Chemistry Form 5 Selected Compounds Of Metals

Selected Compounds Of Metals

(i.e COMPOUNDS OF Na, Mg, Ca, Al, Fe, Zn, Cu AND Pb) METAL OXIDES

Definition; An oxide is binary compound made up of oxygen and other elements, example MgO, PbO, Al2O3, H2O2, NO2, SO2 etc

Therefore metal oxides are binary compounds made up of oxygen and metal, example PbO, FeO,

, etc

NOTE: The binary oxygen – fluorine compounds are not called oxides of fluorine, but are called fluorides of oxygen since fluorine is more electronegative than oxygen (example OF2- oxygen difluorine)

GENERAL METHODS IN PREPARATION OF METAL OXIDES.

There are two methods of preparing metal oxides

1. DIRECT METHOD
2. INDIRECT METHOD

DIRECT METHOD OF PREPARATION OF METAL OXIDES

In this method a metal reacts with a reagent or oxygen or air to give metallic oxide

1. A metal oxide may be prepared by burning a metal in air or oxygen

1. A metal oxide may be prepared by passing steam on red heat metal

1. A metal oxide may be prepared by reacting a metal with an oxidizing agent like HNO3

INDIRECT METHOD OF PREPARATION OF METAL OXIDES

In this method, the metal oxide is obtained by heating carbonates, hydroxides and nitrates etc

Example;

TYPES OF METALLIC OXIDE

The metal oxides may be classified as follows:

1. BASIC OXIDE
2. ACIDIC OXIDES
3. AMPHOTERIC OXIDES
4. PEROXIDES
5. SUPEROXIDE
6. MIXED OXIDES
7. BASIC OXIDES

These are oxides which react with acids to form salt and water only. They also combine with acidic oxides to form salts. Basic oxides may be ionic or covalent

ACIDIC OXIDE

Acidic oxide are formed by metals in their higher oxidation states, example

These oxides are generally covalent in nature. They dissolve in water to form oxy-acids and hence are called acid anhydrides.

1. AMPHOTERIC OXIDE

These are oxides with both basic and acidic properties. They react with both acids and bases. Amphoteric oxides include ZnO, Al2O3, BeO, PbO, SnO2, Cr2O3,

1. AS BASES: They react with acids to form salt and water only

1. AS ACIDIC OXIDES: These react with bases to form salt and water

Sodium metastannate

Sodium ferrite

1. PEROXIDES

Peroxides are compounds containing the peroxide ion Na2O2 Peroxides of alkali metals and alkaline earth metals can be prepared.

1. By heating the metal in the presence of excess oxygen or air

1. By heating the monoxides of the metal alone or in the presence of oxygen/air.

1. By the action of oxygen or air on the metal dissolved in liquid ammonia. This method is for the preparation of K2O2, Rb2O2 and Cs2O2
2. By the action of H2O2 on metallic salt solution in the presence of an alkali.

PROPERTIES OF PEROXIDES

• • Stability of peroxides increases with increasing of the electropositive character of the metal
  • Peroxides are more stable in dry state than when they are in solution form
  • Many peroxides are highly hydrated due to hydrogen bonding,

example Na2O2.8H2O, CaO2.8H2O, BaO2.2H2O etc

• • They dissolved in water to form alkaline solution and hydrogen peroxide

(Ice-cold water)

• • When treated with dilute mineral acids peroxides give H2O2

• • Peroxide give O2 on heating and hence act as oxidizing agents

NB: PbO2 is not peroxide since this oxides does not give water when treated with mineral acids.

1. SUPEROXIDES

These are oxides of metals containing the ion the known superoxides are these of potassium (KO2), Rubidium (RbO2) and Caesium (Cs2O2)

Preparation

Superoxides of K, Rb, and Cs can be prepared by burning the metal in excess of oxygen or air

PROPERTIES OF SUPEROXIDES.

• • They are yellow solids
• The stability of these superoxides is in the order KO2 Rb CsO2
• They are strong oxidizing agents
• They are hydrolyzed by water forming H2O2 and oxygen.

