CBSE Class 7 Science Acids Bases and Salts Notes

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Revision Notes for Class 7 Science Chapter 5 Acids, Bases and Salts

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Chapter 5 Acids, Bases and Salts Notes Class 7 Science

Acid, Base and Salts
Acids and Bases
 
1. Different workers gave different definitions for acids and bases from time to time.
 
i) Arrhenius Theory : Arrhenius defined acids are those compounds which give hydrogen ions in aqueous solution, and bases are those compounds which give hydroxide ions in solutions.e.g.,
Class 7 Science Acid Base and Salts Advanced Notes
The strength of an acid or base depends upon its tendency to furnish H+ or OH ions in solution.
 
This definition can be applied only for those reactions which take place in aqueous solutions.
 
Further this theory does not include substances that do not contain H+ and OH ions but still can neutralize acids and bases.
 
Basicity or protonicity of acids: - It is the number of H+ ions furnished by a molecule of an acid.
 
Monobasic acid : An acid furnishing one H+ ions e.g.HCl, HNO3 etc.
 
Dibasic acid : An acid furnishing two H+ ions e.g. H2SO4, H3PO3 etc.
 
Tribasic acid : An acid furnishing three H+ ions e.g. H3PO4.
Acidity or Hydroxicity of base : It may be defined as the number of OH ions furnished by a molecule of a base.
Monoacidic base : NaOH, KOH
Diacidic base : Ca(OH)2, Ba(OH)2
 
ii) Bronsted Lowry Theory : According to this theory acids are defined as those substances which give up proton and bases are substances which accepts proton.e.g.,
Class 7 Science Acid Base and Salts Advanced Notes
From the above observations we may conclude that every acid has a conjugate base and every base has a conjugate acid.
 
In above equation 1, the conjugate base of acid HCl is Cl and The conjugate acid of water is H3O+ (hydroxonium ion)
 
In the above equation 2, the conjugate acid of base NH3 is NH4+ and the conjugate base of H2O is OH.
 
Thus according to this theory the following conclusions can be drawn.
 
a) A substance can act as an acid/base when another substance capable of accepting a proton and capable of donating a proton exists.
 
b) In aqueous solutions H+ ions exist as hydrated ions or H3O+ (H+. H2O) ions.
 
c) Even ions may act as acids or bases.
 
d) Water, can act both as an acid or base because it can give off a proton as well as can accept a proton.
Class 7 Science Acid Base and Salts Advanced Notes
e) The strength of acids (or bases) depends on the medium which acts as base (or acids)
HCl + H2O → H3O+ + Cl
HCl + NH3 → NH+ + Cl
HCl + Benzene → No reaction
 
Acidic nature of HCl is greatest in ammonia and least in benzene.
 
Classification of Solvents
 
i) Aprotic: Solvents which can neither accept nor donate a proton.e.g. CCl4, CS2, Benzene.
 
ii) Amphiprotic: Solvents which can both accept and donate a proton, e.g. H2O, liquor Ammonia.
 
iii) Protophillic: Solvents which have a greater tendency to accept proton e.g. H2O, R-OH, liquor NH3.
 
Water acts as a very strong base because it has a great tendency to accept proton from mineral acids. The ionization of strong acids in water may be represented as below :
HCl + H2O ƒ H3O+ + Cl
H2SO4 + H2O ƒ H3O+ + HSO4
 
Thus all strong acids (e.g. HNO3, HCl, H2SO4, HClO4) react almost completely to form H3O+ ion.
 
Therefore all strong acids in aqueous solution appear equally strong. i.e. Acidic levels for all acids are same.
 
This is called levelling effect and water is the levelling solvent. Thus the relative acidic strengths or basic strengths in aqueous solutions cannot be compared.
 
When acetic acid is used as solvent instead of water it is found that the acidic strength of the acid lowers and now the given acid in acetic acid becomes weak and thereby different acids will dissociate to different extent in glacial acetic acid. (This is because of poor proton accepting character of acetic acid).
Class 7 Science Acid Base and Salts Advanced Notes
In this reaction the equalibrium does not lie very much to the right and hence strong acids are feebly dissociated.
 
