ACID, BASE & SALT (Sci-X)

Developed by :- NIF NGO EDU-Wing ( 09839281700 )

ACID BASE SALT

Arrhenius Acid Base Theory 1884/87

An acid, according to Arrhenius, is any substance that liberates H+ ions when placed in water. When the H+ concentration is elevated the OH- concentration decreases, this solution is said to be acidic. Similarly, a base is defined as any substance that liberates OH- ions when placed in water. The resulting solution has a higher concentration of OH- ions than H+ ions and is said to be basic, or alkaline.

ACID:HCl(aq)H+(aq) + Cl-(aq)

BASE:NaOH(aq) Na+(aq) + OH-(aq)

Brønsted - Lowry Acid Base Theory 1923

Acids are defined as substances that donate H+ ions, protons and therefore are called proton donators, while bases are substances that accept protons and are defined as proton acceptors. On this basis, the autoionization of water is given as follows:

H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)

In this reaction, one water molecule donates a proton, H+, and behaves as an acid, while the other water molecule accepts the proton and behaves as a base. Since the reaction is written reversible, we see that when it proceeds from right to left, the hydronium ion, H3O+ behaves as an acid, a proton donor while the hydroxide ion, OH- behaves as a base, a proton acceptor.

LEWIS ACID BASE THEORY

The Lewis definition defines a base (referred to as a Lewis base) to be a compound that can donate an electron pair, and an acid (a Lewis acid) to be a compound that can receive this electron pair

Lewis Acids and Bases

  1. An acid is a substance that accepts a lone pair of electrons.
  2. A base is a substance that donates a lone pair electrons.

BUFFER SOLUTIONS( They help to maintain the PH value in a system/Solution )

A buffer is simply a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers work by reacting with any added acid or base to control the pH. For example, let's consider the action of a buffer composed of the weak base ammonia, NH3, and its conjugate acid, NH4+. When HCl is added to that buffer, the NH3 "soaks up" the acid's proton to become NH4+. Because that proton is locked up in the ammonium ion, it proton does not serve to significantly increase the pH of the solution. When NaOH is added to the same buffer, the ammonium ion donates a proton to the base to become ammonia and water. Here the buffer also serves to neutralize the base.

Amphoterism and Water

Substances capable of acting as either an acid or a base are amphoteric. Water is the most important amphoteric substance. It can ionize into hydroxide (OH-, a base) or hydronium (H3O+, an acid).

Ionization of water: 2H2O  H3O+(aq) + OH-(aq)

1. Acids: Acids are sour in taste, turn blue litmus red and give H3O+ ion (hydronium ions in solution. e.g. HCl, H2SO4, HNO3 etcThe word acid comes from Latin ‘ACERE’ which means sour

Properties of Acids:

  • Produce H3O+ ions in water
  • Turns litmus from blue to red
  • Sour taste
  • Double Replacement with solid oxides
  • turns rusting metals into salts
  • Double Replacement with hydroxides
  • neutralizes bases
  • always forms a salt and water
  • Double Replacement with carbonates and bicarbonates
  • turns carbonates into salts and CO2
  • antacids in your stomach remove acid
  • Liberate H2(g) from water in a redox reaction with a metal.
  • this is a part of the activity series for metals
  • drop a metal in an acid and you might get H2, check the activity series

2. Bases: Bases are bitter in taste, have soapy touch, turns red litmus blue and give hydroxide ions (OH-) in solution. Example – NaOH, KOH etc

Properties of Bases:

  • Produces OH- in water
  • Turns litmus from red to blue
  • Slippery feeling
  • Bitter taste
  • Neutralizes acidic solutions
  • always forms a salt and water
  • Breaks down fats and oils
  • which is why many house hold cleaners are basic
  • Will produce soap from fats or oils if heated

3. Salts: - A salt is a compound which is formed by neutralization reaction between an acid and base. For example, sodium chloride is formed by reaction between hydrochloric acid and sodium hydroxide.

Acid + base = Salt + water

Properties Salts:

  • Product of an acid-base reaction
  • Many are soluble
  • for this reason most drugs are a salt, most specifically sodium
  • this gets the drug into the bloodstream as all sodium compounds are soluble
  • if a drug cannot be turned into a salt it will not dissolve in water and is useless to a patient as it will not enter the bloodstream
  • Ionic compounds
  • Salts of weak acid and bases are used to form buffers
  • Hydroscopic: they absorb water very well to form hydrates
  • CaSO4 2H2O (Gypsum)

The most common salt is sodium chloride or table salt which forms by the combination of sodium hydroxide (base) and hydrochloric acid. Other examples include Epsom salts(MgSO4) used in bath salts, ammonium nitrate (NH4NO3) used as fertilizer, and baking soda (NaHCO3) used in cooking.

Acidic, Basic and Neutral Salts

Neutral Salt:Salts produced because of reaction between strong acid and strong base are neutral in nature. The pH of value of such salts is equal to 7, i.e. neutral. Example; Sodium chloride, sodium sulphate, potassium chloride, etc.

Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).

