Form 3 Chemistry – ACIDS, BASES AND SALT

BASE

A base is a substance which contains the oxide ion or hydroxide ion and reacts with acid to give salt and water only

                 Or

A base is a soluble substance which accepts a proton

A soluble base is called Alkali

The Alkali’s are

 The solution of Ca(OH)₂ in water is called lime water

          
Magnesium hydroxide (milk of magnesia ………Mg(OH)₂

It’s also sparingly soluble

The soluble bases when in solution produce OH- as their only negative ion

                

PROPERTIES OF BASES

1. Alkalis have a soapy feeling (slippery)

2. Alkalis have bitter taste

3. Alkalis turn red to blue litmus paper

4. They react with acids and water only

Question: which bases combine to form NaCl?

5. Alkalis yield ammonia when warmed with ammonium salts

           

USES OF BASES

 

QUESTION:

Calculate percentage of hydrogen in compound (HCL)

H  +   Cl

1  +  35.5 = 36.5 (atomic masses)

H%  = 1/36.5  x 100%  

Cl%  = 35.5/36.5 x 100%

What kind of equation is?

Answer: Special case reaction

ACIDS

These are the substances which when in solution produce hydrogen ions as their only positive ions

An acid is a substance which turns blue litmus to red and which contains hydrogen replaceable by a metal

TYPES OF ACIDS

There are two types of acids i) Mineral acids

                                               ii) Organic acids

 A: Mineral Acids

These are derived from substances formed from minerals, for example; from minerals

 
 

The mineral salts that can be derived from the oxides are called oxy – acids (from the acidic/ non – metal oxides)

B. ORGANIC ACIDS

These are derived from plants and animals source animal source (organic sources)

Example;

           
 

 
 

STRENGTH OF ACIDS

Strength of acids are classified into two i) Strong acids

                                                         ii) Weak acids

STRONG ACIDS

These are acids which ionize completely in aqueous solution

WEAK ACIDS

These are acids which ionize partially in aqueous solution

          

All organic acids are weak

QUESTION;

Is phosphoric acid a strong or weak acid? What are strong bases or weak bases?

PROPERTIES OF ACIDS

1. Have sour taste

2. Action on indicators

Indicator is a substance which changes color in acids or bases

 Example; litmus, methyl orange, phenolphthalein etc

 
 

3. Corrosive action
    A concentrated acid is a water molecule aspires. Acid dries any substance of water content in vigorous action, thus burns skin of animals, leaves and stems of plants.

4. Action with metal

Any metal which is above hydrogen in the activity series will replace hydrogen from the acid. The products of these reactions are salt and hydrogen gas

          

5. Action with metal oxides and hydroxides to form salt and water only

                               

6. Action with carbonates and hydrogen carbonates (bicarbonates)

    Acids react with carbonates and bicarbonates producing salt, water and CO₂

7. Action with sulphates

    When acids react with sulphates, SO₂, water and salts are produced

BASICITY OF ACIDS

The basicity of an acid is the numbers of hydrogen ions (H⁺) produced by one mole of an acid in aqueous solution

                                                                                                                                                                                                                                                                              

                              

NB: All organic acids are monobasic

  P^H SCALE;-

It isn’t possible to know exactly how strong an acid or base is by using an indicators. The term PH is used to describe or show the exact strength of an acid or base. It shows the number of hydrogen ion concentration

The P^H scale can be represented diagrammatically as show below;     

                                                           

1-4 Strong acid

5-6 Weak acid

7 Neutral

8-9 Weak alkali

10-14 Strong alkali    

USES OF ACIDS

Acids have the following uses;

a) Preparation of salts, for example; from fertilizers

   From bases / metals.
   Ca(OH)_{2 †} 2HCl → CaCl₂
† 2H₂O
 
   NH₄OH + H₂SO₄ → (NH₄)₂SO₄ + 2H₂O

b) Preparation of other acids;
   
      e.g.Pb(NO₃)₂ + H₂SO₄ → PbSO₄ + 2HNO₃

Example;

c) Manufacture of artificial silk                                                                      

d) Clearing of metals, acids can combine with them to form salts can be easily removed out

e) Necessary for digestion of protein in the stomach.

SALTS

Definition: A salt is a compound consisting of a positive metallic ion (or) and negative ion derived from an acid

Types of salts;

I) Acidic salts

ii) Normal salts

iii) Basic salts

1. Acidic salts
   

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This is the salt which contains part of the hydrogen ions of an acid

Example; Sodium hydrogen – sulphate

               Calcium hydrogen carbonate

     2. Normal salts

These are the salts which contain neither H⁺ ions from the acid nor O^2- or OH^– ions from the base

Example; NaCl, MgCl₂,MgSO₄ ,FeCl₂ ,CaCO₃

     3. Basic salts

These are salts containing or ions from the base  

Example; MgOHCl, ZnOHCl

Magnesium hydroxyl chloride and zinc hydroxyl chloride

O is incorrect because oxygen exists in molecule state but not in atomic

In previous salts was classified according to the nature of salts but in terms of solubility there are two types of salts (i ) Soluble salt   ( ii ) Insoluble salt.

