CBSE Class 7 Science Acids Bases and Salts Notes

Lewis acids

i) Compounds where the central atom has incomplete octet. This means all electron deficient molecules can act as Lewis acids e.g.

BF₃, BBr₃, AlCl₃, FeCl₃, GaCl₃ etc.

ii) Compounds in which the central atom has available vacant orbital e.g. SnF₄, SnCl₂, SnCl₄, PF₃, PF₄, SF₄, TiCl₄ etc.

iii) Simple cations like Ag⁺, Cu²⁺, Al³⁺ etc. e.g. Cu²⁺ + 4NH₃ --------> [Cu (NH₃)₄]²⁺

Lewis Bases

All simple anions are Lewis bases e.g. Cl–, NO₃ –, OH–.

ii) Molecules containing one or more unshared pairs of electrons (Lone pairs) e.g. H₂O, NH₃, ROH, NX₃, R₂S, etc. are Lewis bases

iii) Multiple bonded compounds which can donate the pair of electrons. e.g. C = O, N = O, CH₂ = CH₂, CH = CH.

SALT

A substance which ionizes in water to produce ions other than H⁺ and OH^– is called a salt.

Types of Salts

Neutral Salts: Those salts whose aqueous solutions neither turn blue litmus red nor red litmus blue are called neutral salts. These are prepared by the neutralization of strong acid and strong base. e.g. NaCl, K₂SO₄, KNO₃ etc.

Acidic Salts: Those salts whose aqueous solutions turn blue litmus red are called Acidic salts. These are prepared by neutralisation of strong acid with weak base, e.g., NH₄NO₃, NH₄Cl.

Basic Salts: Those salts whose aqueous solutions turn red litmus blue are called basic salts. These are formed by the neutralization of strong bases with weak acids. e.g. Na₂CO₃, CH₃COONa

Mixed salts: Salts formed by the neutralization of one acid by two bases or one base by two acids are called mixed salts. e.g. CaOCl₂

Double Salts: A compound of two salts whose aqueous solution shows the tests for all constituent ions is called double salt e.g.

Mohr Salt FeSO₄. (NH₄)₂ SO₄. 6H₂O

Potash Alum K₂SO₄. Al₂ (SO₄)₃. 24H₂O

Complex Salts: A compound whose solutions does not give tests for the constituent ions is called a complex salt. e.g.

K₄ [Fe(CN)₆]

Li (AlH₄)

pH : pH scale is used to measure the strengths of acidic and alkaline solutions. It may be defined in number of ways.

pH is defined as the negative logarithm of its [H+] ion concentrations

pH = –log[H⁺] = log 1 / [H⁺]

pH values do not give instantaneous idea above the relative strengths of the solution

pOH: The solution with less than 7 pH are acidic, the solutions with pH value more tan 7 are alkaline and solution with pH value equal to 7 are neutral. It may be defined as the negative logarithms of hydroxyl ions concentration

pOH = –log (OH^–) = log 1 / [OH^- ]

Ionic product of water

[H⁺] [OH^–] = 10^–14 = K_w

–log[H⁺] (+) –log[OH^–] = 14 = PK_w

⇒ pH + pOH = pK_w = 14.

Indicators: Universal indicators give different colours with different strength of acidity and alkalinity. Universal indicators are mixture of indicators to produce a range of colours. It comes in liquid or paper form eg. pH, .

• The different papers colours are:

• Red for strong acids

• Orange / Yellow for weak acids

• Green for neutral substance

• Turquise for weak alkali

• Deep blue for strong alkali

SOME IMPORTANT CHEMICALS

Bleaching Powder (CaOCl₂) 

Preparation In Hasen-Clever Plant

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Properties 1. It is yellowish white solid having chlorine like smell

2. Bleaching powder decomposes to form O₂

                   CoCl₂

2CaOCl₂  -------------> 2CaCl₂ + O₂

3. When reacted with H2O liberates Cl2.

CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂

4. Bleaching powder is decomposed to Cl2 by dilute H2SO4.

CaOCl₂ + H₂SO₄ → CaSO₄ + H₂O + Cl₂

5. It reacts with carbondioxide to form calcium carbonate and Cl2.

CaOCl₂ + CO₂ → CaCO₃ + Cl₂

6. The chlorine liberated in above reaction can be used to oxidise I^– to I₂ thereby liberating I₂. This I₂ can be estimated by Na₂S₂O₃ thus we can measure the available chlorine in bleaching powder.

2KI + Cl₂ → 2KCl + I₂

Uses Bleaching powder is used for the disinfection of drinking water or swimming pool water. For use in outdoor swimming pools, it can be used as a sanitizer in combination with a cyanuric acid stabilizer. The stabilizer will reduce the loss of chlorine because of UV radiation.

Calcium does make the water ‘hard’ and tends to clog up some filters, for this reason Sodium hypochlorite is preferred. Bleaching powder is also used for bleaching cotton and linen and used in the manufacture of chloroform.

Sodium Hydroxide (NaOH)

Preparation 1. Soda lime process, causticisation process or Gossage process

Na₂CO₃ + Ca (OH)₂ ^{80 - 90ºC >} 2NaOH + CaCO₃

2. Castner kellner proces - Electrolytic process

Electrolyte → Brine (NaCl Solution in water)

Cathode → Iron rods and Mercury acts as intermediate cathode by induction.

Reaction NaCl → Na⁺ + Cl^–

At anode                        – Cl– → Cl + e

                                        Cl + Cl → Cl₂

At cathode – Na⁺ + e^– + Hg → Na – Hg

                   2Na – Hg + 2H2O →  2NaOH + H₂ + 2Hg
                   (sodium amalgam)

Properties 1. White hygroscopic solid.

                  2. Sodium hydroxide decomposes on heating to form sodium, hydrogen and oxygen.

                                 _1300ºC
                    2NaOH ----------> 2Na + H₂ + O₂

                 3. It reacts with acids to form salt and water

                     a) NaOH + HCl →  NaCl + H₂O