A: Acids and Bases

How is a salt formed?

A salt is a product formed by combination of acids and bases

How can acids be identified?

By their sour taste

How can bases be identified?

By their soapy touch and bitter taste.

How are acids and bases differentiated?

By indicators

Indicator

An indicator is a chemical which indicates by means of a sharp change in color, the nature of the solution

All bases are either ___ or ___ or ___

metallic oxide (O^2-), metallic hydroxide (OH^-) or ammonia solution

Acids

Acids are compounds which contain one or more hydrogen atoms and when dissolved in water, produce hydronium (H₃O) ions the only positively charged ions

Hydronium ion

The hydrated hydrogen ion that exists in the solution of acids

Types of acids depending on their sources

Organic & Inorganic

Types of acids based on their basicity

Monobasic acids, dibasic, tribasic

Oxyacids

Acids that contain hydrogen, oxygen and some other element

Hydracids

Acids which contain hydrogen and any other non metal

Basicity of an acid

Basicity of an acid is the number of hydronium atoms that can be produced by the ionisation of one molecule of an acid in an aqueous solution

Monobasic acid

Acids which on ionization in water produce one hydronium ion per molecule of the acid.

What do monobasic acids form?

1 normal salt, as they ionize in 1 step

Examples of monbasic acids

HBr, HNO₃, HI, CH₃COOH

What do dibasic acids form?

1 acid salt or 1 normal salt, as they have 2 replaceable hydrogen ions

Examples of dibasic acids

Sulphuric acid, H₂CO₃

Formation of salts with H₂SO₄ on reaction with NaOH

NaOH + H2SO4 → NaHSO4 (Acid salt) + H2O

2NaOH + H2SO4 → Na2SO4 (normal salt) + 2H2O

Why is H3PO3 a dibasic acid?

Because in oxyacids of phosphorus, hydrogen atoms which are attached to oxygen atoms are replacable, but hydrogen atoms directly bonded to phosphorus atoms are not replacable

What do Tribasic acids form?

Tribasic acids form two different acid salts and one normal salt, because they have 3 replacable hydrogen atoms

Formation of salts from H3PO4 on reacting with NaOH

NaOH + H3PO4 → NaH2PO4 + H2O

2NaOH + H3PO4 → Na2HPO4 + 2H2O

3NaOH + H3PO4 → Na3PO4 + 3H2O

Basicity of silicic acid (H4SiO4)?

4

Concentration of an acid

Concentration of an acid means the amount of acid present in a definite amount of its aqueous solution

Concentrated acid

A concentrated acid is an acid which contains a very small amount of water or no water.

Dilute acid

Dilute acid is an acid which contains far more amount of water than its own mass

How to dilute an acid?

An acid can be diluted by pouring small amounts of the acid slowly in water

Organic acids

An organic acid is an acid which is obtained from plants. They contain carbon and hydrogen.

They are weak acids and partially ionize in water

Examples of organic acids

Oxalic acid (COOH)₂, Acetic acid, Formic acid (HCOOH)

Inorganic acids

Inorganic acids are acids which are obtained from minerals. They do not contain carbon. They ionize completely in a solution and produce a large number of hydronium ions

Which inorganic acid contains carbon?

Carbonic acid

Short note on carbonic acid

Carbonic acid is a weak mineral acid. It contains ions as well as molecules. It turns blue litmus pink. It is non-corrosive, and so used in soft drinks.

Prep of acids

By synthesis - binary acids

By action of water on non-metallic or acidic oxides - oxyacids

By oxidation of non metals, eg. sulphur and phosphorus

By displacement

What is NO2 also called and why?

NO2 is also called mixed or double acid anhydride because 2 acids, HNO3 and HNO2 are formed when it reacts with water.

Taste of acids

Sour. HNO3 and H2SO4 should not be tasted because they are highly corrosive in nature.

Acids’ effect on skin

Corrosive action on skin and cause painful blisters.

Conc. H2SO4’s action on skin

Chars the skin black

Conc. HNO3’s action on skin

makes skin yellow

Conc. HCl’s effect on skin

Gives amber color to skin

Which acids are not corrosive?

Carbonic acid and organic acid

Change of litmus on reacting with acids?

