Acids, Bases, And Salts

An acid is a substance which, when dissolved in water produces hydrogen ions, H⁺ as the only positive ion.

Acids are classified into organic and inorganic (mineral) acids. The organic acids occur as natural products in plants and animal materials, while inorganic acids are prepared from inorganic matter. Some examples of organic acids are: ethanoic acid from vinegar, tartaric acid from grapes, lactic acid from milk, citric acid from lime and lemon, amino acid from proteins, fatty acid from fats and oil and ascorbic acid (vitamin c) from oranges, while inorganic acids include hydrochloric acid HCl, trioxonitrate (v) acid HNO3, tetraoxosulphate (Vi) acid H₂SO₄ etc.

Modern definition of an acid

The existence of hydronium (oxonium) ion, H₃O⁺, modifies the earlier definition of an acid. Thus, an acid is also defined as a substance which in aqueous solution produces hydronium (oxonium) ion, H₃O⁺, as the only positive ions.

Note that neutralization is defined as the combination of hydronium ions, H₃O⁺ and hydroxide ions, OH, to form water molecules, H₂O.

Basicity of an acid

Basicity of an acid is the maximum number of hydronium ions which one molecule of the acid is capable of producing in aqueous solution, i.e the number of replaceable hydrogen ions, H⁺ in one molecule of the acid.

Physical properties of acids

1. Dilute acids have sour taste
2. Acids turn blue litmus red
3. The concentrated form of strong acids are corrosive

Chemical properties of acids

1. Reaction with metals

2. Reaction with bases and alkalis

Note: acids react with insoluble bases and alkalis to form salts and water only. The reaction is known as neutralization reaction.

3. Reaction with trioxocarbonate (IV)

Preparation of acids

1. By the reaction between an acid anhydride and water

2. By direct combination of constituent elements in the presence of a suitable catalyst.

3. By displacing a weaker or more volatile acid from its salts by a stronger or less volatile acid

Uses of acid

1. In the manufacture of fertilizers dyes, detergents, explosives, paints, soaps, textiles, drugs, plastics and other chemicals.

2. In dissolving metals or to remove rust 

3. In the production of salts, soft drinks and baking powder.

4. As food preservatives and mild antiseptic or germicide

5. In oil refineries

6. As an electrolyte in lead-acid accumulator

7. As a drying, dehydrating and oxidizing agent

Bases and Alkalis 

A base is a substance which will neutralize an acid to yield salt and water only. Oxides and hydroxides of metals are bases. E.g sodium oxide Na₂O, potassium oxide K₂O and magnesium oxide MgO.

Alkalis

An alkalis is a compound which produces hydroxyl ions, OH^–, when dissolved in water. 

Physical properties of bases/alkalis

1. Alkalis have a bitter taste

2. Alkalis turn red litmus blue

3. Alkalis have soapy feel

4. Concentrated forms of the two caustic Alkalis, NaOH and KOH, have a powerful corrosive action.

Chemical properties of bases / alkalis

1. Reaction with acids 

2. Reaction with ammonium salts

Preparation of bases / alkalis

1. Most metals combine directly with oxygen when heated, to form basic oxides

2. Soluble oxides of metals dissolve in water to give alkalis.

3. Some metals react with water or steam to produce Alkalis and hydrogen.

4. Trioxocarbonates (IV), with the exception of sodium and potassium trioxocarbonate (IV), decompress on heating to yield the oxide of the metal and carbon (IV) oxide gas

5. By heating slaked lime with dilute sodium or potassium trioxocarbonate (IV) 

6. Thermal decomposition of metal hydroxides

7. The insoluble basic hydroxides are prepared by precipitation or double decomposition reaction

8. Ammonium hydroxide (NH₄OH) is prepared by the reactions between any ammonium salts and an alkali to yield ammonia (NH₃) which is then dissolved in water to produce ammonium hydroxide

Uses of bases / Alkalis

1. Sodium hydroxide, NaOH, is used in the manufacture of paper, soaps, dyes and rayon from cellulose.

2. Sodium hydroxide is also used for refining petroleum, production of chemicals and as a drying agent.

3. Potassium hydroxide, KOH, is used in making shaving creams and soft soaps.

4. Calcium hydroxide, Ca(OH)₂ is used in making mortar, plaster of Paris, white-wash, cement, in sugar refining and neutralization of soil acidity.

