CLASS X (ACIDS, BASES & SALTS)

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ACIDS, BASES & SALTS

INTRODUCTION

·         We have always heard of acids, bases and salts in our life.

·         We know acids are the substances which are sour in taste; bases as the substances which are bitter and salts are the substance we use in our food etc.

·         These compounds, acids, bases and salts are interrelated to each other.

·         In this chapter we will learn about these compounds are their properties.

 

ACIDS

·         Acids are the substances which taste sour. Like we have had tomatoes, lemons, vinegar etc. These substances contain acids hence are sour.

·         Acids can be defines as the compounds which release H⁺ ions in water. They turn blue litmus red.

·         Acids conduct electricity as they can dissociates into ions.

·         Acids react with bases to form salts and water, as it is neutralization reaction.

·         It can also be said that acids are the compounds which contains hydrogen that can be replaced partially or completely by a metal or a group of elements acting as a metal to produce salt.

·         There are many substance which have acids in them. Some of these are as follows:

Q. Why is Sulphuric Acid, H₂SO₄ an acid?
Ans: Sulphuric Acid, H₂SO₄ can be called an acid for the following reasons:
i.            It contains Hydrogen atoms in its molecule.

       ii.            The two hydrogen atoms present can be partially or completely removed. eg-

H₂SO₄  + Na à NaHSO₄

H₂SO₄ + 2Na  à Na₂SO₄

H₂SO₄ + NH₃ à NH₄HSO₄

H₂SO₄ + 2NH₃ à (NH₄)₂SO₄

     iii.            It dissolves in water, and this solution turns blue litmus red.

     iv.            It is sour in taste.

       v.            It reacts vigorously with a base to produce salt and water.

H₂SO₄ + NaOH à NaHSO₄ + H₂O

·         The hydrogen atoms present in an acid is called replaceable hydrogen or acidic hydrogen.

·         According to Arrhenius concept, when acid is dissolved in water it dissociates into ions are releases H⁺ ions. eg-

HCl à H⁺ + Cl^-

H₂SO₄ à H⁺ + HSO4^- à H⁺ + SO₄^2-

HNO₃ à H⁺ + NO₃^-

CH₃COOH à H⁺ + CH₃COO^-

 

Ø  CLASSIFICATION OF ACIDS

·         On the basis of the element present in an acid it can be classified as:

1.    Oxo Acids: Acids which contain H and O are called oxo acids. eg- HNO₃ (nitric acid), H₂SO₄ (sulphuric acid) and H₃PO₄ (phosphoric acid) etc.

2.    Hydracids: Acids which contain H and other non-metallic element(s), except O are called hydracids. eg- HCl (hydrochloric acid), HCN (hydrocyanic acid).

Acids can further be classified into:

3.    Organic & Inorganic Acids: Fruits and vegetable which we use in our daily food usually contain natural acids called organic acids. eg- CH₃COOH (acetic acid), HCOOH (formic acid) etc. Chemically prepared acids which we use in laboratories and industries are called inorganic or mineral acids. eg- HNO₃ (nitric acid),   HBr (hydrobromic acid) etc.

4.    Concentrated & Dilute Acids: Acids we use are usually in aqueous solutions, i.e., they are made into solutions by adding water. Now if the concentration of acid is more than water in the solution, we call it concentrated acid, and if the concentration of acid is less than water in the solution, we call it dilute acid. The difference is in the concentration of acid in its solution with water.

5.    Strong & Weak Acids: The strength of an acid is determined by the amount of H+ ions it can lose when dissolved in water. Acids which completely dissociate to give all its H+ ions are called strong acids, whereas acid which partially dissociate to give some of its H+ ions are called weak acids. eg- HCl, HNO3 etc, (strong acids); H2CO3, CH3COOH etc, (weak acids).

 

·         Dilution of acids: While diluting an acid, we add acid to the water not water to the acid. This is so because the dilution of acid is a highly exothermic process if water is added to the acid then water gets vaporised which can spill the acid are cause burn or corrosion.

 

Ø  BASICITY OF AN ACID

·         The basicity of an acid is defined as the number of H-atom contained by an acid. Say HCN has only one H-atom, so its basicity is 1. Basicity of some acids are given below.

