Class 10 Types of chemical equations

Topics to be covered

=> Combination reaction
=> Decomposition reaction
=> Displacement reaction
=> Double displacement reaction
=> Oxidation and reduction

TYPES OF CHEMICAL EQUATION

During a chemical change following observations are made;

1 Atoms in the reactants are rearranged to form one or more different substances.
2 Old bonds are broken, new bonds are formed.
3 Reactants lose their properties to form product of different properties.

𝐍𝐨𝐰 𝐜𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧 𝐜𝐚𝐧 𝐛𝐞 𝐝𝐢𝐯𝐢𝐝𝐞𝐝 𝐢𝐧𝐭𝐨 𝐟𝐨𝐥𝐥𝐨𝐰𝐢𝐧𝐠 𝐰𝐚𝐲𝐬:

color{green}("𝐂𝐨𝐦𝐛𝐢𝐧𝐚𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧:") A reaction in which a single product is formed from two or more reactants is known as a combination reaction. For eg: When carbon is burnt in presence of oxygen , carbon dioxide is formed.

color{red}(C + O_2 → CO_2)

Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

color{red}(underset("(Quick lime)")(CaO(s)) + H_2O (l) → underset("(Slaked lime)") (Ca (OH)_2 (aq)))

In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide. So this is also a combination reaction.

color{purple}♣ color{Violet} " Just for Curious"

A solution of slaked lime produced by the reaction 1.13 is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also color{red}(CaCO_3).

color{red}(underset("(Calcium hydroxide)")(Ca(OH)_2 (aq)) + CO_2(g) → underset("(Calcium carbonate)")(CaCO_3(s)) + H_2O(l))

Let us discuss some more examples of combination reactions.

(i) Burning of coal

color{red}(C(s) + O_2(g) → CO_2(g)) ..................(1.15)

(ii) Formation of water from color{red}(H_2(g)) and color{red}(O_2(g))

color{red}(2H_2(g) + O_2 (g) → 2H_2O(l)) ...............(1.16)

In simple language we can say that when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions. In Activity 1.4, we also observed that a large amount of heat is evolved. This makes the reaction mixture warm. Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. Other examples of exothermic reactions are –

(i) Burning of natural gas

color{red}(CH_4(g) + 2O_2(g) → CO_2 (g) + 2H_2O(g)) ........................(1.17)

(ii) Do you know that respiration is an exothermic process?

Energy is necessary for the survival of life that is obtained from the food we eat. During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy

color{red}(underset("(Glucose)")(C_6H_(12)O_6 (aq)) + 6O_2(aq) → 6CO_2(aq) + 6 H_2O (l) +"energy") ...............(1.18)

(iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

Decomposition reaction

color{green}("𝐃𝐞𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧:") Reactions in which one compound decomposes in two or more compounds or elements are known as decomposition reactions. These reactions are just opposite of combination reaction.For eg:

color{red}(underset("(ferrous sulphate)")(2FeSO_4(s)) overset("Heat")→ underset("Ferric oxide")(Fe_2O_3(s)) +SO_2 + SO_3 (g)) ..........................(1.19)

color{green}("𝐎𝐁𝐒𝐄𝐑𝐕𝐀𝐓𝐈𝐎𝐍:") color{red}(underset("(ferrous sulphate)")(2FeSO_4(s)) overset("Heat")→ underset("Ferric oxide")(Fe_2O_3(s)) +SO_2 + SO_3 (g))

Here a single reactant breaks down to give simpler products. Therefore this is a decomposition reaction. Ferrous sulphate crystals color{red}((FeSO_4 . 7H_2O)) lose water when heated and the green colour of the crystals changes. It then decomposes to ferric oxide color{red}((Fe_2O_3 )), sulphur dioxide color{red}((SO_2 )) and sulphur trioxide color{red}((SO_3 )). Ferric oxide is a solid, while color{red}(SO_2) and color{red}(SO_3) are gases.

color{green}("𝐓𝐲𝐩𝐞𝐬 𝐨𝐟 𝐝𝐞𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧:")

color{green}("𝟏 𝐓𝐡𝐞𝐫𝐦𝐚𝐥 𝐝𝐞𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧:") When a decomposition reaction is carried out by heating, it is called thermal decomposition.For eg:

Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction used in various industries. Calcium oxide is called lime or quick lime.It is used in the manufacturing of cement.

color{red}(underset("Limestone")(CaCO_3 (s)) overset("Heat")→ underset(" Quick lime ")(CaO(s)) + CO_2 (g))

color{green}("𝐎𝐁𝐒𝐄𝐑𝐕𝐀𝐓𝐈𝐎𝐍:") color{red}(underset("(Lead nitrade)")(2Pb(NO_3)_2 (s)) overset(" Heat")→ underset("(Lead oxide)" )(2PbO(s)) + underset("(Nitrogen dioxide)")(4NO_2(g)) + underset("(Oxygen)")(O_2(g))) ...........................(1.21)

We will observe the emission of brown fumes. These fumes are of nitrogen dioxide (color{red}(NO_2) ).

