Language of Chemistry

Language of Chemistry

Question 1

Explain the term 'chemical reaction' with special reference to – 'reactants' and 'products'.

Answer

A chemical reaction involves the breaking of existing bonds between atoms or groups of atoms of the reactants and leading to formation of new bonds forming new substances called the products.

Question 2

Represent the addition of dilute sulphuric acid to zinc by means of a chemical equation. State the function of the arrow between the reactants and products.

Answer

Zn + H₂SO₄ [dil.] ⟶ ZnSO₄ + H₂ [g]

The function of the arrow (⟶) is to show the transformation of reactants into products.

Question 3

A chemical reaction is generally accompanied by certain external indications or characteristics. These include change of –

(a) color

(b) state

(c) smell

(d) evolution of gas

(e) formation of precipitate

(f) evolution or absorption of heat.

With reference to change of color – state the change in color seen when the following are heated –

(i) copper carbonate

(ii) zinc carbonate

(iii) mercury [II] oxide

(iv) lead [IV] oxide

Answer

(i) Green to black.

(ii) White to yellow.

(iii) Red to silvery.

(iv) Brown to yellow.

Question 4

Give a balanced equation for addition of iron to copper [II] sulphate solution. State the change in color seen.

Answer

$\underset{\text{Blue}}{\text{CuSO}_4} + \text{Fe} \longrightarrow \underset{\text{Light green}}{\text{FeSO}_4} + \underset{\text{Brownish red}}{\text{Cu}}$

Colour changes from blue to light green.

Question 5

In certain reactions a change of state is observed i.e. solid to liquid, liquid to gas etc. – State the change of state of the reactants – to give the respective products in each case.

(a) C + 2S ⟶ CS₂

(b) NH₃ + HCl ⇌ NH₄Cl

(c) 2H₂ + O₂ ⟶ 2H₂O

(d) $2\text{H}_2\text{O} \xrightarrow[\text{current}]{\text{electric}} 2\text{H}_2 + \text{O}_2$

Answer

(a) solid to liquid

(b) gas to solid

(c) gas to liquid

(d) liquid to gas

Question 6

In certain reactions a change of smell is observed. Give two examples from daily life of the same.

Answer

1. Fresh bread kept in open for long hours produces a foul smell due to the growth of fungus.
2. Fresh egg gets spoiled and turns into a rotten egg producing foul smell.

Question 7

In certain reactions an insoluble solid called precipitate is formed. State the colour and name of the precipitate formed in each of the following reactions involving addition of:

(a) Dilute hydrochloric acid to silver nitrate

(b) Iron [II] sulphate to sodium hydroxide

(c) Iron [III] chloride to ammonium hydroxide

(d) Copper [II] sulphate to sodium hydroxide

(e) Lead nitrate to ammonium hydroxide

Answer

Question 8

Give examples of exothermic reactions i.e. heat evolved in each of the following:

(a) reaction between a solid and a liquid

(b) reaction between two gases

Give two examples of exothermic reactions seen in everyday life.

Answer

(a) CaO + H₂O ⟶ Ca(OH)₂ + Δ

(b) 2H₂ + O₂ ⟶ 2H₂O + Δ

Two examples of exothermic reactions from everyday life are:

1. Human Body — Many chemical reactions in our body are exothermic and provide energy to survive.
2. Setting of Cement — Cement setting liberates heat hence concrete structures are set slowly.

Question 9

Give one example in each case where supplying energy [given below] is necessary for a chemical reaction.

(a) Heat energy

(b) Light energy

(c) Electrical energy

(d) Pressure

(e) Catalyst

Answer

(a) ZnCO₃ $\xrightarrow{\Delta}$ ZnO + CO₂ [g]

(b) H₂ + Cl₂ $\xrightarrow{\text{sunlight}}$ 2HCl

(c) $2\text{H}_2\text{O} \xrightarrow[\text{current}]{\text{electric}} 2\text{H}_2 + \text{O}_2$

(d) $\text{N}_2 + 3\text{H}_2 \xrightleftharpoons [450\text{\degree C }]{\text{Fe/200 atmos press}} 2\text{NH}_3$ + Δ

(e) 4NH₃ + 5O₂ $\xrightarrow{\text{Pt-800 \degree C}}$ 4NO + 6H₂O + Δ

Question 10

Representation of the results of a chemical change – is a chemical equation.

For the equation: FeCl₃ + 3NH₄OH ⟶ 3NH₄Cl + Fe(OH)₃ ↓

Answer the following:

(a) Write the word equation for the above molecular equation.

(b) Is the equation given above a balanced equation. Give reasons.

(c) Name the reactants and the products in the above equation.

(d) What is the indications of the arrow between the reactants and the products and of the arrow pointing downwards at the end.

Answer

(a) Iron (III) chloride + ammonium hydroxide ⟶ Ammonium chloride + Iron (III) hydroxide

(b) In the given equation:

As, the number of atoms of each element of reactants = the number of atoms of each element of products. Hence, the equation is balanced.

(c) Reactants – Iron (III) Chloride [FeCl₃ ] and Ammonium hydroxide [3NH₄OH],
Products – Ammonium Chloride [3NH₄Cl] and Iron (III) Hydroxide [Fe(OH)₃].

(d) The arrow between reactants and products indicates the chemical reaction taking place in the forward direction i.e., reaction is taking place between reactants and as a result products are formed. The arrow pointing downwards indicates formation of precipitate.

Question 11

Give word equations for the following chemical reactions and give the names of the products formed.

(a) Zn + 2HCl ⟶ ZnCl₂ + H₂

(b) CuO + H₂SO₄ ⟶ CuSO₄ + H₂O

(c) Zn + S ⟶ ZnS

(d) ZnCO₃ $\xrightarrow{\Delta}$ ZnO + CO₂

(e) NH₃ + HCl ⟶ NH₄Cl

Answer

(a) Zinc + dilute Hydrochloric acid ⟶ Zinc chloride + Hydrogen
The products formed are zinc chloride and hydrogen.

