Metals and their Compound

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In today’s class, we will be talking about metals and their compound. Enjoy the class!

Metals and their Compound

Preliminary preparation

Metals are found mixed with earthy materials as ores. Ores are often

1. concentrated and
2. converted to oxides before extraction

The concentration of the ore can be done by any of the following ways:

1. washing away the earthy materials
2. 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
3. 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
K Na Ca Mg	Very reactive	Chlorides	Electrolysis of fused hydroxides and chlorides
		Chlorides & Trioxocarbonates	Electrolysis of fused chlorides
Al Zn Fe Sn Pb	Moderately reactive	Oxides CO32- and Sulphide	Roasting of CO32- & sulphides to form oxides reduction of oxides by C of CO
		sulphides
			Roasting in air
			Heating in air
Cu Hg Ag Au	Least reactive	Free element	Mined as free elements

Evaluation

1. Explain froth floatation
2. State two ways of concentrating an ore

Sodium

Occurrence

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:801^oc) 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        →              Cl_{2 (q)}

Overall electrolysis reaction.

2Na⁺(g)  + 2Cl-(i)        →             2Na_(s)  + Cl_2(g)

Physical properties

1. Silvery solid with metallic luster
2. Floats on water (density of 0.98)
3. very malleable
4. melting point of 97^oC (Low for a metal)
5. Good conductor of heat and electricity.

Evaluation

1. Write the cathodic and anodic half cell equations in the electrolysis of fused NaCl
2. What is the function of CaCl₂ introduced into the electrolytic cell during the electrolysis above?

Chemical properties

1. Reaction with air:

4Na (s)   + O₂(g)       →             2Na₂O(s)

Na₂O(s)  + H₂O(g)        →            2NaOH (aq)

2NaOH (aq) + CO₂(g)        →           Na₂CO_3(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) + O_2(g)          →             Na₂O_2(g)

2. Combination reaction: with H₂, Cl₂, S, P (with non-metals except Boron, carbon & Nitrogen)

2Na_(s)  + H_2(g)    →         2NaCl_(g)

2Na_(s) + Cl_{2(g)        →}           2NaCl_(g)

2Na_(s) + S_(g)        →           Na₂S_(s)

3Na_(s)  + P_{(g)        →}              Na₂P_(s)

In Hg

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

4. With acid:

2Na_(s)  + 2HCl_(aq)       →              2NaCl_(aq)  + H₂O

NB: The reaction is explosive and extremely dangerous

5. With ammonia:

2Na_(s)   + 2NH_{3 (g)}       →           2NaNH_2(s)  + H_{2 (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

1. It is used in manufacturing important compounds such as tetraethyl lead (Iv). (Pb(C₂H₅)₄
2. Sodium vapour lamps (orange-yellow light) are used to light high ways and airports
3. it is used in liquid form as a coolant in nuclear reactors
4. Can be used as a reducing agent in combination with ethanol or sodium amalgam.
5. 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)

Na₂CO₃ exists.

1. As soda ash in the anhydrous state
2. As a monohydrate, Na₂CO₃.H₂

• As a decahydrate (more often) Na₂CO₃.10H₂O called washing soda

The laboratory preparation follows the three equations below:

3. 2NaOH_(aq) + CO_2(g)           →     Na₂CO₃.   +     H₂O_(l)
4. Na₂CO_3(aq) + H₂O  +  CO_2(g)       →            2NaHCO_{3 (s)}
5. 2NaHCO_3(s)   →      NagCO₃ + H₂O_(g)  + CO_2(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.

1. NH₃(aq)  + CO_2(g)   + H₂O       →      NH₄HCO_3(aq)
2. NH₄HCO_3(aq) + NaCl_(aq)        →      NaHCO₃(s)   + NH₄Cl _(aq)

The NaHCO_3(aq)  is filtered, washed and heated to yield anhydrous NaCO₃, steam and CO₂. the CO₂ is used again in the tower.

• 2NaHCO₃(s) ^heat    →          Na₂CO_3(s)  + H₂O  + CO₂ (g)

The soda ash (anh. Na₂CO₃) can be dissolved in hot H₂O and re-crystallized as washing soda

1. Na₂CO_3(s) + 10HsO _(l)

The economic importance of the process.

1. The CO₂ required in the process is obtained by heating CaCO₃ in a lime kiln.

CaCO₃(s)        →               CaO(s)  + CO_2(g)

2. CaO(s) + 2 NH₄Cl(_aq)         →            CaCl_2(aq)   + H₂O _(l)

The NH3 is recycled. The raw materials are NaCl, CaCO₃. The only waste product is CaCl₂.

General evaluation

1. Write an equation to show the reaction of sodium with air.
2. Explain briefly the economics of the Solvay process.
3. Explain the reaction of sodium with cold water.
4. Explain what happens to the following when they are exposed to the atmosphere;     (a) washing soda (b) caustic soda pellets

Weekend assignment

1. Which of these is not an alkali? (a) Sodium hydroxide (b) Potassium hydroxide (c) Aqueous ammonia (d) None of the above
2. Which compound is added to fused NaCl to lower its melting point? (a)CaCO₃ (b) CaCl₂     (c) Ca(OH)₂  (d) Ca(NO₃)₂
3. NaHCO₃ is also called? (a) Chalk (b) Baking powder (c) Lime (d) Slaked lime
4. What is the colour of sodium flame (a) white (b) red (c) black (d) green
5. Sodium reacts explosively with cold water to liberate (a) Hydrogen (b) CO₂ (c) O₂ (d) Na₂

Theory

1. (a) Explain the Solvay process     (b) What is the function of NH₃ in the process
2. Why is it necessary to concentrate the ore before extraction of any metal?

Reading assignment

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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