Blue to Red

Methyle Orange’s change in color on reacting with acids?

Orange to pink

Phenopthalein’s color change on reacting with acids

Remains colorless

Change in color of red cabbage extract?

Red in acidic solutions, turns green in basic solutions

Olfactory indicators with examples

These are the substances whose smell or odour changes in acidic or basic solutions. Examples: onion, vanilla and clove oil.

Example to demonstrate Olfactory indication

Onion has a charecterisitc smell. When a basic solution like NaOH solution is added to a cloth strip treated with onion (or onion extract), then the onion smell cannot be detected.

An acidic soln. doesn’t destroy the smell of onion

Why do acids conduct electricity in the aqueous state?

Because acids ionise in water, producing free ions. These free ions help conduct electricity.

Why isn’t water added to an acid to dilute it?

Because an exothermic reaction occurs due to which so much heat is produced that splashing of acidic solution may occur. Also, the container may break which can be fatal.

Give reaction of NO2 with water

NO2 + H2O → HNO3 + HNO2

Which acid is in vinegar?

Acetic acid

Which acid is present in grapes?

Tartaric acid

Bases

A base is either a metallic oxide or a metallic hydroxide or ammonium hydroxide which reacts with hydronium ions to form a salt and water ONLY(imp.) (neutralization)

Basic oxide

A basic oxide is a metallic oxide which contains the ion O2- and reacts with an acid to form a salt and water only (neutralization)

Basic oxide

A metallic oxide which contains OH- ion and will react with an acid to produce a salt and water only

Alkali

An alkali is a basic HYDROXIDE which when dissolved in water produces OH- ions (hydroxyl ions) as the only negatively charged ions.

Alkalis are water soluble bases.

Soluble metallic hydroxide

Sodium, Potassium, Calcium and Ammonium hydroxides

Strong alkalis

Sodium and potassium hydroxides - because they completely dissociate in their aqueous solution

Weak alkalis

Calcium and Ammonium hydroxide - feebly dissociate in aqueous solutions

Why arent ferric hydroxide and cupric hydroxide alkalis?

Because they are insoluble in water

Strong alkali

An alkali which undergoes almost complete dissociation in aqueous solution to produce hydroxyl ions. Ex: calcium and potassium hydroxides

Weak alkalis

Weak alkalis are alkalis which undergo partial dissociation/ionisation in aqueous solution to produce a low concentration of OH- ions in solution. Ex: calcium and ammonium hydroxides

Acidity of a base

The acidity of a base is the number of hydroxyl ions (OH-) which can be produced per molecule of the base in aqueous solution

OR

the number of hydrogen atoms of an acid with which a molecule of that base will react to (neutralize) produce a salt and base only

Monacidic base

A monoacidic base is a base that ionises in molted(fused) state or in aqueous solutions to produce one OH- ion per molecule of the base.

Example of monacidic base

NaOH

Example of diacidic base

Ca(OH)2, Cu(OH)2

Examples of triacidic base

Al(OH)3, Fe(OH)3

Prep of bases

From metals: reaction with oxygen

Action of water on reactive metals: like Ca, K, Na

Action of water on soluble metallic oxides: Na2O, K2O, CaO

By double decomp.: Salts solution + Base (Alkali)

By action of oxygen on metal sulphides: ZnS, PbS

Decomposition of salts (carbonates and nitrates)

How can ammonia hydroxide be prepared?

By dissolving ammonia gas in water

Physical properties of bases/alkalis

• Sharp and bitter taste

• Change in the color of indicators:

  Litmus-red to blue

  Methyl orange- orange to yellow

  Phenolphthalein-Colorless to Pink

• Insoluble bases don’t affect indicators

• They are soapy substances, i.e., they are slippery to touch.

• Strong electrolytes

• They show a mild corrosive action (slight burn) on the skin

Chemical properties of bases/alkalis

• Strong alkalis absorb carbon dioxide from the air → carbonates

• Neutralization

• Precipitation

• Alkali + Ammonium salt → salt + ammonia + water

Universal indicator

The universal indicator is a mixture of indicator dyes that gives a spectrum of colors depending on how acidic or alkaline a solution is.