Chapter 1

Chemical Reactions and Equations

Students of class 10 preparing for their board examinations must go through NCERT Solutions for a better understanding of concepts and the process of solving a difficult problem. It will help them give their best performance during board exams.

Chemical Reactions and Equations is intended to help students to get accustomed to how a chemical reaction can be represented as an equation which will help us to understand the reaction effectively. NCERT solutions for class 10 science chapter 1 has been provided below to help you understand the subject more efficiently.

Introduction of Chemical Reactions:

It is a reaction in which two elements or compounds react together producing change in physical state, temperature or precipitate resulting in formation of new compounds. Class 10 science chapter 1 notes covers the following topics

Characteristics of Chemical Reactions

The chemical reactions are characterized by

• Change in State: When wax is heated it results in formation of water and gas
• Change in Color: Potassium permanganate is purple in color, when citric acid is added, the solution turns colorless.
• Evolution of Gas: Reaction between sodium carbonate and dilute hydrochloric acid evolves carbon-dioxide gas.
• Change in Temperature: Zinc granules when reacted with dilute sulphuric acid it results in release of heat.
• Formation of precipitate: Sulphuric acid and barium chloride results in formation of barium sulphate

Chemical Equations

The representation of a chemical reaction in the form of symbols and formula is called as chemical equation.

Types of Chemical Equation

The chemical equations can be classified based upon different types of reactions as follows

• Combination reactions: When two elements or two compounds combine together to form a single chemical it is called as combination reaction.
• Decomposition reactions: When the chemical reaction results in splitting of compound into two or more chemicals
• Displacement reactions: The reaction which displaces one chemical with the other less reactive material
• Double Displacement Reactions: The reaction in which the element exchange the ions and results in formation of two new compounds.
• Oxidation: Addition of oxygen or removal of hydrogen
• Reduction: Addition of hydrogen or removal of oxygen

NCERT solutions for class 10 science chapter 1 is explained in simple and easy way to the students to get good hold over the subject.

NCERT solutions for class 10 science chapter 1 Questions:

Q1) Why should a Magnesium ribbon be cleaned before burning in air?

 Solution: If left in unused condition, the uppermost layer of Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer which is a stable compound which prevents any further reactions with Oxygen. The Magnesium brush should be cleaned before using to remove the layer of MgO which formed on the uppermost layer of the brush. After removing the layer the reaction becomes feasible with the atmospheric oxygen.

NCERT solutions for class 10 science chapter 1 Questions:

Q2) Write a balanced equations for the following chemical reactions.

1. i) Hydrogen + Chloride —-> Hydrogen chloride
2. ii) Barium chloride + Aluminium Sulphate —-> Barium sulphate + Aluminum chloride

iii) Sodium + Water —-> Sodium hydroxide + Hydrogen

Solution:

1. i) H2+Cl2 -> 2HCl
2. ii) 3BaCl₂ + Al₂(SO₄)₃ -> 2AlCl₃ + 3BaSO₄

iii) 2Na + 2H₂O -> 2NaOH + H₂

Q3) Write a balanced chemical equation with state symbols for the following reactions

1. i) Solutions of Barium chloride and Sodium sulfate in water react to give solution of Sodium chloride and insoluble Barium sulfate.
2. ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce water and Sodium chloride solution.

Solution:

1. i) BaCl₂ + Na2SO₄ -> BaSO₄ + 2NaCl
2. ii) NaOH + HCl -> NaCl + H₂O

Q4) A solution of a substance ‘X’ is used for whitewashing.

1. i) Name the substance ‘X’ and write its formula.
2. ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Solution:

1. i) The substance ‘X’ which is used in whitewashing is Calcium Oxide or Quick Lime and its formula is CaO.
2. ii) CaO + H2O -> Ca(OH)₂

Q5) Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Solution:

Iron is more reactive than copper so when an iron nail is dipped in copper sulphate solution, the iron displaces copper from the copper sulphate and the colour of the copper sulphate solution changes. The reaction is :

Fe+CuSO4 −>FeSO4+Cu

NCERT solutions for class 10 science chapter 1 Questions:

Q6) Identify the substances that are oxidized and that are reduced in the following equation.

1. i) 4Na(s) + O₂(g) -> 2Na₂O(s)
2. ii) CuO(s)+H₂(g)?Cu(s)+H₂O(l)

Solution:

The Sodium (Na) in the first equation is getting oxidized with the addition of Oxygen (O2) and the Copper (Cu) in the second equation is reduced since the addition of Hydrogen (H2).

