Note on Chemical Reactions

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Types of Chemical Reactions

Transformation of one kind of chemical substance into another kind of chemical substances having different properties is known as a chemical reaction. It is a chemical change which takes place by simple contact, heat, light, pressure etc. Any chemical change, which is represented by symbol and formula is known as a chemical equation. There are many types of chemical reactions. Some important ones are as follows:

1) Combination or synthesis reaction:

When two or more than two elements or simple molecules unite with each other to give large molecule is known as combination reaction. This reaction is carried out by the application of heat, light, electricity, pressure, etc. Example -

When hydrogen reacts with chlorine in the presence of sunlight, we get hydrogen chloride gas which dissolves in water to give hydrochloric acid.

$$H_2 + Cl_2 \xrightarrow {sunlight} 2HCl$$

When magnesium is burnt in air, we get magnesium oxide.

$$2Mg + O_2 \xrightarrow {\Delta} 2MgO$$

2) Decomposition or analysis reaction:

When a large molecule is decomposed into two or more simple molecules, it is known as decomposition reaction. Example-

When potassium chlorate is heated, we get potassium chloride and oxygen gas.

$$2KClO_3 \xrightarrow {\Delta} 2KCl + 3O_2$$

When magnesium bicarbonate is heated, we get magnesium carbonate, water, and carbon dioxide gas.

$$Mg(HCO_3)_2 \xrightarrow {\Delta} MgCO_3 + H_2O + CO_2$$

3) Displacement reaction:

  • Single displacement reaction: Such type of reaction in which more reactive element (Na, K, F, Cl, etc.) displaces less reactive element is known as single substitution (displacement) reaction. Example- When sodium metal is reacted with magnesium chloride, we get sodium chloride and magnesium metal. $$Na + MgCl_2 \xrightarrow {\Delta} NaCl + Mg (\downarrow)$$ When iron is reacted with copper sulphate solution, we get copper metal. $$Fe(s) + CuSO_4(aq.) \xrightarrow {} FeSO_4(aq.) + Cu (\downarrow)$$
  • Double displacement reaction: Such type of reaction in which there is a mutual exchange of cations (e.g- Na+, Ca++, Al+++, etc.) and anions (e.g- Cl-, Br-, I- etc.) is known as double displacement reaction. This reaction is sometimes known as precipitation reaction. Example- When aqueous sodium chloride is reacted with the aqueous solution of silver nitrate, we get a white precipitate of silver chloride and sodium nitrate. $$Na^+Cl^-(aq.) + Ag^+NO_3^--(aq.) \rightarrow AgCl (\downarrow) + NaNO_3(aq.)$$ When barium chloride solution is treated with the dilute sulphuric acid solution, we get a white precipitate of barium sulphate. $$BaCl_2(aq.) + dil.H_2SO_4(aq.) \rightarrow BaSO_4 (\downarrow) + HCl(aq.)$$

4) Neutralisation reaction:

It is a complete reaction of Hydrogen ions (H+ ions) from the acid side and (OH-) from hydroxyl side. Example-

When dil. hydrochloric acid is reacted with sodium hydroxide solution, we get sodium chloride salt and water.

$$NaOH(aq.) + dil.HCl(aq.) \xrightarrow {} NaCl + H_2O$$

5) Hydrolysis reaction:

Any reaction which is carried out by water molecule is called hydrolysis water. (Hydro= water , lysis= break down). Example-

When copper sulphate reacts with water to give cupric hydroxide and sulphuric acid-

$$CuSO_4 + 2H_2O \rightarrow Cu(OH)_2 + H_2SO_4$$

6) Isomerism or rearrangement reaction:

Compounds having same molecular formula but different physical and chemical properties due to different structures (or rearrangement of atoms) are known as isomers and property itself is called as isomerism. Example-

When ammonium cyanate (Inorganic compound) is heated, we get urea.

$$NH_4CNO \xrightarrow {\Delta} NH_2CONH_2$$

7) Polymerisation:

The repeating of same molecule or different molecule at the certain condition of temperature and pressure, to give large molecule is known as polymer and the property is called polymerisation. Example-

When acetylene gas is passed through the red hot tube of iron at 200-300ºC, we get benzene by polymerisation process.

$$3C_2H_2 \xrightarrow [red hot tube] {200-300ºC}C_6H_6$$

8) Condensation reaction:

