Allied solutions simplified middle school chemistry class 8 Solutions for Chapter 6 chemical reactions

Q1) State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include – (a) Colour change (b) Effervescence or gas evolved © Evolution or absorption of heat (d) Formation of a precipitate. With reference to each of the above indications, state the external indication seen during – (i) addition of dilute acid to active metal (ii) Addition of dilute hydrochloric acid to silver nitrate (iii) Addition of water to quicklime (iv) Thermal decomposition of mercury oxide.

Q2) State why a direct combination reaction is called a – ‘synthesis reaction’.

Q3) Differentiate between –

1. Direct combination reaction & a decomposition reaction
2. Displacement reaction & neutralization reaction

Q4) Classify the following reactions into – (a) Direct combination (b) Decomposition (c) Displacement (d) Double displacement. The reactions are – (i) Zinc Hydroxide on heating gives zinc oxide & water (ii) Zinc reacts with copper sulphate to give zinc sulphate & copper (iii) Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide (iv) Molten zinc at high temperatures, burns in air to give zinc oxide.

Q5) Give balance equations for –

1. A direct combination reaction involving two elements, one of which is a non-metal
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 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 sodium salt.

Q6) State what is meant by ‘reactivity series of metals’. With reference to – (a) Water (b) Acids, Explain with suitable examples of how the reactivity of the metals could be differentiated.

Q8) 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 – (i) water is added to quicklime (ii) two neutral gases on passage through an electric arc give nitric oxide (iii) two neutral gases combine to give – a basic gas

Q7) 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 –

1. Nitrogen is one of the reactants
2. Sulphur dioxide as one of the reactants

Q9) Certain thermal decomposition reactions result in the formation of oxides. Give balanced equations for the thermal decomposition of the following which results in the formation of a metallic oxide:

1. Limestone
2. Lead carbonate
3. Calcium nitrate
4. Calcium hydroxide

Q10) State the meaning of the term ‘oxide’. Give a balanced equation for the formation of the following oxides:

1. Sulphur dioxide from a non-metal
2. Zinc oxide from a metal
3. Lead oxide from a mixed oxide

Q11) 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 i.e. (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.

Q12) Give an example of:

1. A peroxide
2. A mixed oxide
3. A dioxide

Q1)

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 and water
2. The neutral gas obtained on thermal decomposition of potassium nitrate.
3. The displaced product of the displacement reaction of sodium with cold water.
4. The catalyst used in the catalytic reaction involving the reactants nitrogen & hydrogen,
5. A neutral oxide which does not react with an acid or a base to give salt & water.

Q2) Complete the statements by filling in the blank with the correct word/words:

1. Direct combination reaction of phosphorus pentoxide with water gives ____( H_3PO_4/H3PO_4
2. Decomposition of silver salts in the presence of sunlight is an example of _____(double decomposition/ photochemical decomposition)
3. The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a ____ (promoter/ catalyst)
4. The reaction of coke with the stream to give water gas is an example of an _____ (exothermic/ endothermic) reaction.
5. The metal which reacts with stream and the reaction is reversible is _____ (calcium/ iron)

Q3) 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.
2. A reaction of direct combination i.e. synthesis in which two gases combine to give another gas – which turns lime water milky.
3. A thermal decomposition reaction in which a metallic nitrate decomposes to give – a basic oxide
4. A catalytic, reversible, exothermic reaction.
5. A displacement reaction in which a metal above hydrogen in the reactivity series, displaces another metal from the solution of its compound.

Q4) Differentiate between the following:

1. Thermal decomposition & thermal dissociation
2. Neutralization reaction & a precipitation reaction
3. Electrolytic decomposition & photochemical decomposition
4. A catalyst & a promoter
5. An acidic oxide & a basic oxide

Q5) Match the chemical reactions in List I with the appropriate answer in List II