Chemical Reactions And Equations Class 10th CBSE Questions And Answers

Chapter 1

                                            Chemical Reactions and Equations

Class X

Exercises Questions And Answers

Questions 1 :

Which of the statements about the reaction below are incorrect?

2Pbo_[s] + C_[s] → 2Pb_[s] + CO_2[g]

(a) Lead is getting reduced.

(b) Carbon dioxide is getting oxidised.

(c) Carbon is getting oxidised.

(d) Lead oxide is getting reduced.

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Answer 1:

(a) and (b)

Questions 2 :

Fe₂O₃ + 2A1 →  AI₂ O₃ + 2Fe

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(c) decomposition reaction.

(d) displacement reaction.

Answer 2:

(d) The given reaction is an example of a displacement reaction.

Question 3:

What happens when dilute hydrochloric acid is added to Iron filings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(c) No reaction takes place.

(d) Iron salt and water are produced.

Answer :

(a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

Fe_[s] + 2HC1_[aq] → FeCl_2[aq] + H₂ ↑

Question 4:

What is a balanced chemical equation? Why should chemical equations be balanced?

Answer :

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation. The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation, Hence, it is for this reason that chemical equations should be balanced.

Question 5:

Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer :

(a) 3H_2[g] + N_2[g]   → 2NH_3[g]

(b) 2H₂ S_[g]  + 3O_2[g]  → 2H₂O_[l]  + 2SO_2[g]

(c) 3BaCl_2[aq]  + Al₂(SO₄)_3(aq)  → 2AlCl_3(aq) + 3BaSO_4(S)

(d) 2K_(S)  +  2H₂O_(l)  →  2KOH_(aq)  + H_2(g)

Question 6:

Balance the following chemical equations.

(a) HNO₃ + Ca(OH)₂ → Ca (NO₃)₂ + H₂O

(b) NaOH + H₂SO₄ → Na₂ SO₄ + H₂O

(c) NaCl + AgNO₃ → AgCI + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCI

Answer:

(a) 2HNO₃ +Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ → Na₂ SO₄ + 2H₂O

(c) NaCl + AgNO₃ → AgCI + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCI

Question 7:

Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer :

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

(c) 2Al + 3CuCI₂ → 2AICI₃ + 3Cu

(d) BaCl₂ + K₂ SO₄ → BaSO₄ + 2KCI

Question 8 :

Write a balanced chemical equation for the following and identify the type of reaction in each case.

• Potassium bromide _(aq) + Barium iodide _(aq) → Potassium iodide _(aq) + Barium bromide _(s)
• Zinc carbonate _(s) → Zinc oxide _(s) + Carbon dioxide_(g)
• Hydrogen _(g) + Chlorine _(g) → Hydrogen chloride _(g)
• Magnesium _(s) + Hydrochloric acid _(aq) → Magnesium chloride _(aq) + Hydrogen _(g)

Answer :

• 2KBr_(aq) + Bal_2(aq) → 2Kl_(aq) + BaBr_2(s); Double displacement reaction
• ZnCO_3(s) → ZnO_(s) + CO_2(g); Decomposition reaction
• H_2(g) + Cl_2(g) → 2HCl_(g); Combination reaction
• Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H_2(g); Displacement reaction

 9. does one mean by exothermic and endothermic reactions? Give examples.

Answer :

Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: Mixture of sodium and chlorine to yield table salt.

Na_(s) + ¹/₂ Cl_2(s) → NaCl_(s) + 411 kj of energy

In other words, combination reactions are exothermic. Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

6CO_2(g) + 6H₂O_(l)      C₆ H₁₂ O_6(aq) + 6O_2(g)                                                                                   

Question 10

Why is respiration considered an exothermic reaction? Explain.