Exercise:

Q1) Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) -> 2Pb(s)+CO₂(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

Solution:

(i) (a) and (b)

Explanation: (a) because Oxygen is being removed and (b) because the removed oxygen from Lead is added to the elemental Carbon.

Q2) Fe₂O₃+2Al -> Al₂O₃+2Fe

The above reaction is an example of a

1. Combination reaction.
2. Double displacement reaction.
3. Decomposition reaction.
4. Displacement reaction.

Soln:

4. Displacement reaction.

Explanation: The Oxygen from the Ferrous oxide is getting displaced to the Aluminium metal to form Aluminium Oxide.

Q3) What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and Iron chloride are produced.
2. Chlorine gas and Iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Solution:

1. Hydrogen gas and Iron chloride are produced.

Explanation: The Chlorine from the Hydrogen chloride is displaced to the Iron fillings undergoing the following reaction.

2HCl+Fe -> FeCl₂+H₂

Q4) What is a balanced chemical equation? Why should a chemical equation be balanced?

Solution:

The chemical reaction in which the number of atoms in reactant and product side of a chemical equation are equal is known as balanced chemical equation. The Law Of Conservation of Mass is obeyed when the chemical equation is balanced. There is no defined method for Balancing a chemical equation and it is a result of purely a trial and error attempt.

Q5) Translate the following statements into chemical equations and 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.

Solution:

(a) Unbalanced: H2+N2→NH3

Balanced: 3H2+N2→2NH3

(b) Unbalanced: H2S+O2→H2O+SO2

Balanced: 2H2S+3O2→2H2O+2SO2

(c)  Unbalanced: BaCl2+Al2(SO4)3→AlCl3+BaSO4

Balanced: 3BaCl2+Al2(SO4)3→2AlCl3+3BaSO4

(d) Unbalanced:K+H2O→KOH+H2

                        Balanced:  2K+2H2O→2KOH+H2

Q6) Balance the following chemical equations.

1. HNO3+Ca(OH)2→Ca(NO3)2+H2O
2. NaOH+H2SO4→Na2SO4+H2O
3. NaCl+AgNO3→AgCl+NaNO3
4. BaCl2+H2SO4→BaSO4+HCl

Soln:

1. 2HNO3+2Ca(OH)2→2Ca(NO3)2+2H2O
2. 6NaOH+3H2SO4→H2SO4+6H2O
3. NaCl+AgNO3→AgCl+NaNO3
4. BaCl2+H2SO4→BaSO4+2HCl

Q7) Write the balanced chemical equation for the following reactions.

1. Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water
2. Zinc + Silver nitrate —-> Zinc nitrate + Silver
3. Aluminium + Copper chloride —-> Aluminium chloride + Copper
4. Barium chloride + Potassium sulphate —-> Barium sulphate + Potassium chloride

Solution:

1. 2Ca(OH)2+2CO2→2CaCO3+2H2O
2. Zn+2AgNO3→Zn(NO3)2+2Ag
3. 2Al+3CuCl3→2AlCl3+3Cu
4. BaCl2+K2SO4→BaSO4+2KCl

Q8) Write a balanced chemical equation for the following and identify the type of reaction of each case

1. KBr+BaI2→KI+BaBr2
2. ZnCO3→ZnO+CO2
3. H2+Cl→HCl
4. Mg+HCl→MgCl2+H2

Soln:

1. 2KBr+BaI2→2KI+BaBr2 (Double Displacement Reaction)
2. ZnCO3→ZnO+CO2 (Decomposition Reaction)
3. H2+Cl→2HCl  (Combination Reaction)
4. Mg+2HCl→MgCl2+H2 (Displacement Reaction)

Q9) What is meant by exothermic and endothermic reactions? Give examples.

Solution:

When energy is absorbed from the surrounding in the form of heat it is known as     an endothermic reaction. (Example: Photosynthesis, melting of ice, evaporation). On the contrary when energy is released from the surrounding in the form of heat it is known as an exothermic reaction is one in which energy is released from the system into the surroundings. (Example: Explosions, concrete setting, nuclear fission and fusion).

Q10) Why is respiration considered to be an exothermic reaction?

