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Notes Of Types Of Chemical Reactions

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Published in: Chemistry
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The note details the types of chemical equations with examples

Joscelin R / Sharjah

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Qualification: B.Sc Chemistry (Hons.)

Teaches: Biology, Chemistry, Hindi, Physics, Science, Maths

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  1. 1. 2. 3. 4. 5. Types of Chemical Reactions Combination Reactions Decomposition Reactions Displacement Reactions Double Displacement Reactions Oxidation Reduction Reactions COMBINATION REACTIONS: Those reactions in which two or more substances combine to form a single substance are called combination reaction. Here two or more elements can combine to form compounds; two or more compounds can combine to form compounds and an element and compound can combine to form compound. Example: CaO(s) 4 H20(l) — calcium water oxide + 02(g) C6H1206(aq) + 602 (aq)— Glucose C02(g) (aq) heat slaked lime 6C02(aq) +6H20 (l) 4 Energy (Respiration is a combination exothermic reaction) In the above reactions energy is released as new compounds are formed hence they are exothermic reactions. DECOMPOSITION REACTION: Those reactions in which a compound splits up into two or more simpler substances are called decomposition reaction. It can be achieved by the application of heat, light or electrici- ty, these provide the energy required to break a compound into two or more simpler com- pounds. Example: 2Fe S04 (s) Ferrous sulphate Ca cog (s) — limestone 1120 (l) 2AgCl (s) — silver chloride Fe203 (s) 4 ferric oxide S02 (g) S03 (g) CaO (s) 4 ccn (g) quick lime 2H2 (g) + 02 (g) (Electrolysis of water) 2Ag (s) + C12 (g) (used in black and white photography)
  2. In the above reactions energy in form of heat, light and electricity is absorbed to bring about decomposition hence they are called endothermic reactions. DISPLACEMENT REACTIONS: Those reactions in which one element takes the place of another element in a compound are known as displacement reaction. Metals have different reactivities, some are more reactive than Hydrogen and some are less reactive than Hydrogen. The arrangement of metals in de- creasing order of their reactivity is called reactivity series of metals. K Na Ca Mg Al Zn Fe Sn m, Hydrogen cu Hg Ag ALI -——decreasing order of reactivity— K is more reactive than Na and so on. In displacement reaction a more reactive metal displa- ces a less reactive metal from its compound. Zn (s) 4 cuS04 )aq) Zn (aq)+ cu(S) Example: Fe (s) 4 cuS04 (aq) FeS04 (aq) + Cu (S) The iron nail kept in CuS04 solution becomes brownish in colour when kept in blue col- oured copper sulphate solution because iron displaces copper and from ferrous sulphate solu- tion and copper deposits on iron nail (brown colour). DOUBLE DISPLACEMENT REACTION: Those reactions where there is an exchange of ions between the reactants are called double displacement reaction. It occurs in a solution and one of the products being insoluble precip- itates out (get separated) hence also called precipitation reaction. Example: Na2 S04 (aq) sodium sulphate BaC12 (aq) barium chloride BaS04 (S) barium sulphate 2NaCl (aq) sodium chloride OXIDATION & REDUCTION REACTION: These reaction are based on addition or removal of oxygen or hydrogen. If a substance gains oxygen in a reaction it is said to be oxidized. 2Cu +02 2CuO here copper gains oxygen so it is said to be oxidised. if a substance loses oxygen during a chemical reaction, it is said to be reduced. Cuo + H2 -heat—--- cu +H20 here when hydrogen gas is passed through copper oxide it loses oxygen and gets reduced to copper.
  3. cuo - -Reduction— [12 ---heat---- Cu 41120 ---Oxidation- The reactions in which one reactant gets reduced and other get oxidised is called redox reac- tions or oxidation reduction reactions. ---REDUCED----—— ZnO + C Zn --REDUCED------ Mn02 4HCl CORROSION: co MnC124 2H20 cn When any metal is attacked by air, moisture, acids around it, it is said to corrode and the process is called corrosion. Corrosion is mainly caused by the oxidation of metals by the oxygen of air. Rusting of iron metal is the most common form of corrosion. when an iron object is exposed to moist air for long time it gets covered by red brown flaky substance called rust. 4Fe 302 2x. H20 2Fe203. xvno. hydrated iron oxide (rust) Black coating on silver and green coating on copper are other examples of corrosion. Corrosion Causes a lot of damage to car bodies, bridges,iron railings and objects made of iron. RANCIDITY: Oxidation has damaging effect on foods containing fats and oils. when the food materials pre- pared in fats and oil are kept for long time it starts giving unpleasant smell and taste. they are said to have become rancid or sour. This is because the fats and oils in the food gets oxidised by the oxygen and their oxidation prod- ucts have unpleasant smell, this process is called rancidity. It can be prevented by adding anti-oxidants to foods containing fats and oils : antioxidant is a substance which prevent oxidation and in their presence the fats and oil in food does not get oxi- dised easily and hence do not turn rancid. examples : butylated hydroxy anisole (BHA) butylat- ed hydroxy toluene (BHT) It can also be prevented by packing food containing fats and oils in nitrogen gas : in presence of unreactive Nitrogen gas there is no oxygen to cause oxidation of food. Example: potato chips are packed in plastic bags with nitrogen gas.

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