Answer :

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

C₆ H₁₂ O_6(aq) +    6O_2(g)  →    6CO_2(g)     +   6H₂O_(l)  +  Energy

↓                          ↓                    ↓                     ↓

Glucose        Oxygen        Carbon dioxide     Water

Question 11 :

Why are decomposition reactions called the opposite of combination reactions? write equations for these reactions.

Answer :

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

Decomposition reactions: AB  + Energy  →  A  +  B

2H₂O_(l)          2H_{2(g)  +} O_2(g)

combination reactions : A  +  B   AB  +  Energy

2H_{2(g)  +} O_2(g)  →  2H₂O_(l)  + Energy

Question 12

Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer :

• Thermal decomposition

2FeSO_4(s) → Fe₂O_3(s) + SO_2(g) +SO_3(g)

• Decomposition by light :

2AgCl_(s)  → 2Ag_(s) + Cl_2(g)

• Decomposition by electricity :

2Al₂O_3(aq)    →         4Al_(s)     +   3O_2(g)

Question 13 :

What is the difference between displacement and double displacement reactions? write equations for these reactions.

Answer :

In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

A+ BX →  AX + B; where A is more reactive than B

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

AB + CD → AD + CB

For example :

Displacement reaction :

CuSO_4(aq)+ Zn_(s)   →    ZnSO_4(aq)  + Cu_(s)

Double displacement reaction :

Na₂SO_4(aq) + BaCl_2(aq)  →   BaSO_4(S)  +2 NaCl_(aq)

Question 14 :

In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. write down the reaction involved.

Answer :

2AgNO_3(aq)     +   Cu_(s)  →     Cu(NO₃)_2(aq)    +    2Ag_(s)

_{Silver nitrate            Copper      Copper nitrete               Silver}

Question 15:

What do you mean by a precipitation reaction? explain by giving examples.

Answer:

A reaction in which insoluble solid (precipitate) is formed is called precipitation reaction.

For examples: 1

Na₂CO_3(aq)     +   CaCl_2(aq)     →     CaCO_3(s)     +     2NaCl_(aq)

Sodium carbonate              calcium chloride                   calcium carbonate          sodium chloride

In this reaction, calcium carbonate is obtained as a precipitate. hence it is a precipitation reaction.

Example: 2

Na₂ SO_4(aq)  +  BaCl_2(aq)  →  BaSO_4(s)    +  2NaCl_(aq)

Sodium sulphate        barium chloride          barium sulphate        sodium chloride

In this reaction, barium sulphate is obtained as a precipitate.

Question 16:

Explain the following terms of gain or loss of oxygen with two examples each.

(a) oxidation

(b) reduction

Answers:

1. oxidation is the gain of oxygen

example:

(i)CO₂   +   H₂    →   CO   +   H₂

(ii) 2Cu  +  O₂  → 2CuO

In equation (i) H2 is oxidized to H2O and inequation (ii) cu is oxidized to CuO;

1. Reduction is the loss of oxygen.

Examples:

• C0₂ +  H₂  →  CO  +  H₂O
• CuO +  H₂ →  Cu  +  H₂O

In equation (i) CO2 is reduced to CO and in equation  (ii) CuO is reduced to Cu.

Question:17

A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer:

‘X’ is copper (Cu) and the black coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

2Cu       +       O₂             2CuO

(Shiny brown in colour)               (Black in colour)

Question:18

Why do we apply paint on iron articles?

Answer:

Iron articles are painted because it prevents them from rusting. When painted, the contact of Iran articles from moisture and air is cut off. Hence rusting is prevented their presence is essential for rusting to take place.

Question:19

Oil and fat containing food items are flushed with nitrogen. Why?

Answer:

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and make them rancid. Thus bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the back rancidity of oil and fat containing food items is avoided.

Question :20

Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Answer:

a)Corrosion:

Corrosion is defined as a process where materials usually metals ,deteriorate as a result of chemical reaction with air, moisture, chemicals, etc.

For example: Iron in the presence of moisture react with oxygen to form hydrated iron oxide.

