ACID, BASES AND SALT

 INTRODUCTION

Acids and bases are two classes of chemical compounds that display generally opposite characteristics. They are so opposite that when they react with each other in a process known as neutralization reaction, a completely new product with entirely different characteristics is formed. This reaction is characteristically very rapid and generally produces salt and water. 

Acids in food 

Acids can be found in many substances, including food such as fruits. Citrus fruits like orange, grape fruit and lemon contain an acid called citric acid.

Acid contained in fruits is called ascorbic acid. Sour milk, palm oil and bees contain lactic acid, palmolic acid and methanoic (or formic) acid respectively. 

Acids in animals and plants 

Acids can also be found in animals. Mammals, for example, have different acids in different parts of the body. The acid found in the stomach is known as hydrochloric acid, which helps in the digestion of food. Acids found in plants and animals are known as organic acids. These acids like many others are everyday acids. 

 Types of acids 

Most organic acids are also referred to as weak acids because they partly dissolve or dissociate in water. They have low hydrogen concentration. 

Weak acids are acids which dissolve or dissociate in water. Apart from these there are also laboratory acids, which include hydrochloric acid, sulphuric acid and nitric acid. 

These acids are called inorganic acids and are normally obtained from mineral sources. They are called laboratory acids because they are commonly produced or used in laboratories. Most inorganic acids are also referred to as strong acids because they dissolve or ionize completely in water. They have high hydrogen concentration. 

Strong acids are acids which do not dissolve or ionize completely in water. 

Definition of acids 

By definition, acids are chemical substances that turn blue litmus paper red. Acids can also be defined as substances that react with some metals to produce hydrogen gas.

 Brønsted-Lowry theory about acids 

A more satisfactory theory was proposed in 1923 by the Danish chemist Johannes Brønsted and independently by Thomas Lowry, a British chemist. This theory is usually known as Brønsted-Lowry concept. With the Brønsted-Lowry concept, we usually refer to a hydrogen ion as a proton. That is because a proton is all that is left when a hydrogen atom loses an electron to become an ion. 

 Brønsted-Lowry definition 

Brønsted-Lowry concept states that an acid is an acid because it provides or donates a proton to something else. 

 An acid is a substance that donates or transfers a proton. When an acid reacts, the proton is transferred from one chemical to another. As will be noted later, the chemical which accepts the proton is a base. When an acid dissolves or dissociates in water it gives a proton to the water. 

HCl + H2O → H3O + + Cl

The Brønsted-Lowry view is that the acid (HCl) gives a proton to water to make two ions, one of which is H3O + . H3O + is called hydronium ion. H2SO4 can also give a proton to water. 

 H2SO4 + H2O → H3O + + HSO4 - 

In this case, the product HSO4 - still has a proton that can be donated to another water molecule.

HSO4 - + H2O → H3O + + SO4 2 HCl can also give a proton to a hydroxide ion (OH- ) rather than water. HCl + OH- → H2O + ClThe first chemical in each of these equations is an acid because they are each giving a proton to something else.

Properties of Acids 

Acids have a number of properties. These properties could be physical or chemical and make acidic substances easy to distinguish. 

Physical properties of acids 

1. Acids are corrosive. 

2. They have a sour taste. 

3. Acids have a pH less than 7. 

4. Acids change litmus (a dye extracted from lichens) red. 

 5. Acids can conduct electricity. 

6. They are soluble in water. 

Chemical properties of acids 

1. Acids react with bases to form salt and water. This process is called neutralisation. 

For example, Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride (salt) and water. 

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Acid + Base Salt + Water Copper oxide reacts with tetraoxosulphate (VI) acid to produce copper (ii) tetraoxosulphate (VI) (salt) and water. CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) Base + Acid = Salt + Water 

2. Acids react with metals (the more reactive metals such as magnesium, calcium and iron) to form hydrogen gas and a salt. 

For example, hydrochloric acid reacts with magnesium to form hydrogen gas and magnesium chloride salt. 

2HCl(aq) + Mg(s) → H2(g) + MgCl2(aq) Acid + metal hydrogen + salt Tetraoxosulphate (VI) acid reacts with iron to produce hydrogen gas and iron (II) tetraoxosulphate H2SO4(aq) + Fe(S) → H2(g) + FeSO4(aq) Acid + metal = hydrogen + salt 

The magnesium and iron metals replace the hydrogen atom of the hydrochloric and tetraoxosulphate acids thereby producing magnesium chloride and iron (II) tetraoxosulphate salts. 