Ionically;

• Superoxides are paramagnetic in nature due to the presence of one unpaired electron in ion.

1. MIXED OXIDES

These are oxides composed of two simple oxides. The two simple oxides may be of the same metal or different metal in different oxidation states, example Red–Lead (Pb3O4) is combination of 2PbO and. Due to this can be written ionically

Also magnetite (Fe3O4) is a combination of FeO and Fe2O3 The mixed oxides with different metals are as follows; Magnesium ferrite (MgFe2O4) MgOFe³⁺2O3

ZnFe2O4 nOFe23+O3 (Zinc ferrite)

PROPERTIES

• It is a brilliant scarlet (bright red) solid insoluble in water
• Red lead behaves chemically as if it were a loose compound of lead monoxide (PbO) and lead dioxide ( PbO2). For example it reacts with dilute HNO3 on warming to give and water whereby PbO2 is left with no reaction

• The reaction above is a redox reaction indicating that is a reducing agent

USES; Red-lead is used as pigment in oil plants

FERROUS –FERRIO OXIDE (Fe3O4) (FeO.Fe2O3)

Fe3O4 occurs naturally as Magnetite. It may be prepared by heating iron with oxygen or steam

PROPERTIES

• The compound is black in color
• Very strongly ferromagnetic
• The compound is inactive chemically
• React with acids as a double oxide giving a mixture of ferrous and ferric salts in solution

HYDROXIDES OF THE METALS

These are compounds of metals which contains hydroxide ions (OH^–) as the only negatively charged ion, example NaOH, Mg(OH)2, Zn(OH, Fe(OH)3

PREPARATION OF METAL HYDROXIDES

There are two methods of preparation of metal hydroxides.

DIRECT METHOD OF PREPARATION OF METAL HYDROXIDES

The hydroxides which can be prepared by this method are those composed of strongly electropositive metals. Example LiOH, NaOH and Ca(OH)2

Example

INDIRECT METHOD OF PREPARATION OF METAL HYDROXIDES

In this method the metal hydroxide is prepared by

• 1. The action of water on the metal oxide;

example

NB: the metal hydroxides which are prepared by action of water on metal oxides are soluble in water.

• 1. Action of calcium hydroxides (milk of lime) on a solution of carbonate, example preparation of NaOH and KOH. These metal hydroxides are prepared by precipitation the unwanted ions and layering a solution of the alkali. For instance when potassium carbonates and (Ca(OH) solution are mixed and then allowed to settle, a solution of potassium hydroxides may be decanted

• 1. Precipitation of a metal hydroxide by adding ammonia solution or sodium hydroxide solution to a solution of salt of the metal

• 1. Electrolysis of a solution of the metal chloride, example preparation of NaOH. Alkali metal chlorides form conducting solutions and since these metals are highly in the electrochemical series, their ions remain in solutions during electrolysis and hydrogen evolved at the cathode. Preferential discharge of chlorides ions enable hydroxyl ions formed by ionization of the water to accumulates in the solution. As a result dilute solution of the metal hydroxide is produced

PROPERTIES OF METAL HYDROXIDES OF THE SELECTED METALS

1. ALKALI METAL HYDROXIDES (MOH) PHYSICAL PROPERTIES

-The hydroxides of group IA metals are white crystalline solids

-They melt at moderate temperature without decomposition except (LiOH)

-They are deliquescent solids

-They are very soluble in water (form alkali solutions)

CHEMICALS PROPERTIES

-The basic strength of the alkali increases down the group, example calcium is the strongest base

-When cold and dilute alkali’s reacts with chlorine to form metal chloride and hydrochlorite

-When hot and concentrated alkali’s react with chlorine to form metal chloride and chlorate (v)

In the two reactions chlorine undergoes disproportion

– Most NaOH and KOH absorbs CO2 from the air whereby a metal carbonates is formed

• The hydroxides of Group IA metal reacts with acids to form salts and water only, example undergo neutralization reaction

USES OF HYDROXIDES OF Na AND K

• 1. Owning to their highly basic character alkali metal hydroxides are used to absorb acidic gases, example CO2
  2. Alkali metal hydroxides are used in neutralization reaction.

