Back to: CHEMISTRY SS3
Welcome to class!
In today’s class, we will be talking about metals and their compound. Enjoy the class!
Metals and their Compound
Metals are found mixed with earthy materials as ores. Ores are often
- concentrated and
- converted to oxides before extraction
The concentration of the ore can be done by any of the following ways:
- washing away the earthy materials
- Froth- flotation (zinc ore): The ore is agitated (churned up) with oil and water to form a froth. The froth is removed by blowing air through it
- By passing magnetic ores through a magnetic separator. The ore will be deflected while the non-magnetic or partially magnetic part of the ore will move on.
Roasting in air
The ore is roasted in air to convert to an oxide. There is no need to roast if the ore is already an oxide.
Principle of extraction of metals
Those metals which are found in a combined state can be extracted through electrolysis or by chemical and thermal methods. The selected method of extraction of any given metal depends on the stability of the ore. The stability of the ore depends on the reactivity of the metal.
|Metal||Reactivity||Most common ore||Method of extraction|
|Chlorides||Electrolysis of fused hydroxides and chlorides|
|Chlorides & Trioxocarbonates||Electrolysis of fused chlorides|
|Moderately reactive||Oxides CO32- and Sulphide||Roasting of CO32- & sulphides to form oxides reduction of oxides by C of CO|
|Roasting in air|
|Heating in air|
Mined as free elements
- Explain froth floatation
- State two ways of concentrating an ore
Sodium occurs as NaCl (rock salt), NaNO and Na2CO3.is often known as Chile saltpetre because it is found abundantly in Chile. It also occurs as borax and in complex trioxosilicates (iv) found in clay soil.
Extraction: It is extracted by electrolysis of fused NaCl using the Downs cell. The cathode is a steel cylinder. Fused NaCl (mp:801oc) is put in the cell where it is heated to keep it molten. CaCl2 is often added to lower the melting point of NaCl to about 600oC. Sodium and chloride are the products.
As the electrolysis progresses, the molten Na collects in the cathode chamber where it gets to the top and is collected through a pipe. A hood guides the gaseous chloride at the anode for collection.
At the cathode: Na+ + e- → Na(s) (Reduction )
At the anode : Cl– → Cl + e- (Oxidation)
Cl + Cl → Cl2 (q)
Overall electrolysis reaction.
2Na+(g) + 2Cl-(i) → 2Na(s) + Cl2(g)
- Silvery solid with metallic luster
- Floats on water (density of 0.98)
- very malleable
- melting point of 97oC (Low for a metal)
- Good conductor of heat and electricity.
- Write the cathodic and anodic half cell equations in the electrolysis of fused NaCl
- What is the function of CaCl2 introduced into the electrolytic cell during the electrolysis above?
- Reaction with air:
4Na (s) + O2(g) → 2Na2O(s)
Na2O(s) + H2O(g) → 2NaOH (aq)
2NaOH (aq) + CO2(g) → Na2CO3(s) + H2O (i)
Sodium tarnishes easily when exposed to air due to the presence of oxygen.
Sodium is stored in paraffin oil, toluene or naphtha to prevent its oxidation by air. In excess air the reaction can be represented as follows:
2Na (g) + O2(g) → Na2O2(g)
- Combination reaction: with H2, Cl2, S, P (with non-metals except Boron, carbon & Nitrogen)
2Na(s) + H2(g) → 2NaCl(g)
2Na(s) + Cl2(g) → 2NaCl(g)
2Na(s) + S(g) → Na2S(s)
3Na(s) + P(g) → Na2P(s)
Na(s) + Hg (l) → Na/Hg(l)
With water: it reacts violently with cold water giving out a lot of heat and liberating hydrogen gas.
2Na + H2O → Na2O + H2
2Na + 2H2O → 2NaOH + H2
- With acid:
2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2O
NB: The reaction is explosive and extremely dangerous
- With ammonia:
2Na(s) + 2NH3 (g) → 2NaNH2(s) + H2 (g)
Test for sodium ions
Flame test: Na compounds give a golden yellow colour for non-luminous flame.
