Chemistry ICSE Class 8 Important Questions Chapter 6 Chemical Reactions

Question 1.
State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include –
(a) Colour change
(b) Effervescene or gas evolved
(c) Evolution or absorption of heat
(d) Formation of a precipitate.
With reference to each of the above indications, state the external indication seen during –

  1. Addition of dilute acid to an active metal
  2. Addition of dilute hydrochloric acid to silver nitrate
  3. Addition of water to quicklime
  4. Thermal decomposition of mercury [II] oxide.

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

  1. Addition of dil. acid to an active metal : Gas is evolved, test tube or flask becomes hot (heat is produced).
  2. Addition of dil. hydrochloric acid to silver nitrate : A white coloured precipitate of AgCl is seen.
  3. Addition of water to quick lime : Hissing sound accompanied by a lot of heat (boiling).
  4. Thermal decomposition of mercury [II] oxides : Red colour of Hg[II]O changes to silvery (Hg metal).

Question 2.
State why a direct combination reaction is called a – ‘synthesis reaction’.
Answer:
When two or more substances [element with element or element with compound or compound with another compound] combine to form a new compound with new properties. Hence direct combination is called ‘synthesis reaction’.

Example :
Hydrogen (a gas) + Oxygen (a gas) combines to form water (liquid).
2H2 + O2 → 2H2O

Question 3.
Differentiate between –
(a) Direct combination reaction & a decomposition reaction
(b) Displacement reaction & a neutralization reaction.
Answer:
(a) Direct combination : “Is a reaction in which two elements ‘ or compound combine to give one new compound.”
C + O2 → CO2
Decomposition reaction : “A chemical compound (breaks up) decomposes into two or more simpler substances.”
Copper carbonate → Copper oxide + carbon dioxide
CuCO3 → CuO + CO2

(b) Displacement reaction : “A chemical reaction in which an element displaces the element from a solution of its compound.”
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 1
In a activitiy series of metals, an element placed higher displaces the element placed below it.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 2
Here Zn is placed above Cu in activity series of metal and is more reactive than copper has displaced Cu from copper sulphate.

Neutralization reaction :
“A reaction between an acid and a base to form salt and water is called neutralization reaction.”
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 3

Question 4.
Classify the following reactions into –
(a) Direct combination
(b) Decomposition
(c) Displacement
(d) Double decomposition
The reactions are –

  1. Zinc hydroxide on heating gives zinc oxide & water
  2. Zinc reacts with copper [II] sulphate to give zinc sulphate & copper
  3. Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide
  4. Molten zinc at high temperatures, burns in air to give zinc oxide.

Answer:

  1. Zinc hydroxide a single compound decomposes into two simpler components zinc oxide and water, hence it is decomposition reaction.
  2. Zinc reacts with copper [II] sulphate to give zinc sulphate and copper is displacement reaction. Zinc being more
  3. reactive than copper displaced copper from copper sulphate. is double decomposition as exchange of ions takes place.
    ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 4

Question 5.
Give balanced equations for –

  1. A direct combination reaction involving two elements, one of which is a nonmetal
  2. A thermal decomposition reaction involving heat on limestone [calcium carbonate]
  3. An electrolytic decomposition reaction involving a neutral liquid
  4. A displacement reaction involving a metal above hydrogen in the activity series with copper [II] sulphate solution
  5. A double decomposition neutralization reaction involving an acid & a base
  6. A white precipitate obtained during a double decomposition reaction involving a silver salt with a sodium salt.

Answer:
1. Balanced chemical equation of : A combination reaction between calcium and oxygen (non-metal) to form calcium oxide.
2Ca + O2 → 2CaO
2. Thermal decomposition of limestone :
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 5

3. Electrolytic decomposition of neutral liquid [water]
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 6

4. Displacement reaction
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 7
[more reactive displaces less reactive Cu]

5. Neutralization of acid with base to form salt and water.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 8

6. Double decomposition reaction
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 9

Question 6.
State what is meant by ‘reactivity series of metals’. With reference to – (a) Water (b) Acids explain with suitable examples how the reactivity of the metals could be differentiated.
Answer:
Activity series of metals : “Is a series of arrangement of metals in decreasing order of their reactivity.” i.e. metals at top are most reactive and least reactive metals are at the bottom of the series. Metal above is more reactive than the lower metal. In other words K is more reactive than all the metals below it and Na is more reactive than all the metals below it.
(a) Action of metals with water : K, Na2 Ca react with cold water.
K : Darts on water surface and react violently.
Na : Revolves on the water surface and bums.
Ca : Sinks in water, react less violently.
Hence, K > Na > Ca calcium is less reactive than K and Na.
Mg and Al react with boiling water/steam.
Hence, Mg and Al are less reactive than K, Na and Ca. Zn, Fe, Pb also react with steam and reaction stops soon. Fe when hot reacts with steam This shows that Mg > Al > Zn > Fe > Pb.
Cu, Hg, Ag, Pt, Au donot react with steam or even when hot are less reactive.

