CBSE Class 10 Chemistry Acids, Bases and Salts Notes Set 25

Acids and Bases: Basic Ideas

1. An acid is a substance that releases H⁺ ions in water. Acids taste sour. Mineral acids come from rocks and minerals (HCl, H₂SO₄, HNO₃); organic acids come from plants and animals (acetic, citric, tartaric, formic, lactic).
2. A base releases OH^– ions in water. Bases taste bitter and feel soapy. A base that dissolves in water is an alkali, so every alkali is a base but not every base is an alkali (for example Fe(OH)₃ is insoluble).
3. Binary acids contain hydrogen and one other element (HCl, HCN). Oxyacids also contain oxygen (H₂SO₄, HNO₃).

Indicators

An indicator changes colour (or smell) in acidic and basic solutions. Litmus is a purple dye from lichen.
Olfactory indicators change smell: onion strips, vanilla essence and clove oil lose their odour in a base but keep it in an acid.

Reactions of Acids and Bases

1. Metal + dilute acid gives salt + hydrogen: Zn + H₂SO₄ gives ZnSO₄ + H₂. Hydrogen burns with a pop sound near a flame.
2. Zinc, tin and aluminium also react with strong alkalis and release hydrogen: Zn + 2NaOH gives Na₂ZnO₂ + H₂.
3. Metal oxide + acid gives salt + water. Black CuO dissolves in dilute HCl to give a blue-green solution of CuCl₂. Metal oxides are generally basic.
4. Non-metal oxide + base gives salt + water: 2NaOH + CO₂ gives Na₂CO₃ + H₂O.
5. Metal carbonate or hydrogencarbonate + acid gives salt + water + carbon dioxide: CaCO₃ + 2HCl gives CaCl₂ + H₂O + CO₂. The gas turns lime water milky; with excess gas the milkiness goes away because soluble Ca(HCO₃)₂ forms.
6. Acid + base gives salt + water (neutralisation): H⁺(aq) + OH^–(aq) gives H₂O.

Strength of Acids and Bases

1. Strong acids ionise almost completely in water (HCl, H₂SO₄, HNO₃). Weak acids ionise only slightly (acetic, carbonic, formic, phosphoric).
2. Strong bases: NaOH, KOH, Ba(OH)₂. Weak bases: NH₄OH, Mg(OH)₂, Cu(OH)₂.
3. Strength depends on the degree of ionisation, not on concentration. A concentrated acetic acid solution is still a weak acid.
4. Basicity of an acid is the number of replaceable hydrogen atoms: HCl is monobasic, H₂SO₄ dibasic, H₃PO₄ tribasic. Acidity of a base is the number of replaceable OH groups: NaOH is monoacidic, Ca(OH)₂ diacidic, Al(OH)₃ triacidic.
5. Acids and bases conduct electricity only in water because they form ions. Glucose and ethanol do not ionise, so their solutions do not conduct. Dry HCl gas shows no acid property; water is needed to release H⁺ (shown as H₃O⁺).

Dilution and Neutralisation in Daily Life

1. Always add the acid slowly to water, with stirring. Dilution is highly exothermic, and adding water to concentrated acid can crack the container.
2. Antacids (milk of magnesia, baking soda) neutralise excess stomach acid. Bee and ant stings contain formic acid and are eased with baking soda; wasp stings are alkaline and are eased with vinegar. Farmers add lime to acidic soil.

Theories of Acids and Bases

1. In a Bronsted-Lowry reaction an acid and base pair differing by one proton is a conjugate pair: HCl and Cl^–, H₃O⁺ and H₂O, NH₄⁺ and NH₃.
2. Species that can both donate and accept protons are amphiprotic (H₂O, NH₃, HSO₄^–).
3. Lewis acids: molecules with an incomplete octet (BF₃, AlCl₃), all cations, and species that can expand their octet (SiF₄, SnCl₄). Lewis bases: molecules with a lone pair (NH₃) and anions (OH^–, Cl^–).

Ionic Product of Water and pH

1. Water ionises slightly: H₂O + H₂O gives H₃O⁺ + OH^–. At 298 K the ionic product is Kw = [H⁺][OH^–] = 1 × 10^–14.
2. pH = –log[H⁺] and pOH = –log[OH^–], so pH + pOH = 14.
3. pH below 7 is acidic, 7 is neutral, above 7 is basic. A fall of one pH unit means [H⁺] becomes 10 times larger.
4. Example: 0.0001 M HCl has [H⁺] = 10^–4 M and pH 4. 0.001 M NaOH has pOH 3 and pH 11.
5. Everyday importance: the body works at pH 7.0 to 7.8; rain below pH 5.6 is acid rain; tooth decay starts when mouth pH falls below 5.5; plants need a suitable soil pH.

Families of Salts

1. Strong acid + strong base gives a neutral salt (pH 7), for example NaCl. Strong acid + weak base gives an acidic salt solution (pH below 7), for example NH₄Cl. Strong base + weak acid gives a basic salt solution (pH above 7), for example CH₃COONa and Na₂CO₃.
2. Common salt is the raw material for NaOH, baking soda, washing soda, bleaching powder and chlorine.

Chemicals from Common Salt

1. Chlor-alkali process: electrolysis of brine. 2NaCl + 2H₂O gives 2NaOH + Cl₂ + H₂. Chlorine forms at the anode, hydrogen at the cathode and NaOH near the cathode.
2. Bleaching powder, CaOCl₂: Ca(OH)₂ + Cl₂ gives CaOCl₂ + H₂O. It is used to bleach cotton, linen and wood pulp, to disinfect drinking water and as an oxidising agent. It slowly loses chlorine on exposure to air.
3. Baking soda, NaHCO₃: made from NaCl + H₂O + CO₂ + NH₃. On heating, 2NaHCO₃ gives Na₂CO₃ + H₂O + CO₂. It is a mild base used in antacids, in baking powder (baking soda + tartaric acid) and in soda-acid fire extinguishers.
4. Washing soda, Na₂CO₃·10H₂O: obtained by recrystallising sodium carbonate, which is made by the Solvay process from brine, ammonia and limestone. It softens hard water and is used in cleaning and in the paper and glass industries. Anhydrous Na₂CO₃ is soda ash.

Water of Crystallisation

1. Water of crystallisation is the fixed number of water molecules in one formula unit of a crystalline salt. Blue copper sulphate, CuSO₄·5H₂O, turns white when heated and blue again when water is added.
2. Gypsum is CaSO₄·2H₂O. Heated at 373 K it gives plaster of Paris, CaSO₄·½H₂O (calcium sulphate hemihydrate): CaSO₄·2H₂O gives CaSO₄·½H₂O + 1½H₂O. Overheating gives anhydrous CaSO₄, which does not set.
3. Plaster of Paris sets to gypsum when mixed with water: CaSO₄·½H₂O + 1½H₂O gives CaSO₄·2H₂O. It is used for plaster casts on fractured bones, toys and decoration, and must be stored in a moisture-proof container.
4. Efflorescence is loss of water of crystallisation in air (washing soda loses nine of its ten water molecules to give Na₂CO₃·H₂O). Deliquescence is absorbing moisture and dissolving (NaOH, KOH, CaCl₂).