GSEB Solutions for Class 9 Science and Technology – Chemical Reactions (English Medium)
A. Oxygen is added or hydrogen is removed
A chemical reaction in which oxygen is added or hydrogen is removed from an element, molecule or a compound is called an oxidation reaction.
A chemical reaction in which hydrogen is added or oxygen is removed from an element, molecule or a compound is called a reduction reaction.
Starch is a complex sugar. During digestion, it gets decomposed to give simple sugars which can easily be absorbed in the body.
Amino acids are building blocks of proteins. Decomposition of proteins yields amino acids.
Fe placed in copper sulphate solution, displaces copper and forms ferrous sulphate and releases free copper particles. This results in the change of blue coloured copper sulphate solution to green coloured ferrous sulphate solution.
A. Calcium oxide
Calcium oxide (CaO) is commonly called quick lime.
Calcium hydroxide [Ca(OH)2] is commonly called slaked lime.
A reducing agent loses an electron during a chemical reaction and gets oxidized.
Starch is a complex sugar molecule. Decomposition of starch yields simple sugars which can easily be absorbed in the body.
In NH3, nitrogen element combines with three atoms of hydrogen, thus indicating that nitrogen is trivalent.
A chemical formula mentions the type of atom and its number present in a compound.
Chemical reaction is a process by which one or more substances may be transformed into 1 or more new substances.
Combining capacity of an atom is the number of electrons an atom must lose or gain to attain a stable configuration. It is also called the valency of an atom.
Formulae of following compounds is :
- Ferric oxide – Fe2O3
- Plumbus chloride – PbCl
- Cuprous iodide – CuI2
- Carbon tetrafluoride – CF4
- Ammonium chloride – NH4Cl
The compounds having a covalent bond are called molecular compounds.
Example – Water is a molecular compound.
Monoatomic positive ions are:
- Sodium ion (Na+)
- Silver ion (Ag+)
Monoatomic negative ions are:
- Fluoride (F–)
Two compounds known by common names are:
- Calcium carbonate commonly called as chalk.
- Sodium hydroxide commonly called as caustic soda.
Three examples of polyatomic negative ions are:
- Phosphate (PO43-)
- Carbonate (CO32-)
The substances taking part in the reaction are called reactants and the substances formed as a result of reaction are called products.
The reaction in which two or more than two elements or compounds combine with one another to form a new compound is called a combination reaction.
A reaction in which one substance is decomposed to form two or more than two substances is called a decomposition reaction.
A reaction in which a more active metal displaces a less active metal from a solution of its salt is called a displacement reaction.
Oxidation reaction is a chemical reaction in which oxygen is added to or hydrogen is removed from an element, molecule or a compound.
Reduction reaction is a chemical reaction in which oxygen is removed from or hydrogen is added to an element, molecule or a compound.
It is necessary to balance a chemical reaction in order to obey the law of conservation of mass.
The decomposition reaction through the passage of electric current is known as electrical decomposition or electrolysis reaction.
Electrolysis of water can be studied using a voltameter.
When electric current is passed through water containing few drops of sulphuric acid, electrolysis of water takes place. Water gets decomposed to dihydrogen and dioxygen as a result of electrolysis.
Utility of thermal decomposition reaction:
Thermal decomposition reaction is used to obtain metals from their oxides, sulphides or salts.
Mono displacement reaction:
When a more active metal displaces a less active metal from its salt solution, it is called mono-displacement method.
Iron is a more active metal than copper. Iron displaces copper from copper sulphate solution and changes the colour of the solution from blue to green.
CuSO4(aq) + Fe(s) → FeSO4(aq) + Cu(s)
Double displacement reaction:
A reaction in which an atom or a group of atoms of one substance are exchanged with an atom or a group of atoms of another substance is called a double displacement reaction.
In a reaction between barium chloride and sodium sulphate, barium ion from barium sulphate displaces sodium ion from sodium sulphate and the two ions exchange places. As a result, a precipitate of barium sulphate and sodium chloride solution is obtained. Hence, here, double displacement reaction takes place.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + NaCl(aq)
In the following reaction
2H2(g)+O2(g) → 2H2O(l)
- 2H2(g) → 4H+ + 4e– – Oxidation reaction
- O2(g) + 4e–→ 2O2- – Reduction reaction
In the 1st reaction, four hydrogen atoms lose one electron each.Thus, it is an oxidation reaction.
In the 2nd reaction, two oxygen atoms receive two electrons each forming O2-. Thus, it is a reduction reaction.
Oxidation and reduction reactions occur simultaneously and thus, it is called a redox reaction.
In the following reaction,
Cu2+(aq)+Zn(s) → Zn2+(aq)+Cu(s)
- Zn(s) → Zn2+(aq) + 2e– – Oxidation Reaction
Oxidation number of Zn is 0 and of Zn2+ is +2
- Cu2+(aq) + 2e–→ Cu(s) – Reduction Reaction
Oxidation number of Cu2+ is +2 and of Cu is 0
The total redox reaction is written as
Cu2+(aq) + Zn(s) → Zn2+(aq) + Cu(s)
+2 0 +2 +0
Hence, the above reaction is balanced from the atom and electric charge point of view.
|Oxidising Agents||Reducing Agents|
|Oxidizing agents are substances which oxidise other elements, molecules or compounds.||Reducing agents are substances which reduce other elements, molecules or compounds.|
|Oxidising agents get reduced during the reaction.||Reducing agents get oxidised during the reaction.|
|Oxidising agents receive an electron or hydrogen during the reaction.||Reducing agents loses an electron or hydrogen during the reaction.|
|Oxidising agents loses oxygen during the reaction.||Reducing agents gains oxygen during the reaction.|
|In the following reaction, Cu2+ is an oxidising agent.
Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)
In the following reaction, Zn(s) is a reducing agent.
Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)
2KMnO4(s) + 16HCl(aq) → 2KCl(aq) + 2MnCl2(aq)+ 8H2O(l) + 5Cl2(g)
- KMnO4 – Oxidising Agent
- O2 – Oxidising Agent
- Cl2 – Oxidising Agent
- FeSO4 – Reducing Agent
- Na – Reducing Agent