Acids with basicity 1 are called monobasic; with 2 dibasic and with 3 tribasic.

 

Ø  PREPARATION OF ACIDS

1.    Synthetic method: In this method acids are prepared by directly combining elements. eg-

electric spark

H2 + Cl2        -->             2HCl

 

S + O₂ à SO₂

2SO₂ + O₂ à 2SO₃

SO₃ + H₂O à H₂SO₄

2.    By dissolving acidic oxides in water: In this method non-metallic oxides, which are acidic in nature are dissolved in water to produce acids. eg-

SO₃ + H₂O à H₂SO₄

CO₂ + H₂O à H₂CO₃

 

Ø  GENERAL PROPERTIES OF AN ACID

1.      They are sour in taste.

2.      They are highly corrosive.

3.      They turn blue litmus red.

4.      Acids show acidic properties in the presence of water.

HCl + H₂O à H₃O⁺ + Cl^-

5.      Their aqueous solution conducts electricity.

6.      They react with certain metals with the evolution of H₂ gas.

2Na + 2HCl à 2NaCl + H₂↑

Fe + 2HCl à FeCl₂ + H₂↑

Mg + H₂SO₄ à MgSO₄ + H₂↑

2Al + 3H₂SO₄ à Al₂(SO₄)₃ + 3H₂↑

Least reactive metals like Hg, Cu, Ag, Au, Pt etc. do not displace H-atom from acids.

Nitric acid only reacts with Mg and Mn to displace H-atom to produce H₂ gas

Mg + 2HNO₃ à Mg(NO₃)₂ + H₂↑

Mn + 2HNO₃ à Mn(NO₃)₂ + H₂↑

7.      They react with bases to from salt and water.

8.      They react with carbonates and hydrogencarbonates (bicarbonates) to form salt, carbon dioxide and water.

Na₂CO₃ + 2HCl à 2NaCl + H₂O + CO₂↑

Na₂CO₃ + H₂SO₄ à Na₂SO₄ + H₂O + CO₂↑

CaCO₃ + 2HCl à CaCl₂ + H₂O + CO₂↑

NaHCO₃ + HCl à NaCl + H₂O + CO₂↑

2NaHCO₃ + H₂SO₄ à Na₂SO₄ + 2H₂O + CO₂↑

Due to the release of CO₂ gas, acids can be used to make fire extinguishers.

9.      Acids react with oxides of metals to form salts and water.

Na₂O + 2HCl à 2NaCl + H₂O

CuO + H₂SO₄ à CuSO₄ + H₂O

CaO + 2HCl à CaCl₂ + H₂O

 

There are many compounds which contain H-atom, but not all act as an acid, this is so because the compounds which loses H⁺ ion in water is termed as acid.

 

Ø  USES OF ACIDS

BASES

·         Bases are the compounds, usually the oxides or hydroxides of metals, which can react with acids to form salt and water.

·         They dissociate in water to form OH^- ions. eg- Na₂O (sodium oxide), CaO (calcium oxide), NaOH (sodium hydroxide). Ca(OH)₂ (calcium hydroxide) etc.

NaOH à Na⁺ + OH^-

KOH à K⁺ + OH^-

Ca(OH)₂ à Ca²⁺ + 2OH^-

·          Various compounds which act like bases, as they form salts with acids but do not give water. eg- NH₃, it reacts with acids to form salt but no water. Whereas NH₄OH an hydroxide of ammonia reacts with acids to form salt and water, hence can be termed as a base.

NH₄OH + HCl à NH₄Cl + H₂O

 

Ø  ALKALIS

·         Those bases which are soluble in water are called alkalis.

·         Bases like NaOH, KOH, Ca(OH)₂ are soluble in water, hence are alkalis, where as bases like Cu(OH)₂, Fe(OH)₃, Al(OH)₃ are not soluble in water hence are not alkalis.

·         So all alkalis are bases but not all bases are alkalis.

·         Examples of some of the acids are given below:

Ø  PREPARATION OF BASES

1.    By the direct union of a metal with oxygen: Some metals when heated in presence of oxygen forms metal oxides which are bases.

4Na + O₂ à 2Na₂O

4K + O₂ à 2K₂O

2Ca + O₂ à 2CaO

These oxides when dissolved in water form hydroxides of metals which are also bases.