(b) Action of metals with acids : As reactivity decreases from top to bottom in metal activity series.
K and Na react with dil. HCl and dil. H2S04 explosively to produce H2
2Na + 2HCl → 2NaCl + H2
Ca, Mg, Al, Zn and iron react less vigorously with decreasing vigour with dil. H2S04 or dil. HCl to produce H2
Ca + H2SO → CaSO2 + H2
Metals below hydrogen do not react with dil. acid and do not displace H2 from it.

Question 7.
A chemical reaction may be ‘reversible’ in nature. State the meaning of the term in italics. Give a reason why a catalyst is used in certain chemical reactions. Give a balanced equation for the following – (a) A reversible catalytic reaction involving
(i) nitrogen as one of the reactants
(ii) sulphur dioxide as one of the reactants.
Answer:
Reversible reaction : “A chemical reaction is said to be reversible in nature if “products formed react together – to form actual reactants depending on the condition of the reaction.”
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 10
A catalyst is used to make the reaction fast or slow down it. When KClO3 is heated, oxygen is produced but reaction is very slow. If we add MnO2 to KClO3 and heat the rate of production of oxygen becomes faster.
Balanced chemical equations for reversible reactions involving,

(a) Nitrogen as one of the reactants
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 11
Here iron is used as catalyst
(b) SO2as one of the reactants.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 12

Question 8.
State which type of chemical reactions proceed with –
(a) Evolution of heat energy
(b) Absorption of heat energy. State in each of the following reactions whether heat is evolved or absorbed –

  1. water is added to quicklime
  2. two neutral gases on passage through an electric arc give nitric oxide
  3. two neutral gases combine to give – a basic gas.

Answer:
(a) Exothermic reactions.
(b) Endothermic reactions.

  1. When water is added to quicklime heat is evolved.
  2. Nitrogen and oxygen.
  3. Nitrogen gas and hydrogen gas give NH3 (ammonia a basic gas).

Question 9.
Certain thermal decomposition reactions, result in formation of oxides. Give balanced equations for the thermal decomposition of the following, which result in formation of a metallic oxide,
(a) Limestone
(b) Lead carbonate
(c) Calcium nitrate
(d) Calcium hydroxide.
Answer:
(a) Limestone CaCO3 decomposes to CaO and CO2
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 13
(b) Lead carbonate PbCO3
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 14
(c) Calcium nitrate Ca[NO3]2
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 15
(d) Calcium hydroxide
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 16

Question 10.
State the meaning of the term ‘oxide’. Give a balanced equation for formation of the following oxides –
(a) Sulphur dioxide from a non-metal
(b) Zinc oxide from a metal
(c) Lead oxide from a mixed oxide.
Answer:
Oxide is product obtained on combining with oxygen.
i. e. combination with oxygen.
OR
Binary compounds of a metallic or non-metallic element with oxygen.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 17

Question 11.
Give two examples each of the following oxides –
(a) Acidic oxides
(b) Basic oxides
(c) Amphoteric oxides
(d) Neutral
oxides. State which of the following oxides Le. (a) to (d)
(i) React with water to give a base
(ii) React with a base to give salt & water
(iii) React with acids & bases to give salt & water.
Answer:
(a) Acidic oxides : Non-metallic oxides are acidic oxides.

  1. SO2
  2. CO2
  3. NO2 are acidic oxides.

(b) Basic oxides : Metallic oxides are basic oxides.

  1. Na2O
  2. K2O
  3. CaO are basic oxides.

(c) Amphoteric oxides : Oxides of metals like Zn, Pb, A1 are amphoteric oxides.

  1. ZnO
  2. PbO
  3. Al2O3 are amphoteric oxides.

(d) Neutral oxides : Oxides like (i) NO (ii) CO (iii) N2O are neutral oxides.

  1. Basic oxides react with water to give bases.
  2. Acidic oxide reacts with base to give salt and water.
  3. Amphoteric oxide reacts with acids and bases to give salt and water.

