CBSE Class 10 Chemistry Acids Bases And Salts Notes Set 03

ACIDS, BASES & SALTS

Acids

According to Arrhenius theory, "An acid is a substance which when dissolved in water, ionizes and releases hydrogen ions [\( H^+(aq) \)] in solution".

Example,
(i) \( HCl(aq) \to H^+(aq) + Cl^-(aq) \)
Hydrochloric acid   Hydrogen ion   Chloride ion
or \( HCl(g) + H_2O \to H_3O^+(aq) + Cl^-(aq) \)
(ii) \( H_2SO_4(aq) \to 2H^+(aq) + SO_4^{2-}(aq) \)
Sulphuric acid   Hydrogen ion   Sulphate ion

Hydrogen ion do not exist as \( H^+ \) ions in solution, they attach themselves to the polar water molecules to form hydronium ions (\( H_3O^+ \) or \( H^+(aq) \)).
\( H^+ + H_2O \to H_3O^+ \)

Properties of acid

• On the basis of sources can be classified as organic or inorganic.
• On the basis of number of displaceable \( H^+ \) ions (basicity) per molecule, can be classified as monobasic, dibasic or tribasic. Eg. \( HCl \), \( H_2SO_4 \) and \( H_3PO_4 \) respectively.
• On the basis of degree of ionisation, can be classified as strong or weak acid. Eg. \( HCl \) and \( CH_3COOH \) respectively.
• On the basis of concentration, can be classified as concentrated and dilute acids.

Bases

According to Arrhenius theory "Those substances which give hydroxide or hydroxyl ion (\( OH^- \)) in their aqueous solution are called bases".

Examples,
(i) \( NaOH(aq) \to Na^+(aq) + OH^-(aq) \)
Sodium hydroxide   Sodium ion   Hydroxyl ion
(ii) \( KOH(aq) \to K^+(aq) + OH^-(aq) \)
Potassium hydroxide   Potassium ion   Hydroxyl ion

Properties of bases

• On the basis of number of hydroxyl ion (\( OH^- \)) produced (acidity), can be of classified as monoacidic, diacidic or triacidic bases. Eg. \( NaOH \), \( Ca(OH)_2 \) & \( Al(OH)_3 \) respectively.
• On the basis of degree of ionization, can be classified as strong base and weak base. Eg. \( NaOH \) & \( NH_4OH \) respectively.
• On the basis of concentration, can be classified as concentrated or dilute bases.

Chemical properties of acids & bases

Indicators

• China rose is a flower which has pink petals. The coloured solution extracted from it is light pink colour. When used as an indicator, its colour changes to green in basic solution and dark pink colour (magenta) in acidic solutions. In neutral solutions, there is no change in colour.
• Litmus solution is a purple dye, which is extracted from lichens. Its colour is purple in a neutral solution.
• Other natural materials like red cabbage leaves, turmeric, coloured petals of Hydrangea, Petunia and Geranium are also used as indicators.

Indicator Changes:
- Blue litmus paper: Blue to Red in acidic; Red to Blue in basic.
- Methyl orange: Orange to red in acidic; Orange to yellow in basic.
- Phenolphthalein: Remains colourless in acidic; Colourless to pink in basic.
- Turmeric paper: No change in colour in acidic; Yellow to red brown in basic.

Reaction of Acids & Bases with Metals

\( \text{Metal} + \text{Acid} \to \text{Metal Salt} + H_2 \)
\( Zn + 2HCl \to ZnCl_2 + H_2 \)

\( \text{Metal} + \text{Alkali} \xrightarrow{\text{warm}} \text{Metal Salt} + H_2 \)
\( Zn + 2NaOH \xrightarrow{\text{warm}} Na_2ZnO_2 + H_2 \)

• \( Al, Sn, Pb, Zn \) form \( NaAlO_2, Na_2SnO_2, Na_2PbO_2 \) and \( Na_2ZnO_2 \), respectively with \( NaOH \) (alkali).
• Evolved \( H_2 \) gas is tested with a burning splinter. It burns with a pop sound.

Reaction of Acid with Metal Carbonates/Hydrogen Carbonates

\( \text{Metal Carbonate/Metal hydrogen carbonate} + \text{Acid} \to \text{Salt} + \text{Carbon dioxide} + \text{Water} \)
\( Na_2CO_3 + 2HCl \to 2NaCl + CO_2 + H_2O \)
\( NaHCO_3 + HCl \to NaCl + CO_2 + H_2O \)

• Evolved \( CO_2 \) gas turns lime water milky.
  \( Ca(OH)_2 + CO_2(g) \to CaCO_3(s) + H_2O \)
  (lime water)   (calcium carbonate - insoluble white ppt.)
• On passing excess \( CO_2 \), the solution becomes clear due to conversion of insoluble \( CaCO_3 \) into soluble calcium hydrogencarbonate.
  \( CaCO_3(s) + H_2O + CO_2(g) \to Ca(HCO_3)_2(aq) \)
  (Calcium hydrogen carbonate - Soluble in water)