Na₂O + H₂O à 2NaOH

K₂O + H₂O à 2KOH

CaO + H₂O à Ca(OH)₂

2.      By heating carbonates of some metals: Carbonates of some metals when heated form metal oxides which are bases.                                               Δ

CaCO₃ à CaO + CO₂↑

                                                                                 Δ

ZnCO₃ à ZnO + CO₂↑

 

Ø  GENERAL PROPERTIES OF BASES

1.      They are bitter in taste.

2.      They are poisonous.

3.      Their solutions give a soapy touch.

4.      Bases when dissolved in water they produce OH^– ions.

NaOH à Na⁺ + OH^-

Ca(OH)₂ à Ca⁺ + 2OH^-

Mg(OH)₂ à Mg²⁺ + 2OH^-

5.      Due to the presence of ions in aqueous solution of bases, they can conduct electricity.

6.      They turn blue litmus red.

7.      They react with acids to undergo neutralization reaction to form salt and water.

NaOH + HCl à NaCl + H₂O

2KOH + H₂SO₄ à K₂SO₄ + 2H₂O

Cu(OH)₂ + H₂SO₄ à CuSO₄ + 2H₂O

 

Q. How does a neutralization reaction occur?

Ans: Lets take the example of NaOH + HCl à NaCl + H₂O

            For an acid or base to work, it has to be made into an aqueous solution. Acids release H⁺ ion        and base release OH^- ion in water. So,

NaOH à Na⁺ + OH^-

HCl à H⁺ + Cl^-

            Now oppositely charged ions attract each other and they combine i.e.,

Na⁺ + OH^- + H⁺ + Cl^- à (Na⁺ + Cl^-) (H⁺ + OH^-) à NaCl + H₂O

8.      The oxides of non-metals are acidic in nature, i.e., when they a dissolved in water they produce acids. When they are reacted with bases they give neutralization reaction to form salt and water.

2NaOH + CO₂ à Na₂CO₃ + H₂O

Ca(OH)₂ + CO₂ à CaCO₃ + H₂O

9.      When bases are heated with an ammonium salt they for salt water and ammonia gas.

NaOH + NH₄Cl + Δ à NaCl + H₂O + NH₃↑

            Ammonia gas is recognized by its pungent smell.

10.  Sometime bases react with certain salts to produce other salts and bases.

6NH₄OH + Al₂(SO₄)₃ à 2Al(OH)₃ + 3(NH₄)₂SO₄

 

Ø  STRONG BASES AND WEAK BASES

·         Strength of a base is determined by the amount of OH^- ions dissociated in water.

·         The bases which completely dissociate when dissolved in water are called strong bases. eg- NaOH, KOH etc.

·         The bases which partially dissociate when dissolved in water are called weak bases. eg- Mg(OH)₃, NH₄OH etc.

 

Ø  COMMON PROPERTIES OF ACIDS AND BASES

·         Both acids and bases conduct electricity when in aqueous solution, as they dissociate into ions.

·         This ability to dissociate into ions in aqueous solution is a common property of acids and bases.

 

Ø  ACIDITY OF A BASE

·         The acidity of a base can be defined as the number of OH^- ions that a base can dissociate in water. Say NaOH it has only one OH^- ion to dissociate hence its acidity will be 1.

·         Acidity of some of the bases are given below:

Base	Acidity
KOH NH4OH Ca(OH)2 Ba(OH)2 Fe(OH)3 Al(OH)3	1 1 2 2 3 3

Bases with acidity 1 are called monoacidic; with 2 diacidic and with 3 triacidic.

 

Ø  USES OF SOME COMMON BASES

·         Uses of some common bases are given below:

 

INDICATORS

·         An acid base indicator is defined as the substance that can assume different colour or smell in acidic and basic medium. Examples of some indicators are given below.

·         Here, litmus is a natural indicator; phenolphthalein and methyl orange are synthetic indicators.

·         Litmus is a natural dye that is made from small plants called lichens. It is naturally purple in colour and turned red by acidifying and blue by alkylation. Filter papers are dipped in this solution to make litmus papers.

·         Phenolphthalein is a colourless compound, its alcoholic solution is used as an indicator which turn red when put in an alkaline solution and shows no change in acidic solution.

