ACIDS, BASES AND SALTS

Transcription

1 Acids are those chemical substances which have a sour taste. Bases are those chemical substances which have a bitter taste. A salt is an ionic compound which dissociates to yield a positive ion other than hydrogen ion (H + ) and negative ion other than hydroxyl ion (OH - ) e.g. NaCl. 1) Indicators - Indicators tell us whether a substance is acidic or basic by change in colour. The three most common indicators are : Litmus, methyl orange and phenolphthalein. The substances whose odour changes in acidic or basic media are called Olfactory indicators. E.g. onion and vanilla. 2) Properties of Acids and Bases a) Reaction with metals - Metals on reacting with acids and bases displaces hydrogen from them, liberating hydrogen gas. It combines with the remaining part of the acids and bases and forms a compound called Salt. Acid + Metal Salt + Hydrogen gas e.g. 2HCl + Na 2NaCl +

2 Base + Metal Salt + Hydrogen gas e.g. 2NaOH + Zn + b) Reaction with Metal carbonates and Metal hydrogencarbonates - Metal carbonates and metal hydrogencarbonates react with acids to give a corressponding salt, carbon dioxide and water. Metal carbonate + Acid (or Metal hydrogen carbonate) Salt + Carbon dioxide + Water e.g. ( ) ( ) ( ) ( ) c) Reaction between acids and bases An acid and a base react to give a salt and water. This reaction is called neutralisation reaction. Acid + Base Salt + Water e.g. ( ) ( ) ( ) All neutralisation reactions are exothermic and non-redox reactions.

3 d) Reaction of metallic oxides with acids Acid neutralises metal oxides to form salt and water. Metallic oxide + Acid Salt + Water e.g. ( ) ( ) ( ) ( ) e) Reaction of a non-metallic oxide with base Bases react with non metallic oxide to give salt and water. Non-metallic oxide + Base Salt + Water e.g. ( ) 3) Common properties of all acids and all bases A common property of all the acids is that they produce hydrogen ions [ ( ) A common property of all the bases is that they produce hydroxide ions [ 4) Strength of acid and base solutions: ph scale The strength of an acid or base is measured on a scale of numbers called the ph scale.

4 The ph scale has values from 0 to 14. Neutral substances have a ph of exactly 7. Acids have a ph of less than 7. Bases have a ph of more than 7. The strength of an acid or a base depends on the number of H + ions and OH - ions produced, respectievely. Universal indicator helps in estimating the strength of an acid or a base. It is a mixture of many different indicators which gives different colours at different ph values. 5) Importance of ph in everyday life Living organisms can survive only a narrow range of ph change. Our body works within a ph range of 7.0 to 7.8. Plants require a specific ph range for healthy growth. If soil is too acidic (having low ph), then its treated with materials like quicklime or slacked lime or chalk. If it s too alkaline then its alkalinity is reduced by adding decaying organic matter. Digestive system Our stomach produces HCl that helps in digestion of food without harming the stomach. - Access acid in stomach causes indigestion. - Antacids are bases that are used as a remedy to neutralise the excess acid in the stomach. E.g. Magnesium hydroxide (Milk of Magnesia)

5 Tooth decay Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating food. - Tooth decay starts when the ph of acid formed in the mouth falls below The best way to prevent tooth decay is to clean the mouth throughly after eating food. Self defence by animals and plants through chemical warfare - Many plants and animals protect themselves from their enemies by injecting painful and irritating acids and bases into their skin. e.g. ant s sting and nettle sting injects methanoic acid. - Rubbing a mild acid or base like vinegar or baking soda, respectievly on the stung area gives relief. 6) Salts Salts are formed by the reaction of acid and base which is known as neutralisation. Salts of strong acids and strong bases are neutral. Salts of strong acid and weak base are acidic. Salts of strong base and weak acid are basic. a) Sodium hydroxide (NaOH) or Caustic soda It is prepared on commercial scale by the electrolysis of strong solution of sodium chloride also called brine. The process is called chlor-alkali process.

6 The overall reaction that takes place is: 2NaCl (aq) + 2H 2 O (l) H 2 (g) + Cl 2 (g) + 2NaOH (aq) Uses: (The three products produced in this process are all useful) - NaOH de-greasing metals, soaps and detergents, paper making, artificial fibres - H 2 fuels, margarine, ammonia for fertilizers - Cl 2 water treatment, swimming pools, PVC, disinfectants, CFCs, pesticides b) Baking soda (NaHCO 3 ) or Sodium hydrogen carbonate It is produced in the following manner- NaCl + H 2 O + CO 2 + NH 3 NH 4 Cl + NaHCO 3 On heating it during cooking, the following reaction takes place- NaHCO 3 Na 2 CO 3 + H 2 O + CO 2 Uses: - Baking powder baking soda acts a leavening agent in baking. - Antacid baking soda reacts with acid due to its alkaline nature and neutralizes acidity. - Fire extinguisher used in soda-fire indicator.

7 c) Bleaching powder (CaOCl 2 ) or Calcium hypochlorite Bleaching powder is produced by the action of chlorine on slacked lime [Ca (OH) 2 ] Ca(OH) 2 + Cl 2 CaOCl 2 + H 2 O Uses: - For bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in industry - As an oxidising agent in many industries - For disinfecting drinking water - d) Washing soda (Na 2 CO 3. 10H 2 O) or Sodium carbonate Sodium carbonate can be obtained by heating baking soda; recrystallization of sodium carbonate gives washing soda. It is also a basic salt. Na 2 CO H 2 O Sodium carbonate Na 2 CO 3. 10H 2 O Hydrated sodium carbonate (washing soda) Uses: - Used in glass, soap and paper industries - Used in manufacture of sodium compounds such as borax - Cleansing agent for domestic purposes - Removing permanent hardness of water

8 e) Plaster of Paris (CaSO 4. ½ H 2 O) It can be obtained by heating gypsum CaSO 4. 2 H 2 O (CaSO 4. 2 H 2 O) + Heat (CaSO 4. ½ H 2 O) + ½ H 2 O Plaster of Paris is a white powder and on mixing with water it changes to gypsum once again giving a hard solid mass. CaSO H 2 O CaSO 4. 2 H 2 O Uses: - Used for making moulds or casts of toys, pottery, ceramics etc. - Used in surgical bandages for setting fractured bones.