Reaction of Acids with Bases

\( \text{Acid} + \text{Base} \to \text{Salt} + H_2O \)
(i) \( HCl + NaOH \to NaCl + H_2O \)
(ii) \( CO_2 + Ca(OH)_2 \to CaCO_3 + H_2O \)
(iii) \( H_2SO_4 + MgO \to MgSO_4 + H_2O \)

• All reactions take place in aqueous solutions.
• Reaction (ii) and (iii) suggest that non-metallic oxides are generally acidic and metallic oxides are generally basic in nature.
• These reactions are generally exothermic.
• All acids release \( H^+ \) ion in their aqueous solutions. \( H^+ \) ion released combines with \( H_2O \) and form hydronium ion (\( H_3O^+ \)). The separation of \( H^+ \) ion from an acid molecule cannot occur in the absence of water. Hence, acids show acidic character only in presence of water.
• As acids and bases dissociate into ions they conduct electricity in their aqueous solution.
• As dilution of acids or bases occur, concentration of ions (\( H_3O^+/OH^- \)) per unit volume decreases.

pH scale

A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in \( OH^- \) ion concentration in the solution, that is, increase in the strength of alkali. Generally paper impregnated with the universal indicator is used for measuring pH.

• The strength of acids and bases depends on the number of \( H^+ \) ions and \( OH^- \) ions produced, respectively. If we take hydrochloric acid and acetic acid of the same concentration, say one molar, then these produce different amounts of hydrogen ions. Acids that give rise to more \( H^+ \) ions are said to be strong acids, and acids that give less \( H^+ \) ions are said to be weak acids.
• Solutions with pH = 0-3 are strongly acidic, with pH = 3–5 are moderately acidic while with pH = 5–7 are weakly acidic.
• Solution with pH = 7–9 are weakly basic, with pH = 9–12 are moderately basic while with pH = 12–14 are strongly basic.
• If pH = 7, then the solution is neutral. It is expressed as \( pH = -\log_{10} [H^+] \).

Importance of pH in everyday life

• Plants and animals are pH sensitive: All living organisms are pH sensitive. e.g. human body works in a narrow pH range of 7.0 to 7.8. Aquatic life gets disturbed due to acid rain (pH below 5.6) when pH of water bodies falls.
• Soil pH and plants: Different plants prefer different pH range for their growth. Soil should be treated with suitable material (acidic/basic) accordingly to obtain required pH of the soil. Acidic soil can be treated with \( CaO \), \( Ca(OH)_2 \) or \( CaCO_3 \) while basic soil can be treated with manure or compost.
• Importance of pH in our digestive system: The acid produced in our stomach kills the bacteria & germs. Excess of acid produced (due to overeating) causes acidity. Antacid such as \( Mg(OH)_2 \), \( NaHCO_3 \), etc., relieves us from such pains.
• pH change as the cause of tooth decay: pH of our mouth is > 7. Food, left in our mouth after eating, gets degraded by bacterial action. This results in lowering the pH to 5.5 (acidic). The acid so formed reacts with calcium phosphate and causes tooth enamel to corrode. Therefore, usage of toothpaste (base) neutralizes excess of acid and prevents tooth decay.
• Self defence by animals and plants through chemical warfare: Sting of honey bees containing formic acid can be neutralized by applying baking soda paste. Also, hair present on nettle leaves inject methanoic acid into the skin which can be neutralized by either applying baking soda or extract of dock plant leaves which grows beside nettle plant.

Salts

Salts are the ionic compounds consisting of two parts, one part carrying a positive charge called positive ion or cation and the other part carrying a negative charge called a negative ion or anion.

Nature of salt produced:
1. \( NaCl + H_2O \) (from \( HCl \) strong acid + \( NaOH \) strong base). Nature: Neutral. Hence, pH = 7.
2. \( NH_4Cl + H_2O \) (from \( HCl \) strong acid + \( NH_4OH \) weak base). Nature: Acidic. Hence, pH < 7.
3. \( Na_2CO_3 + H_2O \) (from \( H_2CO_3 \) weak acid + \( NaOH \) strong base). Nature: Basic. Hence, pH > 7.
4. \( CH_3COONH_4 + H_2O \) (from \( CH_3COOH \) weak acid + \( NH_4OH \) weak base). Nature: Neutral. Hence, pH \(\approx\) 7.