·         Methyl orange is a solid compound, which is dissolved in hot water and filtered to be used as an indicator. It is orange in colour; it turns red in acidic solution and yellow in basic solution.

 

Ø  Household indicators

·         Some useful house hold indicators are given below:

             I.            Turmeric juice: It is yellow in colour and remains the same in acidic or neutral solutions, but turns deep brown in alkaline solution.

          II.            Red cabbage juice: It is purple in colour, it turns red in acidic solution and green in alkaline solution.

·         Some of the household items which are either acidic or basic are given below. They can be used to test the household indicators.

 

Ø  Olfactory indicators

·         Olfactory indicators are substance which changes its smell in acidic and basic medium.

·         Onion: The smell of onion disappears in basic medium, but is retained in acidic medium.

·         Vanilla essence: The smell of vanilla disappears in basic medium, but is retained in acidic medium.

·         Clove oil: Clove oil give different smell in acidic and basic medium.

 

SALTS

·         Salts are the product of neutralization reaction between acids and bases, in which hydrogen of the acid is replaced with the metal.  

·         They are usually ionic compounds.

NaOH + HCl à NaCl + H₂O

H₂SO₄ + NaOH à NaHSO₄ + H₂O

H₂SO₄ + 2NaOH à Na₂SO₄ + 2H₂O

 

Ø  TYPES OF SALTS

1.      Normal or Neutral Salts: Salts which do not have any replicable hydrogen atoms or hydroxyl groups are called normal or neutral salts. eg- Na₂SO₄, NaCl etc.

2.      Acidic Salts: Salts which have one or more replicable hydrogen atom are called acidic salts. eg- NaHSO_4, Na₂HPO₄, NaH₂PO₄ etc.

3.      Basic Salts: Salts which have one or more replicable hydroxyl groups are called basic salts. eg- Pb(OH)Cl, Bi(OH)₂NO₃ etc.

4.      Double Salts: Salts which have two negative and/or position ions, are called double salts. They only exist in solid state, when dissolved in water they dissociate into ions and breaks. eg- CaCO₃.MgCO₃, K₂SO₄.Al₂(SO₄)₂ etc.

K₂SO₄ à 2K⁺ + SO₄^2-

Al₂(SO₄)₃ à 2Al³⁺ + 3SO₄^2-

 

 

Ø  PREPARATION OF SALTS

1.    By the reaction between metals and acids: Certain metals (highly reactive) reacts with HCl or H₂SO₄ to form salts.

Zn + 2HCl à ZnCl₂ + H₂↑

Zn + H₂SO₄ à ZnSO₄ + H₂↑

2.    By the reaction between an acid and a base: Acids reacts with bases to form salt and water.

NaOH + HCl à NaCl + H₂O

CuO + 2HCl à CuCl₂ + H₂O

 

Ø  GENERAL PROPERTIES OF SALTS

1.      Salts react with acids to undergo double displacement to form other salts and acids.

NaCl + H₂SO₄ à NaHSO₄ + HCl

2.      Salts react with bases to undergo double displacement to form other salts and bases.

(NH₄)₂SO₄ + 2NaOH à Na₂SO₄ + 2NH₄OH

3.      Salts reaction with highly reactive metals to undergo displacement to form salt and give out metal.

CuSO₄ + Fe à FeSO₄ + Cu↓

4.      Reaction of salt and water yields various products depending on the salt.

Na₂CO₃ + 2H₂O à 2NaOH + CO₂↑ + H₂O

CH₃COONa + H₂O à CH₃COOH + NaOH

AlCl₃ + 3H₂O à Al(OH)₃ + 3HCl

NH₄Cl + H₂O à NH₄OH + HCl

NaHCO₃, tough it has replicable H-atom in it, but acts as slightly alkaline.

 

Ø  USES OF SALTS

pH SCALE CONCEPT

·         The acidity of a base and the basicity of an acid is measured n terms of H⁺ ion concentration or OH^- ion concentration.

·         The expression in terms of H+ ion concentration is called the pH concept.

·         pH of a solution is the negative logarithm of its H+ ion concentration. pH=-log[H⁺]

·         pOH of a solution is the negative logarithm of its OH^- ion concentration. pOH=-log[OH^-]

·         The pH of a solution indicated how acidic or basic the solution is.