NaOH + HCl⇨NaCl + H2O

Acidic salt:

Salts which are formed after the reaction between a strong acid and weak base are called acidic salt. The pH value of acidic salt is lower than 7. Example: ammonium sulphate, ammonium chloride, etc.

Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).

NH4OH + HCl⇨ NH4Cl + H2O

Basic Salt:Salts which are formed after the reaction between weak acid and strong base are called basic salt. For example; sodium carbonate, sodium acetate, etc.

Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak base).

H2CO3 + 2NaOH ⇨ Na2CO3 + 2H2O

The pH Scale

The pH scale is used to measure the acidity (or basicity) of a solution, especially when the hydrogen ion concentration is very low.

pH – Measurement of strength of Acid and Base

The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than hydroxide ion, the solution is called acidic. If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic. If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called neutral solution.

pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14.

The pH value decreases with increase in hydrogen ion concentration. If the value of pH is 0, this shows maximum hydrogen ion concentration. pH value equal to 14 shows lowest hydrogen ion concentration. pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration.

ACID -CLASSIFICATION OF ACID

On the basis of origin, acids are classified as:

(a). Organic acids: Acids derived from living organisms like plants and animals are called organic acids. They are weak acids and are not harmful for living organisms. For example: citric acid is present in fruits, acetic acid present in vinegar, oxalic acid present in tomato, tartaric acid present in tamarind, lactic acid present in sour milk and curd.(b)Mineral acids: They are also called inorganic acids. They are dangerous and corrosive. Special precautions have to be taken while handling them. For example: sulphuric acid (H2SO4), hydrochloric acid (HCl) etc.

On the basis of their strength, acids are classified as:

(a). Strong acids: Strong acids are those acids which completely dissociate into its ions in aqueous solutions. Example: nitric acid (HNO3) ,sulphuric acid(H2SO4), hydrochloric acid(HCl)(b).Weak acids: Weak acids are those acids which do not completely dissociate into its ions in aqueous solutions. For example: carbonic acid (H2CO3), acetic acid (CH3COOH)

On the basis of their concentration, acids are classified as:

a. Dilute acids: Have a low concentration of acids in aqueous solutions.
b. Concentrated acids: Have a high concentration of acids in aqueous solutions.
Chemical properties of acids

(i) Acids react with active metals to give hydrogen gas.

Zn + H2SO4 = ZnSO4 + H2

(ii) Acids react with metal carbonate and metals hydrogen carbonate to give carbon dioxide.

NaHCO3+HCl = NaCl + H2O+CO2

(iii) Acids react with bases to give salt and water. This reaction is called neutralization reaction.

NaOH +HCl= NaCl + H2O

(iv) Acids react with metals oxides to give salt and water.

CuO + H2SO4 = CuSO4 + H2O

BASE

On the basis of their strength, bases are classified as:
(a). Strong bases: Strong bases are those bases which completely dissociate into its ions in aqueous solutions. Example: sodium hydroxide (NaOH), potassium hydroxide (KOH)(b). Weak bases: Weak bases are those bases which do not completely dissociate into its ions in aqueous solutions and some the part remains undissociated. For example: ammonium hydroxide (NH4OH)
Chemical properties of Bases-

1. Reaction with Metals – Certain metals such as Zinc, Aluminum, Tin react with Alkali solutions on heating and hydrogen gas is evolved2NaOH + Zn = Na2ZnO2 + H2

2. Reaction with acids – Bases react with acids to form salt and water.

KOH + HCl= KCl + H2O

3. Reaction with Non – metallic oxides –Non - metallic oxides are generally acidic in nature. They react with bases to form salt and water.

2NaOH + CO2= Na2CO3 + H2O

On the basis of the number of hydroxide groups present in a base, they can be classified as monobasic (one OH-), dibasic (two OH-), tribasic (three OH-) bases and so on.
INDICATORS
Indicators are those chemical substances which behave differently in acidic and basic medium and help in determining the chemical nature of the substance. Acid base indicators indicate the presence of an acid
or a base by a change in their colour or smell. Indicators can be natural or synthetic.

Synthetic/Chemical Acid Base Indicators
Phenolphthalein: Phenolphthalein remains color-less in acids but turn pink in bases.
Methyl orange: Methyl orange turns pink in acids and becomes yellow in bases.

Natural Indicators

Litmus: Litmus is a natural indicator. Litmus solution is a purple dyewhich is extracted from lichen. Acids turn blue litmus red. Bases turnred litmus blue. Water is essential for acids and bases to change thecolour of litmus paper. Remember that litmus paper will act as anindicator only if either the litmus paper is moist or the acid or base is in the form of aqueous solution. This is because acids and bases release H+ and OH- ions respectively in aqueous solutions.

Olfactory Indicators

Onion: Smell of onion diminishes in a base and remains as it is in an acid..

Vanilla essence: The odour of vanilla essence disappears when it is added to a base. The odour of vanilla essence persists when it is added to an acid.

Turmeric: In acids, yellow colour of turmeric remains yellow. In bases, yellow colour of turmeric turns red.

Red cabbage:The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.