When two or more molecules condense to each other by losing water molecule, it is known as a condensation reaction. Example-

$$H_2SO_4 + H_2SO_4 \xrightarrow [-H_2O] {condensation} H_2S_2O_7$$

Factors responsible for a chemical reaction

There are many factors responsible for a chemical reaction. Some of the methods of bringing a chemical reaction are as follows:

1) By simple contact: Chemical reaction between some chemical substances takes place when they are brought together in contact. Example: When granulated zinc is treated with dil. sulphuric acid, we get zinc sulphate and hydrogen gas by simple contact.

$$ Zn(s) + dil. H_2SO_4 \xrightarrow {} ZnSO_4 (aq.) + H_2 (g) $$

2) By contact with an aqueous solution: Some of the reactions are carried out in the presence of aqueous solution. Example-

$$NaCl(s) + AgNO_3(s) \xrightarrow {} NO REACTION$$

$$ NaCl(aq.) +AgNO_3(aq.) \xrightarrow {} AgCl(s) + NaNO_3 (aq.)$$

• $$BaCl_2(aq.) + dil. H_2SO_4 \xrightarrow {} BaSO_4(\downarrow) + HCl (aq.)$$

3) By application of heat: Generally, heat increases the rate of chemical reaction. Some reaction takes place in the presence of heat. Example: When potassium chlorate is heated, we get potassium chloride and oxygen gas.

$$2KClO_3 \xrightarrow {\Delta} 2KCl + 3O_2$$

When lead nitrate is heated, we get lead oxide, nitrogen peroxide and oxygen gas.

$$2Pb(NO_3)_2 \xrightarrow {\Delta} 2PbO + $NO_2 + O_2$$

4) By application of light: Certain reactions take place in the presence of sunlight. Example-

When carbon dioxide is reacted with moisture in the presence of sunlight and chlorophyll, we get glucose and oxygen gas.

$$6CO_2 + 6H_2O \xrightarrow [chlorophyll] {sunlight} C_6H_12O_6 + 6CO_2 $$

When hydrogen reacts with chlorine in the presence of sunlight, we get hydrogen chloride gas.

$$H_2 + Cl_2 \xrightarrow{sunlight} 2HCl$$

5) By application of pressure: Certain reaction takes place in the presence of pressure. Example- For the synthesis of ammonia by Haber's process, 200-800 atm. pressure is required to get a good yield of ammonia.

$$N_2 + 3H_2 \xrightarrow{200-300 atm. pressure} 2NH_3$$

6) By application of electricity: Some of the reaction takes place in the presence of electricity. Example- Extraction of sodium from fused sodium chloride.

$$ 2NaCl \xrightarrow [current]{Electric} 2Na^+ + 2Cl^-$$

7) By application of catalyst: Those chemical substances, which increase or decrease the rate of the reaction without any change in its chemical composition are known as a catalyst and the process itself is known as catalysis. Example- Use of manganese dioxide in lab preparation of oxygen with potassium chlorate acts as a positive catalyst because it increases the rate of reaction.

$$ 2KClO_3 + MnO_2 \xrightarrow [200-300ºC]{\Delta} 2KCl + MnO_2 + 3O_2$$

8) Sound: Acetylene gas is decomposed when mercuric fulminate produces sound.

$$C_2H_2 \xrightarrow [Sound] {Hg(ONC)_2} 2C+ H_2 $$

References:

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).

  • Transformation of one kind of chemical substance into another kind of chemical substances having different properties is known as a chemical reaction. 
  • When two or more than two elements or simple molecules unite with each other to give large molecule is known as combination reaction.
  • When a large molecule is decomposed into two or more simple molecules, it is known as decomposition reaction.
  • Such type of reaction in which more reactive element (Na, K, F, Cl, e.t.c.) displaces less reactive element is known as single substitution (displacement) reaction. 
  • Such type of reaction in which there is a mutual exchange of cations (e.g- Na+, Ca++, Al+++, etc.) and anions (e.g- Cl-, Br-, I- etc.) is known as double displacement reaction. 
  • Neutralisation  is a complete reaction of Hydrogen ions (H+ ions) from the acid side and (OH-) from hydroxyl side.
  • Any reaction which is carried out by water molecule is called hydrolysis water. 
  • When two or more molecules condense to each other by losing water molecule, it is known as a condensation reaction.
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