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chemistry-Acid, base and salts

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Introduction

Acids and bases are terms that form part of our daily language.

They are substances that are part of our daily lives and are found in every where; in fruit juice,

our blood, cellular fluid, digestive juice, the food we eat, in the kitchen and in the laboratory.

Some of the acids and base that occur in nature include the hydrochloric acid

in the human digestive juice, acetic acid in vinegar,citric acid in lemon,

the amino acids of the protein in our body etc.

The known classes of acids are mineral acids (inorganic acid), e.g,

hydrochloric acid (HCL), nitric acid (HNO3), sulphuric acid (H2SO4)

and organic acids, e.g, acetic acid, citric acid, lactic acid found in milk,

fatty acids found in fats and oil, etc.

They are examples of naturally occurring organic acids.

One of the areas where acids and bases are of immense application is in the soap and paper production industries where large quantities of NaOH are used daily.

Sulphuric acid is an important component in the manufacture of car battery,

while nitrate and phosphate salts are important component in the manufacture of car battery,

while nitrate and phosphate salts are important components in fertilizers.

The term salt is often referred to as sodium chloride (the common table salt),

a product from acid-base reaction that is used for food seasoning and preservation.

Salt is one of the products , apart from water that is formed when an acid is neutralized by a base.

Examples include sodium sulphate, potassium nitrate, calcium bisulphate,

ammonium chloride, sodium carbonate, etc.

A salt consists of a cation in ion-pair association

with and anion to give a compound with a zero net charge.

Properties of and acid

An acid in an aqueous medium will exhibit some or all of the following properties.

Some of these properties can be observed in non-aqueous medium also.

  1. It will have a sour taste. The sour taste of lemon is due to citric acid in it.
  2. An acid ionizes in solution to produce H+ ions.
  3. An aqueous solution of an acid that is totally or partially ionized will conduct electric current.
  4. It is corrosive to the skin.
  5. It can change the colour of a wet litmus paper from blue to red.
  6. It will liberate CO2 from carbonates e.g.

CaCO3(s) + 2HCL(aq) —–> CaCl2(aq) + H20(l) + CO2(g)

      7.  The non-oxidizing acids like HCL, will liberate H2

from metals that are above hydrogen in the reactivity series.

2HCL(aq) + Zn(s) ——-> ZnCl2(aq) + H2(g)

HNO3 is an oxidizing acid and does not liberate H2 from metals.

     8.  Acids react with metals oxides and hydroxides to form salts and water only.

This reaction is called neutralization.

HCL(aq) + NaOH(aq) —–> NaCl(aq) + H2O(l)

Properties of base

Some examples of bases include sodium hydroxide (NaOH),

potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)2).

Some oxides that possess basic properties include sodium oxide(Na2O), calcium oxide (CaO) etc.

In aqueous solutions a base will exhibit the following properties:

  1. It will have a bitter taste.
  2. A base ionizes in solution to produce OH ions.
  3. An aqueous solution of a base that is totally or partially ionized will conduct electric current.
  4. It is corrosive to the skin.
  5. It can change the colour of a wet litmus paper from red to blue.
  6. Bases react with acids to form salts and water only.

This reaction is called neutralization.

KOH(aq) + HNO3(aq) —-> KNO3(aq) + H2O(l)

    7. A base will present a soapy feeling when touched.

The Arrhenius Theory

The properties presented above for acids and bases formed the basis for the formulation

of the arrhenius theory about acid and bases in 1884 by Svante Arrhenius.

According to this theory an acid is defined as a substance that produces hydrogen ion

in an aqueous medium and a base is a substance that produces hydroxyl ion in an aqueous medium.

This theory adequately explains the reaction between mineral acids like HCl and H2SO4,

and bases such as NaOH and KOH. Thus, neutralization ,

according to Arrhenius theory can be defined as the reaction

between a proton H+ and a hydroxyl group OH to form water H2O.

H+(aq) + OH(aq) ——> H20(l)

Other studies have revealed that the proton H+ generated in solution in hydrated due to the attraction

between H+ and oxygen atom of the water molecule to form the hydronium ion H3O+.

The oxygen atom of the water molecule is partially negatively charged.

Hence, hydration reaction between H+ and H20 is,

H+(aq) + H2O(l) —-> H3O+(aq)

Acids under this classification are often referred to as protonic acids.

The relative strengths of some acids are presented on table below:

and HI is the strongest acid.

The strength of the acid decreases down the table and water is the weakest acid among the acids on the table.

Relative strengths of some acids


                                                                                                                                                                                                                                   Acid Name                                                                                  Formula


Hydroiodic acid                                                                                                     HI                                                          (strongest acid)

Perchloric acid                                                                                                      HCLO4                                                                                                                                                       

Hydrobromic acid                                                                                                 HBr                                            

Hydrochloric acid                                                                                                 HCl

Sulphuric acid                                                                                                        H2SO4

Chloric acid                                                                                                            HCLO

Nitric acid                                                                                                               HNO3

Hydrofluoric acid                                                                                                   HF

Nitrous acid                                                                                                            HNO2

Acetic acid                                                                                                               CH3COOH

Carbonic acid                                                                                                         H2CO3

Hydrosulphuric acid                                                                                                H2S

Water                                                                                                                           H20

AMPHOTERISM

The term amphoterism is used for the description of a substance

that is capable of reacting either as an acid or as as base.

Many metal oxides and hydroxides exhibit amphoteric properties e.g,

aluminium hydroxide.

1
Hydroiodic acid
 (strongest acid)
HI
2
Perchloric acid
HCLO4
3
Hydrobromic acid
HBR
4

Hydrochloric acid

HCL
5
Sulphuric acid
   H2SO4

 

1
Chloric acid
   HCLO
2
Hydrofluoric acid
HF
3
Nitrous acid
HNO2
4
Acetic acid
CH3COOH
5
Carbonic Acid
H2CO3
1
Hydrosulphuric acid
H2S
2
Water
H20
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