Acids, Bases and Salts
Category : Banking
Introduction
Properties of acids and bases
(i) Action of metals: Metals generally react with dilute acids to form their respective salt and hydrogen.
\[Metal+Acid\to Salt+Hydrogen\]
(ii) Action with metal oxides (Basic oxides) Metal oxides are generally basic oxides. These oxides get neutralised when they react with acids. These reactions are mostly carried upon heating e.g.
\[Basicoxide+Acid\to Salt+Water\]
\[\left( Neutralization\text{ }reaction \right)\]
(iii) Action with metal carbonates and metal hydrogen carbonates Acids react with carbonates and hydrogen carbonates to form their respective salts, water and carbon dioxide gas.
\[Carbonate/bicarbonate+Acid\to Salt+water+carbon\,\,dioxide.\]
(i) Reaction of metals with bases: Metals (e.g. Zn, Al, Sn) dissolve in NaOH (an alkali) to liberate hydrogen gas.
\[Zn+2NaOH\to N{{a}_{2}}Zn{{O}_{2}}+{{H}_{2}}\]
\[Sod.Zincate\]
(ii) Action with acids: Bases combine with acids to form salt and water only. It is a neutralisation reaction.
\[Base+Acid\to Salt+Water\]
Non - metallic oxides react in the same way hence non- metallic oxides are acidic in nature.
The strength of an acid or a base can be easily estimated by making use of universal indicator which is a mixture of several indicators. The universal indicator show different colours at different concentrations of hydrogen ions in solution.
pH Scale
For acids pH< 7
For bases pH > 7
For neutral substances pH = 7
(i) Blood pH: For proper functioning our body needs to maintain blood pH between 7.35 and 7.45. Values of blood pH greater than 7.8 or less than 6.8 often results in death.
(ii) Acid rain: When pH of rain water is less than 5.6, it is called acid rain, when acid rain flows into rivers, it lowers the pH of river water.
(iii) pH in our digestive system: We know that hydrochloric acid (HC1) produced in our stomach helps in digestion of food without harming stomach. However excess of acid causes indigestion and leads to pain as well as irritation. To get rid of this people use bases called "antacids".
(iv) pH of the soil: For their healthy growth plants require a specific pH. Soils with high peat content or iron minerals or with rotting vegetation tend to become acidic and the soil pH can reach as low as 4.
(v) pH change as the cause of tooth decay: Tooth decay starts when the pH of mouth is lower than 5.5.
(vi) Self defence by animals and plants through chemical warfare: Bee-sting leaves an acid (formic acid or methanoic acid, HCOOH) which causes pains and irritation. Use of mild base like baking soda on the stung area gives relley.
Salts
(i) Sodium hydroxide (NaOH) or Caustic soda: It is prepared on commercial scale by the electrolysis of strong solution of sodium chloride (NaCI) also called brine. The process is called chlor-alkali process.
The overall reaction taking place is:\[2NaCl(aq)+2{{H}_{2}}O(l)\to {{H}_{2}}(g)+C{{l}_{2}}(g)+2NaOH(aq)\]
(a) Sodium hydroxide is most used base in the laboratory.
(b) It is used in many industries, mostly as strong chemical base in manufacture of pulp and paper, textiles, drinking water, soap and detergents etc.
(ii) Baking soda. Sodium hydrogen carbonate, \[(NaHC{{O}_{3}})\] \[NaCl+{{H}_{2}}O+C{{O}_{2}}+N{{H}_{3}}\to N{{H}_{4}}Cl+NaHC{{O}_{3}}\]
When heated the following reaction occurs \[2NaHC{{O}_{3}}\xrightarrow{heat}N{{a}_{2}}C{{O}_{3}}+{{H}_{2}}O+C{{O}_{2}}\]
The above reaction occurs when baking soda is heated during cooking.
(a) In baking powder: The most practical use of baking soda is as a leavening agent in baking.
(b) As an antacid: Baking soda reacts with acid due to its alkaline nature and neutralizes acidity (i.e. acts as an antacid)
(c) In fire extinguishers: It is used in soda-acid fire extinguisher
(iii) Washing soda, \[N{{a}_{2}}C{{O}_{2}}.10{{H}_{2}}O\], Sodium carbonate Sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.
\[\underset{Sodium\text{ }carbonate}{\mathop{N{{a}_{2}}C{{O}_{3}}+10{{H}_{2}}O}}\,\to \]
\[\underset{Hydrated\text{ }sodium\,\,carbonate\text{ }\left( Washing\text{ }soda \right)}{\mathop{N{{a}_{2}}C{{O}_{3}}\cdot 10{{H}_{2}}O}}\,\]
(a) Sodium carbonate (washing soda) is used in glass, soap and paper industries.
(b) It is used for removing permanent hardness of water.
(iv) Bleaching powder: Calcium hypochlorite is a chemical compound with formula.\[CaOC{{l}_{2}}\] It is a yellowish powder with smell of chlorine. It is widely used for water treatment and as a bleaching agent (bleaching powder)\[2Ca{{(OH)}_{2}}+2C{{l}_{2}}\to CaOC{{l}_{2}}+CaC{{l}_{2}}+2{{H}_{2}}O\] Calcium hypochlorite is used for the disinfection of drinking water or swimming pool water.
(v) Plaster of Paris, \[CaS{{O}_{4}}.1/2{{H}_{2}}O\] It can be obtained by heating gypsum (\[CaS{{O}_{4}}.2{{H}_{2}}O\])
\[(CaS{{O}_{4}}.2{{H}_{2}}O)+heat\to (CaS{{O}_{4}}.\frac{1}{2}{{H}_{2}}O)+\frac{3}{2}{{H}_{2}}O\]
Plaster of parts is a white powder and on mixing with water it changes to gypsum once again giving a hard solid mass \[CaS{{O}_{4}}.\frac{1}{2}{{H}_{2}}O+1\frac{1}{2}{{H}_{2}}O\to CaS{{O}_{4}}.2{{H}_{2}}O\]
(a) for making moulds or casts for toys, pottery, cermics etc.
(b) in surgical bandages for setting fractured bones.
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