Question 12.
Give one example each of –
(a) A peroxide
(b) A mixed oxide
(c) A dioxide.
Answer:
Examples of :
(a) A peroxide – Na2O2 (Sodium peroxide)
(b) A mixed oxide – Pb3O2
(c) A dioxide – PbO2

Objective Type Questions

Question 1.
Select the correct answer from A, B, C, D and E for each statement given below:
A: Iron
B: Carbonic acid
C: Hydrogen
D: Oxygen
E: Carbon monoxide

1. The product formed during direct combination reaction of carbon dioxide & water.
Answer:
B; Carbonic acid

2. The neutral gas obtained on thermal decomposition of potassium nitrate.
Answer:
D: Oxygen

3. The displaced product of the displacement reaction of sodium with cold water.
Answer:
C: Hydrogen

4. The catalyst used in the catalystic reaction involving the reactants nitrogen & hydrogen.
Answer:
A Iron

5. A neutral oxide which does not react with an acid or a base to give salt & water.
Answer:
E: Carbon monoxide

Question 2.
Complete the statements by Ailing in the blank with the correct words :
1. Direct combination reaction of phosphorus pentoxide with water gives …………… [H3PO3/H3PO4].
Answer:
Direct combination reaction of phosphorus pentoxide with water gives H3PO4.

2. Decomposition of silver salts in the presence of sunlight is an example of …………… [double decomposition/ photochemial decomposition].
Answer:
Decomposition of silver salts in the presence of sunlight is an example of photochemial decomposition.

3. The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a …………… [promoter/catalyst].
Answer:
The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a promoter.

4. The reaction of coke with steam to give water gas is an example of an …………… [exothermic/endothermic] reaction.
Answer:
The reaction of coke with steam to give water gas is an example of an endothermic reaction.

5. The metal which reacts with steam and the reaction is reversible is …………… [calciumiron].
Answer:
The metal which reacts with steam and the reaction is reversible is iron.

Question 3.
Give a balanced equation for each of the following types of reactions:
1. A thermal decomposition reaction in which a compound decomposes to give two new compounds.
Answer:
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 18

2. A reaction of direct combination i.e. synthesis In which two gases combine to give another gas – which turns lime
water milky.
Answer:
Synthesis when two gases combine to give a gas which turns lime water milky.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 19

3. A thermal decomposition reaction in which a metallic nitrate decomposes to give – a basic oxide.
Answer:
Metallic nitrate is calcium nitrate on thermal decomposition gives CaO which is a basic oxide.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 20

4. A catalytic, reversible, exothermic reaction.
Answer:
A catalytic, reversible, exothermic reaction.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 21

5. A displacement reaction in which a metal above hydrogen in the reactivity series, displaces another metal from the solution of its compound.
Answer:
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 22

Question 4.
Differentiate between the following :
1. Thermal decomposition & thermal dissociation.
Answer:
Thermal decomposition and thermal dissociation. Decomposition of a compound on heating into two elements or element and compound or into two new compounds. It is irreversible reaction. Thermal dissocation is the decomposition of a substance into two or more simpler substances on heating but reaction is reversible.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 23

2. Neutralization reaction & a precipitation reaction.
Answer:
Formation of salt and water when an acid reacts with a base is called neutralisation.
Precipitation : Formation of semi solid (insoluble) product in called precipitation.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 24

3. Electrolytic decomposition & photochemical decomposition.
Answer:
Electrolytic decomposition : “A decomposition reaction which is brought about by – passage of electric current.”
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 25
Photochemical decomposition : “A decomposition reaction which takes place in presence of light.” e. g. decomposition of silver salts in presence of sun light.

4. A catalyst & a promoter.
Answer:
Catalyst : “A substance which alters the rate of chemical reaction.” e. g. MnO2, V2O2, Fe, Pt. etc.
The positive catalyst : A catalyst which speeds up the reaction.
Negative catalyst : A catalyst which reduces the rate of reaction, e.g. alcohol. Enzymes are biological catalysts present in human body and accelerate the biochemical reactions in the body.
Promotors is a catalyst to catalyst, i.e. a substance which ‘ increases the activity of a catalyst.
Molybdenum [MO] is added to the catalyst iron [Fe] to increase its efficiency.

5. An acidic oxide & a basic oxide.
Answer:
Acidic oxide : Non-metal oxide is called acidic oxide i.e. SO2, NO2 when acidic oxide dissolves in water forms acid.
Basic oxide : Metallic oxide is called basic oxide, e.g. CaO, Na2O basic oxide dissolved in water is called alkali.

Question 5.
Match the chemical reactions in List I with the appropriate answer in List II.
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 26
Answer:
ICSE Class 8 Chemistry Important Questions Chapter 6 Chemical Reactions - 27

ICSE Class 8 Chemistry Important Questions

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