3. Acids react with carbonates to form carbon dioxide gas, water and a salt. 

For example, hydrochloric acid reacts with calcium trioxocarbonate to produce carbon (II) oxide, water and calcium chloride salt. 

2HCl(aq) + CaCO3(s) → CO2(g)+H2O(l)+ CaCl2(aq) Acid + calcium trioxocarbonate = carbon (II) oxide + water + salt Trioxonitrate(V) acid reacts with sodium hydrogentrioxocarbonate (IV) to produce carbon(II) oxide, sodium trioxonitrate(V) salt and water. 

2HNO3 + NaHCO3 → CO3 + Na(NO3)2 + 2H2O Acid +sodium trioxocarbonate = sodium (II) oxide + water + salt 

Uses of acids 

1. Acids are normally used in industries, household cleaning product, daily products, and soft drinks. 

 2. We can get acidic or sour taste from most food items such as lemons, vinegar, cream, yogurt, cottage cheese, etc. 

3. A wasp sting, which is alkali, can be neutralized with a weak acid such as vinegar and lemon juice. 

4. Sulphuric acid is widely used in car batteries as wet cell battery electrolyte. 

5. Phosphoric acid is normally used in cola soft drinks. 

6. Dilute hydrochloric acid in the stomach helps in digestion. 

7. Industrial hydrochloric acid is used to make glue and for cleaning steel. 

8. Sulphuric acid is used to manufacture fertilizers, paints and dyes, detergents, plastics, paper and explosives. 

9. Sulphuric acid is also used in refining petroleum. 

10. Nitric acid is used in cleaning and purifying gold and silver. 

11. It is also used in manufacturing fertilizers, drugs, paints and dyes.  

BASES

Bases are the opposite of acids. Bases are basic because they take or accept protons. 

 Hydroxide ion, for example can accept a proton to form water. OH- + H+ → H2O 

Brønsted-Lowry theory about bases 

Brønsted and Lowry realized that not all bases had to have a hydroxide ion. As long as something can accept a proton it is a base. So anything, hydroxide or not, that can accept a proton is a base under the Brønsted-Lowry definition. The water molecules that accept protons when HCl dissolves in water are acting as bases. Ammonia can accept or react with hydrogen ion to give ammonium ion NH4 + . NH3 + H+ → NH4 + Carbonate ion can accept a hydrogen ion, or accept a proton, to become bicarbonate ion.

 CO3 2- + H+ → HCO3 - 

Also, water molecules, as mentioned before, can act as a base by accepting protons. 

H2O + H+ → H3O  

Hydroxide, ammonia, carbonate and water are all Brønsted-Lowry bases. Be sure to note the distinction between ammonia and ammonium. NH3 is ammonia and NH4 + is ammonium. They sound very much the same and their formulas are very similar, but their chemical properties are far different. They are different because one has one more proton than the other. Ammonia is a base and ammonium is an acid. 

 Examples of bases 

Some bases are often found in household cleaners, they help clean grease from windows and floors and are found in the soap we use everyday. 

 Some other examples of basic substances are toothpaste, egg whites, dishwashing liquids and household ammonia. 

 Bases like potassium hydroxide are normally obtained when wood, cocoa pod, plants etc are burned.

 Other bases, such sodium hydroxide are produced industrially from salt. 

 Alkalis 

 Bases which are soluble in water are called alkalis. Examples of alkalis are ammonium hydroxide, sodium hydroxide, potassium hydroxide and calcium hydroxide. A base is any substance that accepts protons. 

Types of bases 

Bases can be organic or inorganic. 

Organic bases: 

These are bases obtained from natural source, such as ashes of plant, cocoa pods etc. Most organic bases are weak bases. 

Inorganic bases: These are bases obtained from mineral sources; examples are calcium hydroxide (Ca(OH)2), aluminium oxide (Al2O3), and milk of magnesia (Mg(OH)2). Inorganic bases such as NaOH, Ca(OH)2 etc. are strong bases. 

Properties of bases 

Like acids, bases also have properties which are either physical or chemical. 