China Rose: China rose is another natural indicator. China rose solution gives dark pink (magenta) colour with acid and green colour with base.
Some Important Points
1- pH of our digestive system: Our stomach produces hydrochloric acid for digestion of food. But during indigestion, excess of acid is produced in the stomach and therefore, the pH decreases. This causes pain and
irritation. So, to neutralise this excess acid, a mild base is used. This mild base works as an antacid. An antacid is any substance, generallya base or basic salt, which counteracts stomach acidity.
2- Tooth decay: Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel is made up of calcium phosphate which is the hardest substance in the body. It does not dissolve in water, but is
corroded when the pH in the mouth is below 5.5. If food particles remain in the mouth after eating, bacteria present in our mouth produce acid by degradation of sugar. This decreases the pH of mouth and hence tooth decay occurs. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralize the excess acid and prevent tooth decay.

Some Important Chemical Compounds

(1) Common Salt (NaCl)

Sodium chloride is known as common salt .Its main source is sea water. It also exists in the form of rocks and sodium chloride obtained from rocks is called rock salt. Common salt is an important component of our food. It is also used for preparing sodium hydroxide, baking soda, washing soda etc.

(2) Sodium Hydroxide or Caustic Soda (NaOH)

Sodium hydroxide is prepared by passing electricity through an aqueous solution of sodium chloride (also known as brine).

2NaCl (aq) + 2H2O (l) 2NaOH (aq) + Cl2 (g) + H2 (g)

This process is known as chlor -alkali process due to the formation of chlorine and sodium hydroxide (an alkali) as the products.

Properties –

1. It is a white translucent solid.

2. It is readily soluble in water to give a strongly alkaline solution.

3. Crystals of sodium hydroxide are deliquescent.

(3) Bleaching Powder (CaOCl2)

Its chemical name is calcium oxychloride .It is prepared by passing chlorine gas through dry slaked lime.

Ca(OH)2 + Cl2⇨ CaOCl2 + H2O

Uses-

1. For bleaching cotton and linen in textile industry.

2. For bleaching wood pulp in paper industry.

3. For disinfecting drinking water.

(4) Baking soda (NaHCO3)

Its chemical name is sodium hydrogen carbonate .It is prepared by passing CO2 gas through brine solution saturated with ammonia.

NaCl + H2O + CO2 + NH3 = NH4Cl + NaHCO3

Ammonium chloride Sodium Hydrogen carbonate

The precipitated sodium hydrogen carbonate is filtered off.

Properties –

1. It is a white crystalline solid, sparingly soluble in water at room temperature.

2. Its aqueous solution is weakly alkaline due to hydrolysis.

3. On heating, it decomposes to give sodium carbonate and carbon dioxide.

2NaHCO3 = Na2CO3 + H2O + CO2

It reacts with acids to give CO2 gas.

NaHCO3 + HCl= NaCl + H2O + CO2

Uses-

1. It is used as a component of baking powder. In addition to sodium hydrogen carbonate baking soda contains tartaric acid.

2. It is used in soda- acid fire extinguisher.

3. It acts as mild antiseptic and antacid.

(5) Washing soda (Na2 CO3 .10 H2O)

Its chemical name is sodium carbonate decahydrate. It is obtained by heating baking soda in turn is obtained by passing CO2 gas through sodium chloride solution saturated with ammonia .

NaCl + H2O + CO2 + NH3 = NH4Cl + NaHCO3

2NaHCO3 = Na2CO3 +H2O + CO2

Recrystallisation of sodium carbonates gives washing soda.

Na2CO3 +10 H2O = Na2CO3 .10 H2O

Uses-

1. It is used in glass, soap and paper industries. 2. It is used for removing permanent hardness of water. 3. It can be used as a cleaning agent for domestic purposes.

(f)Plaster of Paris (CaSO4.1/2 H2O)

Its chemical name is calcium sulphate hemihydrate. It is obtained by heating Gypsum upto 373 K.

Plaster of Paris

Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.CaSO4.2H2O + Heat ⇨ CaSO4.(0.5)H2O + (1.5)H2O

After addition of water Plaster of Paris is again converted into gypsum.CaSO4.(0.5)H2O + (1.5)H2O ⇨ CaSO4.2H2O

Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.

Uses –

1. It is used by Doctors for setting fractured bones.

2. It is used for making statues, models and other decorative materials.

IMPORTANT POINTS

Acid & Base and Fire Extinguisher

Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produce is poured over fire. As carbon dioxide does not support burning, it puts off the fire.

Aqua Regia Contains concentrated Nitric acid and concentrated Hydrochloric acid in the ratio 1:3. Aqua Regia means 'Royal Water' in Latin. It is used to dissolve noble metals like gold and platinum.

Water of crystallization: Many salts contain water molecule and are known as hydrated salts. The water molecule present in salt is known as water of crystallization.

Examples: Copper sulphatepentahydrate (CuSO4.5H2O):Bluecolour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate. After adding water; anhydrous copper sulphate becomes blue again. CuSO4.5H2O + heat ⇨ CuSO4

Ferrous Sulphateheptahydrate (FeSO4.7H2O): The green colour of Ferrous sulphateheptahydrate; commonly known as ferrous sulphate; is due to the presence of 7 molecules of water in it.