SOLUBILITY RULE states;

Soluble salts are;-            

a) All salts of (made of) K, Na and NH⁺₄

Example; KNO₃,KCl ,K₂SO₄ ,K₃PO₄ ,K₂CO₃,NaHCO₃

This is an acidic salt (because it gives out 2 positive ions of Na and H)

1. All nitrates
   

   Example; KNO₃
   

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ii) All hydrogen carbonates

Example; NaHCO₃

b) All chlorides except                                

         i) Silver chloride (AgCl)

         ii) Mercury (I) Chloride (HgCl)           iii) Lead (II) chloride (PbCl₂) which is soluble in hot water

N.B: In reactions the compound above are considered as solids (s)

c) All sulphates except

            i) – Barium sulphate.

            ii) – Lead (II) sulphate.

            iii) – Calcium sulphate is slightly soluble in water.

Insoluble salt are;

a) All carbonates except those of K, Na, NH₄

b) All hydroxyl except those of K, Na, and Ca (which is slightly soluble in water)

   Salt of strong acid and bases do not react with water

Solubility rule (summary)

i) All salts of K, Na and are soluble

ii) All nitrates are soluble

iii) All sulphates are soluble except those Ba, lead (II) and Ca

Vi) All chlorides are soluble except those Ag, lead (ii) and mercury (x)

All carbonates and hydroxides are insoluble except those of K, Na,

NB: – All acids are soluble unless when concentrated (aq)  

       – All oxides are labeled by solids (s)

Examples;

METHODS OF PREPARING SALTS

A: SOLUBLE SALTS

i) Direct combination of elements

  Example; Burning Mg in chlorine gas

                   Mg + Cl₂ → MgCl₂

                     Pass hot Al in Chlorine gas.

                   2Al_(hot) + 3Cl → 2AlCl₃

ii) Addition of a metal to a dilute acid. But this method is restricted to metals above hydrogen in the reactivity series.

A metal below hydrogen cannot displace hydrogen

Example;
              Zn + 2HCl₂ + H_2(g)

              Na + H₂SO₄ → Na₂SO₄ + H_2(g)

                  
 

1. Addition of carbonates to a dilute acid.
   

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          Products = salt, water and carbon dioxide.
          
          CaCO_3(s) + 2HCl → CaCl_2(aq) + H₂O_(l) + CO_2(g) 

                        
       2. Addition of an oxide to a dilute acid.

        CaO + H₂SO₄ → CaSO₄ + H₂O_(l) + H₂O        

  v) The general method for preparing soluble salts is called crystallization, for example; Preparation of ZnSO₄

Procedure;

a) Add excess Zn to dilute in a beaker Add few crystals of and few drops of concentrated to speed up the reaction rate

1. If the reaction is still slow warm the mixture
   
2. When the reaction is over, filter and put the filtrate in an evaporating dish
   

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c) Crystallization

The filtrate is evaporated and cooled. Check if the crystals are formed. If they are formed stop evaporating and leave it for clay.

The water attached or associated to the crystals is called water of crystallization.

    ZnSO₄.7H₂O is called zinc sulphate hepta hydrate.

B. INSOLUBLE SALTS

The insoluble salts are prepared by ionic precipitation also called double decomposition

Here two soluble salts are mixed and react by interchanging their radicals ion forming both soluble and insoluble salts.

   BaCl_2(aq) + Na₂SO_4(aq) → BaSO_4(s) +2NaCl
  
   Ba²⁺ _{(aq) +} 2Cl_(aq) + 2Na⁺_(aq) + SO₄^2-_(aq) → BaSO_4(s) + 2Na⁺_(aq) + 2Cl^–_(aq)

   Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_4(s)

              
 

QUESTION;

Ions to get solid, what is the name of the reaction? It’s a double decomposition reaction or ionic precipitation.

PROPERTIES OF SALTS

1. COLOUR

Some salts are coloured

For example; i) Iron (ii) salts are green

     ii) Iron (ii) salts are yellow

     iii) Iron (ii) salts are pale blue

     iv) Nickel (ii) salts are green

     v) Cobalt (ii) salts are pink

The colours of the above mentioned salts are due to the color of hydrated ion

Coloured salts are formed only in the group of transition elements. All elements in first 20 elements are not colored.

