ICSE Class 8 Chemistry Notes Chapter 6 Chemical Reactions

ICSE Chemistry Class 8 Notes Chapter 6 Chemical Reactions

→ Chemical reaction : “Is a chemical change in which matter changes into a new subtance or substances.”

→ A chemical change is accompained by –

• change of colour
• evolution of gas
• absorption of heat
• formation of ppt. or evolution of heat.

→ Direct combination or synthesis : “When two substances (elements or compounds) combine to produce a new substance.”
Fe + S → Iron sulphide (Fes)

→ Decomposition reaction : “A chemical compound breaks (decomposes) into two simpler substances.”
\(2 \mathrm{HgO} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{Hg}+\mathrm{O}_{2}\)

→ Displacement reaction (or substitution) : “One element displaces another element from its compound.”

→ Double-decomposition or double displacement reaction :
“Compounds exchange their radicals.”

AgNO₂ + KCl → AgCl ↓ + KNO₃
AgNO₂ + HCl → AgCl ↓+ HNO₃

→ Thermal-decomposition : Decomposition with the help of heat.” ‘
\(2 \mathrm{KClO}_{3} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{KCl}+3 \mathrm{O}_{2}\)

→ Thermal-dissociation : “Decomposition of a compound on heating, but reaction is reversible.”

→ Electrolytic decomposition : “A decomposition reaction by passage of current electricity

→ Photochemical decomposition : “A decomposition reaction which occurs in presence of light.”

→ Neutralization : “When acid combines with a base, interchange of radicals takes place to form a salt and water.”

→ Precipitation reaction : “Reaction between two substances (compounds) in liquid form to produce a ppt. (semi solid) by
exchange of radicals.” It is represented by ↓

→ Reactivity series of metals : “Is the series of arrangement of metals in descending order, i.e. most reactive metal K at the top and noble metals like Pt and Au at the bottom.”

→ Reversible reaction : “Is a chemical reaction in which the products formed react together to form the original reactants depending upon the conditions of the reaction.” It is represented by ⇌

→ Catalytic reaction : “A chemical reaction in which a catalyst is used to – Alter the rate of reaction.”

→ Catalyst : “Is a substance which increases or decreases the rate of production of product.” e.g. MnO₂, V₂O₅

→ Promotor : “Is a substance which increases the efficiency of a catalyst.” e.g. MO is added as promotor to iron while preparing NH3 from N₂ and H₂.

→ Positive catalyst : “Is a catalyst which speeds up the rate of reaction.” e.g. MnO₂, V₂O₅, Pt. etc.

→ Negative catalyst : “A catalyst which reduces the rate reaction.” i.e. Alcohol.

→ Enzymes : Pepsin, lipase like substances which accelerate the rate of biochemical reactions in the human body and act as catalysts.”

→ Exothermic reaction : “A chemical reaction which proceeds with evolution of heat.”
CaO + H₂O → Ca(OH)₂ + Heat

→ Endothermic reaction : A chemical reaction which proceeds with – Absorption of Heat.”

→ Non-metal oxides are acidic oxides when put in water produce acids.

→ Metal oxides are basic oxides, when put in water produce base.

→ Amphoteric oxides are metal oxides which react with acids and bases to give salt and water, e.g. ZnO, PbO, Al₂O₃.

→ Neutral oxides are non-metal oxides, donot react with acids or bases, e.g. NO, CO, N₂O.

ICSE Class 8 Chemistry Notes