides

✦ CO₂ and SO₂ escape as gasses

✦ The acidic silicon and phosphorus oxides reaction with calcium oxide to form slag, mainly calcium silicate

✦ Slag floats on the iron and the molten iron is removed

Extraction of Aluminium

✦ Electrolysis is used to extract Aluminium from its ore Bauxite

✦ Bauxite is first purified to get Aluminium Oxide (Al₂O₃)

✦ Aluminum oxide is dissolved in molten cryolite to lower its melting point, improve conductivity and reduce cost

✦ The dissolved Aluminium Oxide in Cryolite solution is taken as the electrolyte at a temperature of 1000 degrees.

✦ Graphite electrodes are used

✦ The Oxygen ions go to anode and lose electrons to produce Oxygen gas

✦ Some of the oxygen reacts with the graphite electrodes to produce carbon dioxide causing the electrodes to be used up

✦ Aluminium gains electrons at the cathode and molten Aluminum is produced

✦ The molten Aluminium is taken out and more Aluminium oxide is added

✦ Aluminium is used in air-plane bodies due to high strength with low density

Bio-leaching and Phytomining:

• These methods involve using living organisms, such as bacteria or plants, to extract metals from low-grade ores.

• Bio-leaching involves bacteria that secrete chemicals to dissolve metals from ores.

• Phytomining uses plants that absorb metals from the soil through their roots, which are then harvested and burned to extract the metal.

Equilibria

Dynamic Equilibrium

✦ When the rate of the forward reaction is equal to the rate of the backward direction, the reaction is at equilibrium

✦ It can only occur in a closed system where none of the reactants or products can escape the reacting apparatus

✦ A change in the temperature, pressure, concentration, or amount of reactants and products can change the position of the equilibrium

✦ If the position of equilibrium shifts towards the forward reaction, the product concentration is higher and the rate of the forward reaction is greater

✦ If the position of equilibrium shifts towards the backward reaction, the reactant concentration and rate of backward reaction is higher

Le Chatelier’s principle and position of equilibrium

✦ The addition of a catalyst does not impact the position of the equilibrium

✦ A catalyst only lowers the activation energy and speeds up the rate of reaction equally for both the forward and backward reaction, maintaining the equilibrium

✦ Temperature favours endothermic reaction

✦ An increase in temperature will increase the rate of the endothermic reaction and decrease the rate of the exothermic reaction, shifting the equilibrium towards the endothermic reaction

✦ Decreasing the temperature will increase rate of exothermic reaction and reduce rate of endothermic reaction, shifting equilibrium towards exothermic

✦ Pressure favors less number of moles

✦ Increasing the pressure will increase the rate of reaction and shift equilibrium to the side with the lesser number of moles of gas

✦ Decreasing pressure will shift equilibrium towards the side with larger number of moles

✦ Increasing concentration of reactants shifts equilibrium towards forward reaction

✦ Increasing concentration of products shifts equilibrium towards backward reaction

✦ Decreasing concentration of reactants shifts equilibrium towards backward reaction

✦ Decreasing concentration of products shifts equilibrium towards forward reaction

✦ Concentration is increased or decreased by adding or removing some of the reactants or products