Other salts are not colored, for example; salts of Na, K, Ca, Pb, Zn, Al, Mg

2. Hydrolysis

This is the reaction of salts and water giving an acid or alkaline solution

 Example:

  NH₄Cl_(aq) + H₂O → NH₄OH_(aq) + HCl_(aq)

  Na₂CO_3(aq) + H₂O_(L) →  NaOH_(aq) + H₂CO_3(aq)

a) All salts of weak bases and strong acids hydrolyze to give acid solution

    Example; NH₄, Cl, FeCl₂, CuCl₂,Al₂(SO₄) _etc.

b) All salts of strong bases and weak acids are hydrolyzed to give an alkaline solution

Example; Na₂CO₃ ,CH₃COONa, etc.

N.B: Salts of strong bases and strong acids are not undergoing hydrolysis. They only ionize when in solution:

         Example; NaCl + H₂O → There’s no reaction.

3.
EXPOSURE TO AIR

When salts are exposed to air they either lose water of crystallization or absorb water from the atmosphere

a) Hygroscopic

Is the action of absorbing water from the atmosphere without changing into solution NaCl, anhydrous copper II sulphate (CuSO₄) and NaNO₃ others are concentrated and copper II sulphate

b)
Deliquescent

This is the action of absorbing water from the atmosphere by a solid to form a solution

            Example; MgCl₂, CaCl₂, FeCl₃, Ca(NO₃)₂

            Others are the hydroxides of K and Na

c) Fluorescence

This is the action of giving up water of crystallization of the solid to the atmosphere

Examples;

 i) Hydrate sodium carbonate
 
      ^Na₂^CO₃^.10H₂^{O → Na}₂^CO₃^.H₂^{O + 9H}₂^O

ii) Hydrated sodium sulphate

        
    Na₂SO₄.10H₂O → Na₂SO₄ + 10H₂O

iii) Hydrated magnesium sulphate

       
         MgSO₄.7H₂O → MgSO₄.H₂O + 6H₂O           

4.
HEAT EFFECTS ON SALTS

Different salts behave differently on heating. Most of the hydrated salts loose water of crystallization when heated. The anhydrous salts undergo chemical change when heated

     

c) When iron (ii) sulphate is heated strongly it decomposes to form black iron (iii) oxide, and SO₃ and SO_2(g)

             
 

                        2FeSO₄ → Fe₂O_3(s) + SO_3(g) + SO_2(g)
                            White      black

d) Iron (iii) sulphate when heated strongly it decomposes to form black iron (iii) oxide and SO₃ only.

                     Fe₂(SO₄)_3(s) → Fe₂O_3(s) + 3SO_3(g)   

                  White                     black

II) Chlorides

All chlorides of metals are hydrated except those of K, Na, Pb, Hg and Ag when heated, chlorides undergo a chemical change called hydrolysis (i.e. they don’t form anhydrous chlorides) in which hydrogen chloride gas (and water involve and a basic salts of chloride or oxide are formed       

Example;

          MgCl₂.6H₂O_(s)  MgOHCl_(s) + HCl_(g) + 5H₂O_(g)           

           
          AlCl₃.6H₂O_(s)
AlO_3(s) + 8H₂O_(g) + 6HCl_(g)                                                                                    
          
           ZnCl₂.6H₂O_(s)
  ZnOHCl_(s) + HCl + 5H₂O_(g)
       
             
             Ammonium chloride sublimes when heated

            NH₄Cl_(s)
NH_3(g) + HCl_(g)

                    
 

III) Carbonates and hydrogen carbonates

Carbonates of sodium and potassium are unaffected by heat (even at very high temperature)

Ammonium carbonate decomposes readily on heating to form NH_3(g), CO_2(g) and H₂O_(g)                                               
             (NH₄)₂CO3_(s)
2NH_3(g) + CO_2(g) + H₂0_(g)

           
            All other carbonates decompose to give oxide and CO₂

                Al₂(CO₃)₃
Al₂O₃ + CO₂

               K2CO3 NO Reaction

            All hydrogen carbonates decompose to give metal carbonates water and CO₂ on heating

           
                Mg(HCO₃)₂ → MgCO₃ + H₂O + CO₂
        
                      2NaHCO₃ → Na₂CO₃ +H₂O + CO₂  

IV) Nitrates

 
 

    Na and K nitrates decompose when heated to give corresponding nitrite and oxygen

    NaNO₃ → NaNO₂ + O₂

    KNO₃ → KNO₂ + O₂

– Ammonium nitrate decomposes on heating to give dinitrogen oxide and water

  
     NH₄NO₃ → N₂O + H₂O
 

 – While the nitrite gives nitrogen and water

   NH₄NO₂ → N₂ + H₂O

 – The metal nitrates (i.e. those of Ca, Mg, Al, Zn, Fe, Pb, Cu) decompose on heating to give the corresponding oxide, nitrogen dioxide and oxygen

 
   Ca(NO3)_2(s) → CaO_(s) + NO_2(g) + O_2(g)

   Mg(NO₃)_2(s) → MgO_(s) + NO_2(g) + O_2(g)

  – Nitrates of heavy metals (Ag and Hg) decompose to give a metal, NO₂ and 0₂ on heating.