Example

• 1. Alkali metal hydroxides are used in precipitation reaction

Example

• 1. Caustic soda (NaOH) is used in the manufacture of silk, paper and soap
  2. Caustic potash (KOH) is used to manufacture soft soaps

ALKALINE EARTH METAL HYDROXIDES (M(OH)2) PHYSICAL PROPERTIES

• They are white crystalline solids
• Solubilities increases considerably down the group from beryllium hydroxides (Be) to barium hydroxides (Ba). Beryllium hydroxide is insoluble in water.

Solubility of calcium hydroxide decreases with rise in temperature, the others increase, magnesium slightly but strontium and barium hydroxide greatly. Increase in solubility down the group is due to the fact that lattice energy decreases faster than hydration energy (Be() is essentially covalent because of the high polarizing effect of the small

• Group IIA hydroxides are much less soluble

-The hydroxides of Na and K precipitates some metals from their soluble salts (example, Aqueous solutions of their salts) as hydroxides

– Both NaOH and KOH liberates ammonia gas when added to ammonium salts

REACTION WITH AMPHOTERIC METALS

Zinc, Aluminium, Lead and Tin react with hydroxides of sodium and potassium to form complexes, example aluminate, plumbate, zincate and stumnate.

Aluminate ion

Zincate ion

Plumbate ion

REACTION WITH CARBONDIOXIDE

When CO2 is bubbled through aqueous solutions of the NaOH and KOH the carbonates are formed,With excess of the CO2the hydrogen carbonates are formed.

Of group IA elements due to the decrease in metallic character of the elements (example Group IA elements are more electropositive than their corresponding Group IIA elements). Also the decrease in solubility may be due to decrease ionic character of the hydroxides from Group IA to Group IIA

NB; A Suspension of slaked lime(calcium hydroxide) in water is called Milk of lime

CHEMICAL PROPERTIES

1. ACTION WITH ACIDS AND ALKALIS
   • Beryllium hydroxide is amphoteric. It reacts with excess sodium hydroxide forming a solution of sodium beryllate

Sodium beryllate

The other hydroxide of group IIA metals do not react with alkalis but react with acids to form salt and water only

ACTIONS WITH CARBON DIOXIDE

• • Moist hydroxides absorb CO2 from air forming carbonates

• When is bubbled through lime water (Ca(OH)2) white precipitate of CaCO3 are formed. This causes the lime water to turn milky. The milky colour disappears when excess CO2 in bubbled through it. The milky colour disappears because calcium carbonates is converted into calcium hydrogen carbonate which is soluble in water

Clean solution White precipitate (milky)

(Milky) (Excess) Clear solution

ACTION OF HEAT

The temperature at which the hydroxides begin to decompose increases down the group from about 300⁰C for beryllium hydroxide and magnesium hydroxide to about 700⁰C for barium hydroxide.

Example

ACTION WITH AMMONIUM SALTS

All the hydroxides except Be(OH) 2 react with aqueous ammonium salts to give ammonia gas. The ammonia gas is easily identified because it turn alkaline to litmus paper.

Example

ACTION WITH SULPHUDIOXIDE

Sulphur dioxide turn lime water milky due to calcium sulphite formed. When excess SO2 is added the milky colour disappears (example, a clear solution is formed). The milky colour disappears due to the formation of calcium bisulphite which is soluble in water.

USES

1. Lime water is used to test for carbon dioxide.
2. A suspension of Magnesium hydroxide in water (milk of magnesium)is used as an ant-acid.
3. Ca(OH)2 is used in making builders mortar (mixture of slaked lime, sand and water).
4. A mixture of Ca(OH)2 is used in making bleaching powder.
5. Ca(OH)2 is used for neutralizing acids in the soil.
6. A mixture Ca(OH)2 and water (white wash) is used for coating walls and ceiling.
7. Ca(OH)2 is used in water softening.

1. Ca(OH2) is used in sugar refining filtered.