4Fe  +  3O₂  +  nH₂O  →    2Fe₂O₃ .nH₂ O

Hydrated iron oxide

(This hydrated iron oxide is rust.)

1. b) Rancidity:

the process of oxidation of fats and oils that can be easily noticed by the change in the taste and smell is known as rancidity. for example the taste and smell of butter changes when kept for for long time.

Rancidity can be avoided by:

1. Storing food in air tight containers
2. Storing food in refrigerators
3. Adding oxidizing agents
4. Storing food in an environment of nitrogen.

Intext Questions     (Page No. 6)

Question 1 :

Why should a magnesium ribbon be cleaned before burning in air?

Answer :

Magnesium is very reactive metal like (Na Ca, etc). When it expose to air it reacts with oxygen to form a layer magnesium oxide (MgO) on its surface.

Mg         +   O₂           →      2MgO

              _{Magnesium             oxygen                    Magnesium oxide}

This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be used for the reaction.

Question 2 :

Write the balanced equation for the following chemical reactions.

• Hydrogen + chlorine → Hydrogen chloride
• Barium chloride + Aluminum sulphate → Barium sulphate + Aluminum chloride
• Sodium + water →  Sodium hydroxide + Hydrogen

Answer :

• H₂  +  Cl₂ → 2HCl

• 3BaCl₂  +  Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃

• 2Na +  2H₂O  →   2NaOH   +  H₂

Question 3 :

Write a balanced chemical equation with state symbols for the following reactions?

• Solutions of barium chloride and sodium sulfate in water react to give the insoluble barium sulfate and the solutions of sodium chloride
• Sodium hydroxide solution (in water ) react with hydrochloric acid solution (in water ) to produce sodium chloride solution and water

Answers:

• BaCl₂  + Na₂SO₄ → BaSO₄  + 2NaCl
• NaOH + HCl → NaCl + H₂O

Page no:10

Question:1

A solution of a substance`X’ is used for white washing. (i)Name of the substance`X’ and write its formula.

(ii)Write the reaction of the substance`X’ named in (i)  above the water.

Answer:

• The substance X is a calcium oxide. Its chemical formula is Cao
• Calcium oxide react vigorously with water to form calcium hydroxide (slaked lime).

CaO  +  H₂O  →  Ca(OH₎₂

Question:2

Why is the amount of gas collected in one of the test tube in activity 1.7 double of the amount collected in the other? Name this gas.

Answer:

During the electrolysis of water, hydrogen and oxygen is get separated by the electricity. Water (H₂O) contains two parts of hydrogen and one part of oxygen. Since hydrogen goes to one test tube and oxygen goes to another. the amount of gas collected in one of the test tube is double of the amount collected in the other.

Page no:13

Question :1

Why does the colour of the copper sulphate solution change when an iron nail is dipped in it.

Answer :

When an iron nail is dipped in a copper sulphate solution, Iron (which is more reactive than copper) displaces copper from copper sulphate solution forming iron sulphate which is green in color.

CuSO₄ + Fe → FeSO₄   + Cu

• Therefore the blue colour of copper sulphate solution fades and green color appears.

Question :2

Give an example of a double displacement reaction other than the one given activity 1.10.

Answer:

Sodium carbonate react with the calcium chloride to form calcium carbonate and sodium chloride.

Na₂CO₃ + CaCl₂ →  CaCO₃ + 2NaCl

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence it is a double displacement reaction.

1. Question:3

Identify the substances that are oxidized and the substance that are reduced in the following reactions.

• 4Na_(s) +    O_2(g)  →   Na₂O_(s)
• CuO_(s) +     H_2(g)  →  Cu_(s}   +  H₂O_(l)

Answer:

• Sodium (Na) is oxidized as it gains oxygen and the oxygen gets reduced.
• Copper oxide (CuO) is reduced to copper (Cu) while hydrogen(H₂) gets oxidized to water.