   AgNO₃ → Ag + NO₂ + O₂

    Hg(NO₃)₂ → Hg + 2NO₂ + O₂  

 
 

V) Hydroxides

The hydroxide of Na and K are stable to heat ie. Don’t decompose on heat even at very high temperature.

All other metal hydroxides decompose on heating to give the corresponding oxides and water vapor

         KOH No Reaction

    Mg(OH)₂
MgO + H₂O

5. SOLUBILITY AND SOLUBILITY CURVES

Solubility of a salt in a liquid is the maximum amount of the salt that will dissolve in 100cm³ of a liquid at any given temperature

Solubility Curve

These are the graphs which show the variation of solubility with temperature

Solubility of a salt increases with the increase in temperature.

The steeper the solubility curve the more soluble the salt and the easier it is to crystallize that salt

               
                 SOLUBILITY AND SOLUBILITY CURVES.

  

The diagram above is a graph of solubility against temperature. The vertical component is a solubility of substance in gram per dm³ , where by horizontal component is temperature of a substance. The graph drawn in the plane is analysis of two related variables (solubility and temperature). The change of solubility related to change of temperature.
    From the accumulated data due to the test of salts NaCl, KCl and KNO₃ analyzed in the graph as drawn above.
(i) A graph of NaCl interpret that, it has a constant solubility at any temperature. The increase or decrease of temperature does not affect its solubility.

(ii) A graph of KCl is a smooth linear graph which interpret that for every change of temperature give the effect change of solubility. Therefore KCl is more soluble than NaCl.

(iii) A graph of KNO₃ is a curved graph interpret that the change of temperature gives a rapid  solubility. Therefore a salt KNO₃ is more soluble than NaCl and KCl.

Assignment;
Take a salt of NaNO_3(S)  to examine its solubility.

Procedure
(i) Measure 1litre of distilled water and pour it into the beaker.

(ii) Measure 1g of NaNO₃ by electronic balance and pour into the beaker.

(iii) By heat source, tripod stand, wire gauze, retort stand and thermometer make connection and heat both water and sample salt.

(iv) Watch a disappearance of salt particles until all finish and record the temperature as follows;
     

 

(v) Repeat procedure (ii) for except 2,3,4,5,6,7 and 8

(vi) Draw a graph of solubility against temperature.

             USES OF SALT IN DAILY LIFE
Salt is essential for life, because it has more than 14000 users in daily. But the common uses categorized into Food, Agriculture,Water conditioning, High way Deicing and Industrial chemicals.

1. FOOD
   
    i) A salt of sodium chloride is mixed with food as flavoring, (common salt).

   ii) A salt of sodium chloride (NaCl)is mixed in the food industrial product as both flavoring and preservative.

   iii) A salt of sodium bicarbonate (NaHCO₃) ‘bicarb’ is used in cooking as raising agent for cakes, bread e.t.c.
    
        Baking powder
        Is a mixture of sodium hydrogen carbonate and tartaric acid, The mixture helps to keep the PH neutral.

2. AGRICULTURE
    A salt is very important in agriculture since used as land additive nutrients. A fertilizers of Ammonium sulphate (NH₄SO_4(S)) and Sodium nitrate (NaNO₃) salt are used to make land fertility to help healthier growing of crops.

3. WATER CONDITIONING
   
    i) In the purification of urban water (Permutit process),the salt of Aluminum silicate is used to remove permanent hardness of water (Al(SiO₃)₃).

    ii) Sodium carbonate (Na₂CO_3(S)) called washing soda, is used to make water soft by replacing calcium ions by sodium ions.
   
    iii) Sodium chloride is used in a water softener to generate the ion exchange column.

4. HIGH WAY DEICING
   
    A salt of sodium chloride (NaCl) is mixed with grit and spread on roads to prevent road freezing in cold condition.

5. INDUSTRIAL USES
  
   i) The salt of potassium Iodide (KI) is added to sodium chloride (common salt) to prevent a lack of Iodine in the diet.

   ii) The salt of sodium carbonate is used in the manufacture of glass.