5. Ammonium hydroxide, NH₄OH, is used for bleaching cloth, as a detergent and solvent for grease.

6. Magnesium hydroxide, Mg(OH)₂ is used as anti acid in toothpaste and medically as mild laxative.

Measurement of acidity and alkalinity

The pH scale

The pH scale is a number scale from 0 to 14, used to measure the degree of acidity or alkalinity of a solution. The pH of a solution can be measured with a pH meter or by the use of universal indicators. A solution with a pH value of 7 is neutral, i.e neither acidic nor alkaline. A solution with a pH value of 7 is neutral, i.e neither acidic nor alkaline. A pH with less than 7 is acidic while one with pH more than 7 is alkaline.

Acid base titration

Importance of pH

1. The knowledge of pH value is useful in the control of public supply by the water chemist, to avoid poisoning from the lead pipes, which occurs if water becomes too acidic or alkaline.
2. It is also useful in medicine and pharmacy.
3. pH value is also put into use in agriculture most plants grow well in soils with pH of 7 – 8.
4. The regulation of pH is important in sewage disposal, treatment of industrial waste and environmental protection of rivers.

Salts

Salts is a compound derived from an acid by replacing all or part of the ionizable hydrogen contained in the acid by a Metallic or ammonium ions. Most salts are neutralization products of acids and bases, i.e Acid + Base > salt + water.

Types of salt 

Normal, acid, basic, double and complex salts.

Normal salts are formed when all the replaceable hydrogen ions in an acid have been completely replaced by metallic ions or a base.

Acid salts are formed when the replaceable hydrogen ions in the acid are only partially replaced by a metal.

Basic salts are formed when there is an insufficient supply of acid for the complete neutralization of the base.

Double salts occur when hot concentrated equimolar aqueous solutions of two simple salts are mixed together in the ratio of their relative molecular masses and allowed to cool.

Complex salts contain complex ions, i.e ions consisting of charged groups of atoms.

Uses of salts

Salts are used in the manufacture of many industrial, agriculture and consumer substances like chlorine gas, fertilizers and laxative. They are also used as food preservatives, drying agents and antifreeze.

Water of crystallization

Many salts combine chemically with water to form hydrated salts. Hydrated salts contain a fixed definite amount of water, known as the water of crystallization.

Characteristics of some salts 

1. Efflorescence: some hydrated salts, when exposed to the atmosphere, give off part or all of their water of crystallization spontaneously to form a lower dehydrated or anhydrous salt. Such salt is known as efflorescent and phenomenon is known as efflorescence.

2. Deliquescence: some compounds tend to absorb so much water from the atmosphere that they dissolve in it to form an aqueous solution of the compounds. This phenomenon is known as deliquescence and substance is said to be deliquescent.

3. Hygroscopy: hygroscopic compounds also absorb water when exposed to the atmosphere but will form solutions rather they become moist or sticky.

4. Drying agents: hygroscopic and deliquescence compounds are very useful as drying agents. In chemical processes, they are called dehydration agents or desiccants because of their great affinity for water. They are used to dry gases in the laboratory.

*Desiccator

Source: www.commons.wikimedia.org 

Past questions

1. Sodium sulphate decahydrate (Na₂SO₄10H₂O) on exposure to air loses all its water of crystallization. The process of loss is known as? (Jamb 1983)

A. Efflorescence

B. Hygroscopy

C. Deliquescence

D. Effervescence

E. Dehydration

Answer: A

2. Sodium hydroxide (NaOH) pellets are? (Jamb 1984)

A. Deliquescent

B. Hygroscopic

C. Efflorescent

D. Hydrated

E. Fluorescent

Answer: A

3. Which salt loses its water of crystallization to the atmosphere on exposure, the process is said to be? ( Jamb 2002)

A. Efflorescence

B. Deliquescence

C. Effervescence

D. Fluorescence

Answer: A

4. Which of the following ions is acidic? ( Jamb 2003)

A. S^2-

B. H₃O⁺

C. NO^–₃

D. K⁺

Answer: B

5. A weak acid is one which? (Wassce 1995)

A. Is not corrosive

B. Is slightly ionize in water 

C. Does not produce salts with Alkalis

D. Does not conduct an electric current in aqueous solution

E. Forms acid salts with bases

Answer: B

6. Explain the term pH? (Wassce 1995)

Answer: pH is a measure of the acidity or alkalinity of a solution.

7. If a solution has a pH of 2, it can be concluded that it?  ( Wassce 2000)

A. Is a weak electrolyte

B. Has hydrogen ion concentration of 0.2moldm^-3 

C. Is twice as acidic as a solution of pH 1

D. Will produce effervescence with magnesium ribbon.

Answer: D

8. A substance is said to be hygroscopic if it absorbs? (Wassce 2002)

A. Water from the atmosphere to form a solution

B. Heat from the surrounding

C. Carbon (IV) oxide from the atmosphere

D. Moisture from the atmosphere without dissolving

Answer: D