Physical properties of bases 

1. Bases taste bitter. 

2. Bases feel soapy and slippery. 

3. Bases have a pH over 7. 

4. They change litmus paper to blue. 

5. They are very corrosive in concentrated or solid form. 

Chemical properties of bases 

1. Bases react with acids to form a salt and water. This process is called neutralization reaction.

For example, calcium hydroxide reacts with hydrochloric acid to produce calcium chloride and water, CaOH(aq) + HCl(aq) → CaCl(aq) + H2O(l) Base + acid = salt + water 

2. Bases react with ammonium salt to release ammonia gas, salt and water. 

For example, ammonium chloride reacts with calcium hydroxide to produce ammonia gas and salt. 2NH4Cl(s) + Ca(OH)2(s)→2NH3(g)+CaCl2(s) + 2H2O(l) Ammonium + base = ammonia gas + salt + water 

Uses of bases 

1. Ammonium hydroxide, frequently called ammonia, is used in the preparation of important related compounds such as nitric acid and ammonium chloride. 

2. Ammonia is also used as a cleaning agent, refrigerant and for softening hard water. 

3. Sodium hydroxide is used in the manufacture of soap, rayon, and paper. Strong solutions of this base are very caustic; that is, they are extremely harmful to the skin. 

4. Calcium hydroxide, commonly known as slaked lime, is used in the preparation of plaster and mortar. 7. It is also used to neutralize acidic soil. 

5. Water solutions of calcium hydroxide, called limewater, can be used in the lab as a test for the presence of carbon dioxide.

SALTS

Many chemical compounds may be classified as salts. The salt most familiar to most people is table salt - sodium chloride (NaCl). Baking soda is the salt sodium bicarbonate. 

Magnesium sulphate, also called Epsom salts, is often found in the home. 

Salts are ionic compounds that are composed of metallic ions and non-metallic ions. 

For example, sodium chloride (NaCl) is composed of metallic sodium ions and non-metallic chloride ions. Some salts are composed of metallic polyatomic ions and non-metallic polyatomic ions (ammonium nitrate is composed of ammonium ions and nitrate ions). 

Properties of Salts 

Physical properties 

1. Salts are soluble in water. 

2. Salts are usually odourless. 

3. Molten salts conduct electricity. 

4. Normal salt (NaCl) is colourless. However, different salts exist in different colours. (For example, sodium chromate is yellow, potassium dichromate is orange, nickel chloride hexahydrate is green, etc.). 

5. Salts are crystalline in nature. That is they are like shards of broken glasses. This is because the ions in a solid salt are usually arranged in a definite crystalline structure; each positive ion is associated with a fixed number of negative ions and vice versa. 

Chemical properties 

1. Salts are ionic compounds. That is they are made up of ions rather than molecules. In NaCl, for example, a fixed proportions of sodium ion (Na+ ) and Chlorine ion (Cl- ) come together to form the salt. 

2. They are composed of related number of cations (positively charged ions) and anions (negatively charged ions). 

3. Salt are electrically neutral. That is they have no charge. 

4. Salts dissolve or dissociate in water. Salts consist of tightly bonded ions. In water, these bonds are weakened and the ions become mobile. In water, for example, sodium chloride ionizes, or dissociates like this: NaCl (s) → Na 1+ (aq) + Cl 1- (aq) 

5. Salts react with water to form acids and bases in a process called hydrolysis reaction. For example, when sodium carbonate (Na2 CO3) is dissolved in water, carbonic acid (H2 CO3) and sodium hydroxide (2NaOH) are formed. Na2 CO3 (aq) +2 H2O (l) →H2 CO3 (aq) +2NaOH (aq) 

NB: Since acids and bases react to form water and salt (neutralization reactions) hydrolysis reactions are the reverse of neutralization reactions. A salt that includes water in it solid crystalline form is called a hydrate. 

Salt in the sea 

The salty taste of sea water is due to the presence of salts such as sodium chloride and magnesium bromide. There are many different salts present in sea water:

Types of salts 

Based on the above properties of salts, we can deduce four main types of salts: acidic salts, basic salts, normal (neutral) salts and double salts.

Acidic salts 

This is the type of salt in which not all the protons or hydrogen ions are replaced or donated. 