(b) Copper (II) oxide + Sulphuric Acid ⟶ Copper (II) sulphate + water
The products formed are Copper (II) sulphate and water

(c) Zinc + Sulphur ⟶ Zinc Sulphide
The product formed is Zinc Sulphide.

(d) Zinc carbonate ⟶ Zinc oxide + Carbon dioxide
The products formed are Zinc oxide and Carbon dioxide

(e) Ammonia + Hydrogen chloride ⟶ Ammonium chloride
The product formed is Ammonium chloride.

Objective Type Questions

Question 1

Select the correct answer from A, B, C, D and E for each statement given below:

A: Hydrogen

B: Copper [II] oxide

C: Oxygen

D: Copper [II] hydroxide

E: Ammonia

1. The black residue obtained on heating copper carbonate.

2. The basic gas obtained when two gases react in presence of a catalyst and the reaction is exothermic.

3. The gas obtained when potassium chlorate is heated in the presence of a catalyst.

4. The pale blue precipitate obtained when copper [II] sulphate reacts with sodium hydroxide.

5. The gas which reacts with oxygen to give a neutral liquid.

Answer

1. B: Copper [II] oxide

2. E: Ammonia

3. C: Oxygen

4. D: Copper [II] hydroxide

5. A: Hydrogen

Question 2

Give a word equation for each of the following reactions.

1. Two solids which combine on heating, to give a liquid.

2. Two gasses which combine to give a solid.

3. A compound which on heating shows – color change from brown to yellow.

4. A salt which reacts with ammonium hydroxide to give a reddish-brown precipitate.

5. An exothermic reaction where both the reactants are gases, one of which is oxygen.

Answer

1. Carbon + Sulphur ⟶ Carbon disulphide

2. Ammonia + Hydrogen chloride ⟶ Ammonium chloride

3. Lead [IV] oxide ⟶ Lead monoxide + Oxygen

4. Iron (II) chloride + Ammonium hydroxide ⟶ Ammonium chloride + Iron (III) hydroxide

5. Hydrogen + Oxygen ⟶ Water + Δ

Question 3

Name the following:

1. The silvery residue obtained on heating mercury [II] oxide.

2. The gas evolved when dilute acid is added to chalk [limestone].

3. Two non-metal which react explosively when brought in close contact.

4. Two gases which react under pressure in presence of a catalyst at elevated temperatures to give a gaseous product.

5. A catalyst which increases the rate of a chemical reaction.

Answer

1. Mercury [Hg]

2. Carbon dioxide [CO₂]

3. Iodine and phosphorus

4. Nitrogen [N₂] and hydrogen [H₂]

5. Manganese dioxide [MnO₂]

Question 4

Give reasons for the following:

1. Even though rusting is an exothermic reaction, the heat evolved when an iron nail rusts are unnoticeable.

2. Electrolytic decomposition of water is a reaction involving a change of state.

3. A characteristic change of colour is seen when a piece of iron is added to copper [II] sulphate solution.

4. The catalytic oxidation of sulphur dioxide to sulphur trioxide is represented by a double arrow [⇌] between reactants and products, formed in the chemical reaction.

5. Silver salts are kept in dark coloured bottles.

Answer

1. Rusting of iron is a slow reaction and the heat evolved is very less, hence it is unnoticeable.

2. In electrolytic decomposition of water, water which is in liquid state decomposes to form hydrogen and oxygen which are in gaseous state. Hence, it is a reaction involving a change of state from liquid to gas.

3. Copper [II] sulphate solution which is blue in colour, reacts with iron to form pale green iron [II] sulphate solution and reddish-brown metallic copper. Hence, a characteristic change of colour is seen in this reaction.

4. The double arrow indicates that the reaction is reversible i.e., the products formed will recombine to give the reactants. The sulphur trioxide formed can decompose back to sulphur dioxide and oxygen. This is represented in the equation by the double arrow [⇌].

5. Silver salts decompose in presence of sunlight, hence they are kept in dark coloured bottles.

Question 5

Write word equations for the following chemical reactions given below. Also state the observation seen in each case.

1. ZnCO₃ $\xrightarrow{\Delta}$ ZnO + CO₂

2. NaCl + AgNO₃ ⟶ AgCl ↓ + NaNO₃

3. Na₂SO₃ + 2HCl $\xrightarrow{\Delta}$ 2NaCl + H₂O + SO₂

4. $2\text{SO}_2 + \text{O}_2 \xrightleftharpoons [450\text{\degree C }]{\text{V}_2\text{O}_5} 2\text{SO}_3$

5. Pb(NO₃)₂ + 2NH₄OH ⟶ 2NH₄NO₃ + Pb(OH)₂ ↓

Answer

1. Zinc carbonate ⟶ Zinc oxide + Carbon dioxide.
   White coloured zinc carbonate changes to yellow coloured zinc oxide.

2. Sodium Chloride + Silver nitrate ⟶ Silver chloride + Sodium nitrate.
   A milky white coloured precipitate of silver chloride is formed when silver nitrate solution is added to sodium chloride solution.

3. Sodium sulphite + Hydrochloric acid ⟶ Sodium chloride + Water + Sulphur dioxide
   Effervescence of pungent smelling gas sulphur dioxide are seen.

4. Sulphur dioxide + Oxygen ⇌ Sulphur trioxide
   It is an exothermic reaction and heat is evolved.

5. Lead nitrate + Ammonium hydroxide ⟶ Ammonium nitrate + Lead hydroxide.
   Chalky white precipitate of lead hydroxide is a formed.