·         In a solution, concentration of H+ ion and OH- ion is very less, as pH is negative logarithm, hence the value comes out positive and the inconvenience of working with small number is eliminated.

·         The pH is only a number; it has no unit to it.

·         For neutral solution, [H⁺]=1.0x10^-7; pH=-log(1.0x10^-7)=7

·         For acidic solution, [H⁺]>1.0x10^-7; pH<7

·         For basic solution, [H⁺]<1.0x10^-7; pH>7

·         The pH sale is a scale of 15 points starting from 0 to 14; in which 0 to 7 is acidic, 7 is neutral and 7 to 14 is basic.

·         The pH indicator solution shows a different colour for different pH values.

·         pH scale and colour chart is shown below:

·         The lower the value, more acidic the solution is.

·         The higher the value, more basic the solution is.

·         Pure water has the pH of 7, which is neutral.

·         pH values of some common solutions are given below:

Ø  ROLE OF pH IN EVERYDAY LIFE (IMPORTANCE OF pH) (EFFECT OF pH IN OUR DAILY LIFE)

1.    In our digestive system: HCl produced in our stomach helps in the digestion of food without causing any harm to the stomach. Sometime the amount of acid goes more than the limit due to indigestion, causing pain, irritation and acidity in our stomach. In order to neutralize this we take mild bases called antacids. Milk of magnesia is one such medicine, which is made up of Mg(OH)₂ and MgCO₃.

2.    Acids cause tooth decay: When we eat sugary food, it gets stuck in our teeth. This is degraded by the bacteria present in our mouth to produce acid called sugar acid, when causes tooth decay. To neutralize this we brush out teeth with toothpaste which contain slight amount of bases. Neem sticks also contain slight amount of alkalis and saliva itself is an alkali.

3.    Acid is produced in fatigued muscle: When we exercise, anaerobic metabolism occurs due to lack of oxygen in the muscle cells which leads to the formation of lactic acid, which causes stiffness and pain in the muscles.

4.    Some animal and plant contain acids: Insects like bee and ants have formic acid in their sting, hence when they sting it causes a burning sensation. Nettle leaves are stinging, when in contact with them it starts to burn and itch. Baking soda (NaHCO₃), which is mildly basic can be used to get relief from these burns.

5.    Brilliance of old copper vessels can be returned: The glow of copper vessels disappear after certain time of use due to oxidation. Acids can be used to remove these layers of oxides to bring back the brilliance of the copper vessel.

6.    pH of Soil: Soil is generally acidic. Plants require a definite pH range to grow, they cannot grow in alkaline pH. Now pH of the soil has to be maintained. If the pH of the soil is too high, it can be neutralized using quicklime, slaked lime or calcium hydroxide.

7.    Colour of hydrangea: The colour of hydrangea depends on the pH of the soil, if the pH is acidic (5.5 or lower) the colour is blue and if the pH is alkaline (6.5 or higher) it is pink, the colour purple is the pH is between (5.5 and 6.5).

8.    Acid rain: When gases present in air like CO₂, NO₂ and SO₂ etc. mixes with water vapours in clouds they form acids like H₂CO₃, HNO₃ and H₂SO₄, which causes acid rain.

9.    pH of humans: For the body of humans and other animas to function properly it has to undergo a log of biochemical reactions, which can undergo under a very certain range of pH between 7.35 to 7.45.

 

Ø  pH OF SALTS

1.      Salts of strong acids and strong bases: These are neutral salts with pH=7. eg- NaCl, KNO₃ and Na₂SO₄ etc.

2.      Salts of strong acids and weak bases: These are slightly acidic salts with pH<7. eg- AlCl₃, CuSO₄, ZnSO₄ etc.

3.      Salts of weak acids and strong bases: These are slightly basic salts with pH>7. eg- CH₃COONa, Na₂CO₃ and NaHCO₃ etc.

If you find these notes really helpful and easy to understand. Do like, comment and share. Keep following Gone Nuclear for more notes on CBSE IX, X, XI & XII. Follow us on Facebook on Gone Nuclear

Atish J. Bain

HOD (Dept. of Chemistry)

St. Thomas High School (10+2)

Dhanbad, Jharkhand