Examples are: 

1. Potassium hydrogen tetraoxosulphate (VI), KHSO4 

2. Sodium hydroxide trioxocarbonate (IV), NaHCO3 

3. Sodium dihydrogen tetraoxosulphate (IV), NaH2PO4 

4. Sodium hydrogen chloride, NaHCl 

Basic salts 

Basic salts are form when there is a neutralization reaction between a strong base and a weak acid. Examples are: 

1. Sodium hydroxide carbonate, Na(OH)CO3 

2. Magnesium hydroxide chloride, Mg(OH)Cl 

3. Calcium hydroxide carbonate, Ca(OH)CO3 

4. Lead hydroxide carbonate, Pb(OH)NO3 

Normal salts 

Normal salts are formed when there is a perfect neutralization. That is when the strength of both the acids and bases involved are the same and all the protons are donated. Examples are: 

1. Sodium chloride, NaCl 

2. Magnesium Chloride, MgCl 

3. Potassium carbonate, K2CO3 

4. Potassium nitrate, KNO3 

5. Ammonium chloride, NH4Cl 

6. Sodium sulphate, Na2SO4 

Double salts 

These types of salts are formed when two salts are mixed together. They are usually referred to as alums and generally assume the properties of the constituent salts. Examples are: 

1. Calcium magnesium carbonate CaMg(CO3)2 

2. Potassium sodium tartrate and bromide 

3. Calcium potassium alum, Ca(SO4)K2SO424H2O 

4. Aluminium potassium alum, AlSO4K2SO424H2O 

Complex salts 

This type of salt contains two different types of metal ions one of which does not dissociate or dissolve in water. Examples include: 

1. Potassium ferricyanide, K3Fe(CN)6 

2. Potassium mercury iodide,K2HgI4 

3. Chromium ammonium chloride, Cr(NH3)6Cl3

Preparation of Salts 

As you might have observed, all the chemical properties of acid discussed above give rise to the formation of salt. You may have to take another look at those properties. This shows that there are at least three methods of preparing salt: 

 Neutralization of acid and base 

When an acid and base react, they counteract each other; that is, they neutralize each other. Such a reaction known as a neutralization reaction results in the formation of water and a salt. For example, when sodium hydroxide and hydrochloric acid react, water and the salt sodium chloride are formed. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Base + Acid = Salt + Water 

This occurs because the hydrochloric acid and the sodium hydroxide first ionize, and then react. 

The compounds ionize releasing hydrogen, chloride, sodium, and hydroxide ions. Since these are mobile in solution, hydrogen ions meet hydroxide ions and unite to form water. At the same time, sodium ions and chloride ions remain as aqueous salt. 

This method of preparing salt is also called the titration method. 

Acid and metal 

Acids react with metals (the more reactive metals such as magnesium, calcium and iron) to form hydrogen gas and a salt. 2HCl(aq) + Mg(s) → H2(g) + MgCl2(aq) Acid + metal = hydrogen + salt Metal oxide and acid When a metal oxide reacts with an acid, a salt is formed. For example, when calcium oxide reacts with nitric acid, the salt calcium nitrate is formed. CaO + 2HNO3  Ca(NO2)3 + H2O Metal oxide + acid = salt + water Direct combination When a metal reacts with a non-metal, a salt is generally formed. 

For example, when the metal magnesium is burned in chlorine gas, the salt magnesium chloride is formed. Mg + Cl2 → MgCl2 

Salts produced by the above methods are known as soluble salts. 

Precipitation method 

This method is used to produce salt which is insoluble in water. A solutions of two soluble salts which contain the ions of insoluble salts, (example lead), are mixed together and the precipitate formed is washed with distilled water and dried. 

For example lead nitrate (PbNO3) and sodium carbonate (NaCO3), both of which are soluble salts, give rise to the insoluble salt lead carbonate. 

PbNO3(aq) + NaCO3(aq) → PbCO3(s) + NaCO3(aq) Other examples of insoluble salts are calcium tetraoxosulphate (VI), CaSO4; lead (II) chloride, PbCl2; silver chloride, AgCl; and lead (II) tetraoxsulphate (VI), PbSO4. 

The precipitation method is usually referred to as the double decomposition. 

 That is: AB + YZ → AZ + BY