The degree of dissociations of a number of acids dissolved in glacial acetic acid has been determined and it is found that the acidic strength order is
HClO4 > HI > HBr > H2SO4 > HCl > HNO3 > H3O+ > HSO– > H3PO4 > HF > CH3COOH > H2CO3 > H2S> NH4+ > HCN
 
Lewis Concept of Acids And Bases
 
According to Lewis, an acid is any species which is capable of accepting a pair of electrons and a base is a species that is having a lone pair of electrons for donations.
 
This statement covers a wide range of acids and bases. Thus all Bronsted-Lowry bases are Lewis bases while all Bronsted-Lowry acids are not Lewis acids.
 
Hence this theory includes acids which do not donate proton or liberate H+ (aprotic)
Class 7 Science Acid Base and Salts Advanced 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

pOHThe 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] = 10–14Kw

–log[H+] (+) –log[OH] = 14 = PKw

 pH + pOH = pKw = 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

PreparatioIn 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 + H2→ 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

UseBleaching 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)

Preparatio1. 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)

Anode → Graphite

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

                     b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
 
 
 

Q1Write the properties of acids.

Ans 1. Acids are corrosive in nature and have a sour taste.

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

3. Acids react with metals to form hydrogen gas and a salts

4. Acids react with bases to forma salt and water. This process is called neutralization

Q2Mention the sources of common acids.

Ans Sulphur, Hydrochloric and Nitric acids are commonly used inorganic acids. They are also called mineral acids and are seen in chemistry laboratories. Organic acids occur naturally in fruits and vegetables. e.g.

Source                                                                     Organic acid

Citrus fruits like lemon, Oranges                             Citric acid

Milk                                                                          Lactic acid

Vinegar                                                                    Acetic acid

Grapes                                                                    Tartaric acid

Vitamin C                                                                Ascorbic acid

Q3Write the properties of bases.

Ans 1. They are soapy or slippery to touch.

2. They turn red litmus blue.

3. They taste bitter (ever get soap in your mouth?) .

4. They react with acids and form salt and water.

Q4What are indicators . mention few common indicators and their colour changes in acids and bases

Ans. A chemical that changes colour and helps in identification of an acidic, basic and neutral substances. Indicators can be either natural (extracted from plants) or synthetic Many natural dyes found in fruits, vegetables, and flowers act as indicators, too. For example, red cabbage extracts changes red in acidic solution and blue in basic solution.

Common indicators include:

Litmus: A dye extracted from various species of lichen that changes colour in acidic and basic medium

Phenolphthalein: A synthetic indicator that changes colour in the basic range.

Methyl orange: A synthetic indicator that changes colour in the acid.

CBSE Class 7 Science - Acids, Bases and Salts_1

Q5Write the differences between acids and bases.

Ans.

CBSE Class 7 Science - Acids, Bases and Salts_2

Q6 How are salts formed?

Ans A Salt results when an acid reacts with a base

Q7How can we cure an ant bite?

Ans When an ant stings on the skin and baking soda is applied on the skin for relief. Ant sting is acidic due to formic acid present in ant sting and baking soda is basic.When baking soda is rubbed on the affected area neutralization reaction takes place

Q8Why and when does a doctor recommend an antacid ?

Ans .Antacids are basic in nature. When someone suffers from acidity, some antacids are given to him bya doctor to eat so that neutralization can take place in the stomach of the patient.

Q9Why should acidic factory wastes be treated before their disposal?

Ans .When acidic wastes from factories are disposed off in rivers, they are first treated with bases so as to neutralize the wastes first and prevent the river water from getting acidic as acidic substances may harm the aquatic life.

Q10 How do farmers treat the soil before sowing seeds?

Ans . Acidic soil is treated with bases like slaked lime which is actually Calcium hydroxide or chalk powder is added to such soils.

Basic soils are neutralized by adding large quantities acidic substances like manure.

Please click the link below to download Class 7 Science Acid Base and Salts Notes

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