Chemicals from common salt (NaCl)

NaOH - Chloralkali process
Electrolysis of brine (\( NaCl + H_2O \)):
- At Anode: \( Cl_2 \) gas is evolved.
- At Cathode: \( H_2 \) gas is evolved.
- Near Cathode: \( NaOH \) solution is formed.
Reaction: \( Cl_2 + Ca(OH)_2 \to CaOCl_2 + H_2O \) (Formation of Bleaching powder).
Also, \( H_2 + Cl_2 \to 2HCl \).
Reaction near cathode: \( NaOH + Cl_2 \to NaOCl \) (Bleach).

Uses of Bleaching powder

(i) for bleaching cotton and linen in the textile industry, for bleaching of wood pulp in paper factories and for bleaching washed clothes in laundry;

(ii) as an oxidising agent in many chemical industries; and

(iii) for disinfecting drinking water to make it free of germs.

Baking soda (\( NaHCO_3 \)) & Washing soda (\( Na_2CO_3 \)) - Solvay’s process

Step I: \( CaCO_3 \xrightarrow{\Delta} CaO + CO_2(g) \)
\( NaCl + H_2O + NH_3 + CO_2 \to NH_4Cl(aq) + NaHCO_3(s) \)

Uses of sodium hydrogencarbonate (\( NaHCO_3 \))

(i) For making baking powder, which is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place:
\( NaHCO_3 + H^+ \to CO_2 + H_2O + \text{Sodium salt of acid} \)
Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.

(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

(iii) It is also used in soda-acid fire extinguishers.

\( NaHCO_3 \) can be used to neutralize an acid because it is a mild non-corrosive base due to the presence of \( HCO_3^- \) ion.

Washing Soda:
Step II: \( 2NaHCO_3 \to Na_2CO_3 + H_2O + CO_2 \) (Formation of anhydrous sodium carbonate or soda ash).
Step III: \( Na_2CO_3(s) + 10H_2O(l) \to Na_2CO_3 \cdot 10H_2O \) (Crystallization to Sodium carbonate deca hydrate).

Uses of washing soda

(a) Washing soda (or sodium carbonate) is used for washing clothes (laundry purposes).

(b) Washing soda is used for softening hard water.

(c) Sodium carbonate (soda ash) is used for the manufacture of detergents.

(d) Sodium carbonate is used for the manufacture of many important compounds, such as borax (\( Na_2B_4O_7 \)), hypo (\( Na_2S_2O_3 \cdot 5H_2O \)), etc.

(e) Sodium carbonate is also used in paper, glass, soap and paint industries.

Are the crystals of salts really dry?

Crystals of some salts contain certain amount of associated water. The water associated with the crystal (or molecule) of any salt is called water of crystallisation. The salt containing water of crystallisation are called hydrated salts. Water of crystallization is the fixed number of water molecules present in one formula unit of a crystalline salt. e.g., \( CuSO_4 \cdot 5H_2O \) (blue), \( FeSO_4 \cdot 7H_2O \) (green).

• Crystalline salts on heating loose their water of crystallization. This can be seen by observing the colour change in the compound on heating.
  e.g. \( CuSO_4 \cdot 5H_2O \text{ (Blue)} \xrightarrow{\Delta} CuSO_4 + 5H_2O \text{ (White)} \)

Plaster of Paris (\( CaSO_4 \cdot \frac{1}{2}H_2O \))

Preparation:
\( 2[CaSO_4 \cdot 2H_2O] \xrightarrow{373 K} (CaSO_4)_2 \cdot H_2O + 3H_2O \)
(Gypsum)   (Plaster of Paris)

• Gypsum requires controlled heating at 373 K to avoid complete loss of water resulting into \( CaSO_4 \) (anhydrous calcium sulphate) which does not set into a hard mass when water is mixed.
  \( CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \to CaSO_4 \cdot 2H_2O \)
  (Plaster of Paris)   (Water)   (Gypsum - Hard mass)
• If heated more than 373 K:
  \( CaSO_4 \cdot 2H_2O \xrightarrow{> 373 K} CaSO_4 + 2H_2O \)
  (Anhydrous calcium sulphate or Dead burnt plaster)

Properties of Plaster of Paris

(1) Plaster of Paris is a white, odourless powder.

(2) At room temperature, Plaster of Paris absorbs water and a large amount of heat is liberated.

(3) When mixed with a limited amount of water (50% by mass), it forms a plastic mass, evolves heat and quickly sets to a hard porous mass within minutes. This is called the setting process. During setting, a slight expansion in volume occurs. It is due to this that it fills the mould completely and gives sharp impression.

Uses of Plaster of Paris

(1) Plaster of Paris is used in making casts and patterns for moulds and statues.

(2) Plaster of Paris is used as cement in ornamental casting and for making decorative materials.

(3) Plaster of Paris is used as a fire proofing material and for making chalks.

(4) Plaster of Paris is used in hospitals for immobilising the affected part in case of bone fracture or strain.

(5) Plaster of Paris (POP) is used to fill small gaps on walls & roofs.