6.  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
             

 

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

 

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           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.

A)   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

   

B) INDIRECT METHOD OF PREPARATION OF METAL HYDROXIDES

  In this method the metal hydroxide is prepared by

a) 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.

b) 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

                                         

c)  Precipitation of a metal hydroxide  by adding ammonia solution or sodium  hydroxide solution  to a solution of salt of the metal

              

             

              

              
             

  d)  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.

 

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      Example

   

1. Alkali metal hydroxides are used in precipitation reaction

 

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       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

 

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 2. 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 CO₂
the 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

 

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                                                                      Sodium beryllate

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

              

              

 2. 
ACTIONS WITH CARBON DIOXIDE

•  Moist hydroxides absorb CO₂ from air forming carbonates

 

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–  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

 3.  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

           

4.  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            

          

          

 5. 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.

      

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