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Revision of Acids, Bases And Salts For NDA

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Acids

`=>` Acids have a sour taste; for example, vinegar owes its sourness to acetic acid, and lemons and other citrus fruits contain citric acid.

`=>` Acids cause color changes in plant dyes; for example, they change the color of litmus from blue to red.

`=>` According to Arrhenius, an acid is a compound that releases `H^+` ions in water;

e.g. `HCl -> H^+ Cl^-`

`=>` According to Bronsted-Lowry concept, an acid is any molecule or ion that can donate a proton (`H^+`)

e.g. `undersettext(Acid)(CH_3COOH) → undersettext(Conjugate base)(CH_3COO^(-))+undersettext(Proton)(H^+)`

similarly `undersettext(Acid)(HCl) → undersettext(Conjugate base)(Cl^(-)) + undersettext(Proton)(H^+)`

A conjugate base is obtained, when an acid give up a proton (`H^+` ).

`undersettext(Conjugate base) ( text(Acid)) - H^+ → text(Conjugate base)`.

The acid and base which differ by a proton are said to form a conjugate acid-base pair. Generally, a strong acid has a weak conjugate base and a weak acid has a strong conjugate base.

`=>` According to Lewis concept, an acid is an electron-pair acceptor i.e. these are generally positively charged or electron deficient species. e.g.` Bf_3, AlCl_3, Na^+ , K^+, PF_3 , SF_4` etc.

Classification of Acids

Acids can be classified in the following classes:

Class I : On the basis of degree of dissociation :

(i) Strong acids : Dissociates completely even in concentrated solution, e.g. `HCl, HNO_3 , H_2SO_4, HClO_4` etc.

`HCl + H_2O -> H^+ (aq) + Cl^- (aq)`

(ii) Weak acids : Dissociates partially when dissolved in water, e.g. acetic acid `(CH_3COOH)`, formic acid `(HCOOH)` etc.

`CH_3 COOH + H_2 O ⇌ CH_3COO^- (aq)+ H^+ (aq)`

Class II : On the basis of the presence of oxygen or hydrogen in the acid :

(i) Oxyacids : Acids containing both oxygen and hydrogen and also having another non-metallic element, e.g. sulphuric acid (`H_2SO_4`), nitric acid (`HNO_3`) etc.

(ii) Hydracids : Acids containing oxygen. Besides hydrogen they contain other non-metallic elements like hydrochloric acid (`HCl`), hydrofluoric acid (`HF`) etc.



Basicity of Acid

The number of replaceable protons present in an acid is known as basicity of the acid.

`=>` Monobasic acid : has only one replaceable hydrogen ion and can combine with one hydroxyl (`-OH`) group to give one kind of salt. e.g. `HCl, HNO_3, CH_3COOH` etc.

`=>` Boric acid is a weak monobasic acid. It is not able to release `H^+` ions on its own. It receives `OH^-` ions from water molecule to complete its octet and in turn release `H^+` ions.

`=>` Dibasic acid : has two replaceable hydrogen ions and hence can combine with two hydroxyl groups to give two kinds of salts. e.g. `H_2 SO_4, H_2 SO_3 , (COOH)_2` etc.

`=>` Tribasic acid : has three replaceable hydrogen ions and can combine with three hydroxyl groups to give three kinds of salts. e.g. `H_3 PO_4` etc.

Factors Affecting Acidic Strength of Acids

`=>` Acidic strength of acids increases with increase in the size of atom.

e.g. `HF < HCl < HI`

`=>` Acidic strength of acids increases with increase in electronegativity of elements.

e.g. `H - CH_3 < H - NH_2 < H - OH < H - F`

`=>` Among oxyacids of element, acidic strength increases with increase in the oxidation state of central element.

e.g. `underset (+1) (HClO_1) < underset (+3) (HClO_2 ) < underset (+5) (HClO_3) < underset (+7) HClO_4`

Properties of Acids

Acids have the following general properties

`=>` Acids liberates hydrogen with reactive metals like `Zn, Fe, Na`.

`Zn + 2 HCl -> ZnCl_2 + H_2 ↑`

`=>` Acids produces `CO_2` gas with carbonates and bicarbonates which turns lime water milky.

`undersettext { Marble (calcium carbonate)} ( CaCO_3 ) +undersettext ( Sulphuric acid ) (H_2SO_4) -> undersettext(Calcium suphate ) (CaSO_4) + undersettext (water) (H_2O) + undersettext (carbon dioxide) (CO_2 ↑)`

`=>` Acid reacts with base to give salt and water. It is known as neutralisation reaction.

`HCl (aq) + NaOH (aq) -> NaCl (aq) + H_2 O(l)`

`=>` Acids react with metallic oxides to form salt and water.

`Na_2 O + 2HCl -> undersettext (salt) (2 Na Cl) + H_2O`

Above reaction is similar to neutralisation reaction (Acid-base reaction). Therefore metallic oxides are also called basic oxides.

`=>` Aqueous solution of acids conduct electricity.

`=>` A concentrated solution of 3 part conc. `HCl` and 1 part conc. `HNO_3` is called aqua-regia. All metals dissolve in it.

`\ \ \ \ \ \ \ \ text(Some Naturally Occurring Acids)`

Natural source Acids
Vinegar Acetic acid
Orange/Lemon Citric acid
Tamarind Tartaric acid
Tomato Oxalic acid
Curd/Milk Lactic acid
Nettle sting Methanoic acid ( formic acid)
Ant sting Methanoic acid ( formic acid )
Apple Maleic acid
Curd Lactic acid

Uses of Some Important Acids :

`=>` Acetic acid is used in food processing, in the manufacture of acetone and as a solvent.

`=>` Boric acid is used as an antiseptic for eye wash.

`=>` Nitric acid is used in manufacture of artificial silk, dyes, drugs, perfumes, explosives (TNT), fertilisers and in purification of silver and gold.

`=>` Hydrofluoric acid is used in etching of glass and in pickling of alloy steel.

`=>` Sulphuric acid (king of chemicals) is used in manufacture of hydrochloric acid, nitric acid, fertilisers, explosives, paints and pigments, coaltar, dyes, lead storage batteries and as an oxidising and dehydrating agent.

`=>` Hydrochloric acid is the main constituent of gastric acid and is used in manufacture of chlorine, chlorides and in
textile, dyeing and tanning industry.
 
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