Note: Potassium gives a lilac (pale purple) flame which looks common (deep red) when viewed through a blue gas.
Uses of sodium
- It is used in manufacturing important compounds such as tetraethyl lead (Iv). (Pb(C2H5)4
- Sodium vapour lamps (orange-yellow light) are used to light high ways and airports
- it is used in liquid form as a coolant in nuclear reactors
- Can be used as a reducing agent in combination with ethanol or sodium amalgam.
- Can be sued in the extraction of titanium to reduce titanium tetrachloride to the metal.
Compound of sodium
The following are compounds of sodium: Na2O, Na2O2, NaOH, NaCl, Na2SO4, NaNO3, NaCO3.
Sodium trioxocarbonate (Na2CO3)
- As soda ash in the anhydrous state
- As a monohydrate, Na2CO3.H2
- As a decahydrate (more often) Na2CO3.10H2O called washing soda
The laboratory preparation follows the three equations below:
- 2NaOH(aq) + CO2(g) → Na2CO3. + H2O(l)
- Na2CO3(aq) + H2O + CO2(g) → 2NaHCO3 (s)
- 2NaHCO3(s) → NagCO3 + H2O(g) + CO2(g)
The NaHCO formed as a white ppt is filtered off, washed and heated to give the anhydrous Na2CO3. Na2CO3 is prepared industrially using the Solvay process.
Solvay process – industrial preparation
A concentrated sodium chloride solution is saturated with NH3 gas to produce ammoniacal brine this is allowed to fall into the top of a large tower. As the solution passes through a series of baffle-plates (baffles) it reacts with CO2 which is forced up the tower under pressure.
- NH3(aq) + CO2(g) + H2O → NH4HCO3(aq)
- NH4HCO3(aq) + NaCl(aq) → NaHCO3(s) + NH4Cl (aq)
The NaHCO3(aq) is filtered, washed and heated to yield anhydrous NaCO3, steam and CO2. the CO2 is used again in the tower.
- 2NaHCO3(s) heat → Na2CO3(s) + H2O + CO2 (g)
The soda ash (anh. Na2CO3) can be dissolved in hot H2O and re-crystallized as washing soda
- Na2CO3(s) + 10HsO (l)
The economic importance of the process.
- The CO2 required in the process is obtained by heating CaCO3 in a lime kiln.
CaCO3(s) → CaO(s) + CO2(g)
- CaO(s) + 2 NH4Cl(aq) → CaCl2(aq) + H2O (l)
The NH3 is recycled. The raw materials are NaCl, CaCO3. The only waste product is CaCl2.
- Write an equation to show the reaction of sodium with air.
- Explain briefly the economics of the Solvay process.
- Explain the reaction of sodium with cold water.
- Explain what happens to the following when they are exposed to the atmosphere; (a) washing soda (b) caustic soda pellets
- Which of these is not an alkali? (a) Sodium hydroxide (b) Potassium hydroxide (c) Aqueous ammonia (d) None of the above
- Which compound is added to fused NaCl to lower its melting point? (a)CaCO3 (b) CaCl2 (c) Ca(OH)2 (d) Ca(NO3)2
- NaHCO3 is also called? (a) Chalk (b) Baking powder (c) Lime (d) Slaked lime
- What is the colour of sodium flame (a) white (b) red (c) black (d) green
- Sodium reacts explosively with cold water to liberate (a) Hydrogen (b) CO2 (c) O2 (d) Na2
- (a) Explain the Solvay process (b) What is the function of NH3 in the process
- Why is it necessary to concentrate the ore before extraction of any metal?
New School Chemistry by O.Y Ababio pages 419-442.
In our next class, we will be talking about the Extraction of metals. We hope you enjoyed the class.
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