This is also an example of thermal decomposition reaction.

color{green}("𝟐 𝐄𝐋𝐄𝐂𝐓𝐑𝐎𝐋𝐘𝐓𝐈𝐂 𝐃𝐄𝐂𝐎𝐌𝐏𝐎𝐒𝐈𝐓𝐈𝐎𝐍 𝐑𝐄𝐀𝐂𝐓𝐈𝐎𝐍:")

Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as electrolytic decomposition. This is also known as electrolysis.

color{red}(underset("Water")(2H_2O(l)) overset(" Electricity")→ underset("Hydrogen")(2H_2(g)) + underset("Oxygen")(O_2(g)))

color{green}("𝟑 𝐏𝐡𝐨𝐭𝐨𝐥𝐲𝐬𝐢𝐬 𝐨𝐫 𝐩𝐡𝐨𝐭𝐨𝐝𝐞𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧:")

Reactions in which compound decomposes because of sunlight is called photodecomposition.

color{green}("𝐎𝐛𝐬𝐞𝐫𝐯𝐚𝐭𝐢𝐨𝐧:") White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

color{red}(2Ag Cl (s) overset("Sunlight")→ 2 Ag(s) + Cl_2(s))

Silver bromide also behaves in the same way.

color{red}(2AgBr(s) overset("Sunlight")→ 2Ag(s) + Br_2(g))

The above reactions are used in black and white photography.

The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.

DISPLACEMENT REACTION:

Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound.For eg:

color{red}(Fe(s) + CuSO_4(aq) → FeSO_4(g) + Cu(s))

color{red}(Pb(s)+ CuCl_2 (aq) → PbCl_2 (aq) + Cu(s))

color{green}("𝐎𝐁𝐒𝐄𝐑𝐕𝐀𝐓𝐈𝐎𝐍 :") We will observe that iron nail becomes brownish in colour and blue colour of copper sulphate solution fades away.

color{red}(Fe(s) +underset("(Copper sulphate)")(CuSO_4(aq)) → underset("(Iron sulphate)")(FeSO_4(aq)) +Cu(s)) ....................(1.24)

Here, iron has displaced or removed another element, copper, from copper sulphate solution. This reaction is known as displacement reaction.
More examples of displacement reaction are:

color{red}(Zn(s) +underset("(Copper sulphate)")(CuSO_4(aq)) → underset("(Zinc sulphate)")(ZnSO_4(aq)) + Cu (s)) ....................(1.25)

color{red}(Pb(s) + underset("(Copper chloride)")(CuCl_2) → underset("(Lead chloride)")(PbCl_2(aq)) + Cu(s)) ...............................(1.26)

Zinc and lead are more reactive elements than copper. They displace copper from its compounds.

Double Displacement Reaction

Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions

color{green}("𝐎𝐁𝐒𝐄𝐑𝐕𝐀𝐓𝐈𝐎𝐍 :") A white substance, which is insoluble in water, is formed. This insoluble substance formed is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.

color{red}(underset("(Sodium sulphate)")(Na_2SO_4(aq)) + underset("(Barium chloride)")(BaCl_2 (aq)) → underset("(Barium sulphate)")(BaSO_4(s)) + underset("(Sodium chloride)")(2NaCl(aq)))

The white precipitate of color{red}(BaSO_4) is formed by the reaction of color{red}(SO_4^(2-)) and color{red}(Ba^(2+)). The other product formed is sodium chloride which remains in the solution. Such reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

Oxidation and Reduction

color{green}("𝐎𝐗𝐈𝐃𝐀𝐓𝐈𝐎𝐍:") Addition of oxygen or removal of hydrogen from a compound is known as oxidation.

color{green}("𝐑𝐄𝐃𝐔𝐂𝐓𝐈𝐎𝐍:") Addition of hydrogen or removal of oxygen from a compound is known as reduction.

color{green}("𝐎𝐗𝐈𝐃𝐈𝐙𝐈𝐍𝐆 𝐀𝐆𝐄𝐍𝐓:") Compounds which can add oxygen or remove hydrogen are known as oxidizing agent.

color{green}("𝐑𝐄𝐃𝐔𝐂𝐈𝐍𝐆 𝐀𝐆𝐄𝐍𝐓:") Compounds which can add hydrogen or remove oxygen are known as reducing agent.

color{green}("𝐑𝐄𝐃𝐎𝐗 𝐑𝐄𝐀𝐂𝐓𝐈𝐎𝐍:") In a chemical reactions in which oxidation and reduction takes place simultaneously are called redox reactions.

color{green}("𝐎𝐒𝐄𝐑𝐕𝐀𝐓𝐈𝐎𝐍:") The surface of copper powder becomes coated with black copper(II) oxide. This is because oxygen is added to copper and copper oxide is formed.

color{red}(2Cu + O_2 overset("Heat")→ 2CuO)

If hydrogen gas is passed over this heated material color{red}((CuO)), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

color{red}(CuO + H_2 overset("Heat")→ Cu + H_2O)

During this reaction (1.29), the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidized.Such reactions are example of oxidation-reduction reactions or redox reactions.

𝐒𝐨𝐦𝐞 𝐨𝐭𝐡𝐞𝐫 𝐞𝐱𝐚𝐦𝐩𝐥𝐞𝐬 𝐨𝐟 𝐫𝐞𝐝𝐨𝐱 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧𝐬 𝐚𝐫𝐞 :

color{red}(ZnO + C → Zn + CO)

color{red}(MnO_2 + 4 HCl → MnCl_2 + 2H_2O + Cl_2)

In reaction (1.31) carbon is oxidised to color{red}(CO) and color{red}(ZnO) is reduced to color{red}(Zn). In reaction (1.32) color{red}(HCl) is oxidised to color{red}(Cl_2) whereas color{red}(MnO_2) is reduced to color{red}(MnCl_2) .