Solution:

For the continuity of life we require energy. This energy is obtained from the food we eat. Through the process of digestion the food molecules are broken down into simpler molecule like glucose. These substances form carbon dioxide, water and certain amount of energy (Respiration process) after coming in contact with the oxygen present in our body cells. Since the energy is in the form of heat (that maintains our body temperature) the respiration is considered to be an exothermic reaction. The reaction taking place is:

C6H12O6+6O2→6CO2+6H2O+Energy

Q11) Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.

Solution:

The reaction between two or more molecules to form a larger molecule is known as combination reaction whereas the splitting of a larger molecules into two or more smaller molecules is known as decomposition reaction. That essentially explains that the decomposition reaction is the opposite of the combination reaction.

In most of the cases the decomposition reaction is endothermic since heat from the surrounding or induced heat is used to break the bonds of the larger molecule. Few examples of decomposition reactions are:

• ZnCO3→ZnO+CO2
• CaCO3+Energy→CaO+CO2
• 2HgO→2Hg+O2

Q12) Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity.

Solution:

(a) Thermal decomposition reaction (Thermolysis)

Potassium chlorate decomposes into potassium chloride and oxygen when heated strongly. The preparation of oxygen is done through this reaction.

  2KClO3+Heat→2KCl+3O2

(b) Electrolytic decomposition reaction (Electrolysis)

Decomposition of sodium chloride:  Molten sodium chloride decomposes into sodium and chlorine when electricity is passed through it.

2NaCl→electricity2Na+Cl2

(c) Photodecomposition reaction (Photolysis)

Decomposition of hydrogen peroxide: Hydrogen peroxide decomposes into water and oxygen in the presence of light.

2H2O2→light2H2O+O2

Q13) What is the difference between displacement and double displacement reactions? Write relevant equations for the above.

Solution:

When a more reactive substance displaces a less reactive one from its salt solution it is known as a displacement reaction whereas the reaction where a mutual exchange of ions happens between two compounds is known as double displacement.

Only a single displacement takes place in a displacement reaction and two displacements take place between the molecules in  double displacement reaction.

Example:

Displacement reaction

Mg+2HCl→MgCl2+H2

Double displacement reaction

2KBr+BaI2→2KI+BaBr2

Q14) In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.

Solution:

Cu(s)+2AgNO3(aq)→Cu(NO3)2(aq)+2Ag(s)

Q15) What do you mean by precipitation reaction? Explain by giving examples.

Solution:

A double displacement reaction takes place in which the ions are exchanged between the compounds when two solutions containing soluble salts are combined. One of such compound formed settles down at the bottom of the container because it is in solid form (that is insoluble). The solid formed is known as the precipitate and the term precipitation reaction is given to the respective reaction. Few examples of precipitation reactions are:

• CdSO4(aq)+K2S(aq)→CdS(s)+K2SO4(aq)
• 2NaOH(aq)+MgCl2(aq)→2NaCl(aq)+Mg(OH)2(s)

Q16) Explain the following in terms of gain of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Solution:

(a) When oxygen is added to the element in a chemical reaction, to form its respective oxide it is the element being oxidised.

Example:

4Na(s)+O2(g)→2Na2O(s) H2S+O2→H2O+SO2

(b) When the oxygen is being removed from the compound in a chemical reaction, then it is said to be reduced.

Example:

CuO(s)+H2(g)→Cu(s)+H2O(l) 2HgO→2Hg+O2

Q17) A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Solution:

Copper metal (Cu) is the shiny brown coloured element. It reacts with the atmospheric oxygen to form copper oxide when the metal is heated in air. Copper Oxide is the black coloured compound is the.

2Cu(s)+O2(g)→2CuO(s)

Q18) Why do we apply paint on iron articles?

Solution:

To prevent the iron articles getting rusted they are painted which is the best way to protect the iron. If the metal is left unpainted, it comes in contact with the atmospheric oxygen and in the presence of moisture it forms Iron (III) oxide i.e rust. By painting the surface it does not come in contact with both the moisture and air which prevents rusting.

Q19) Oil and Fat containing food items are flushed with Nitrogen. Why?

Solution:

The primary purpose of filling nitrogen into the food packets which contains oil and fat items is to prevent rancidity which occurs when the oil or fat reacts with the oxygen letting out an unpleasant smell and taste. To prevent this unpleasant smell nitrogen is filled which creates an unreactive surrounding preventing rancidity. Therefore by flushing Nitrogen, an unreactive surrounding is created thus preventing rancidity.

Q20) Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Solution:

(a)When a refined metal is oxidised by the atmospheric oxygen to form a more stable compound like oxides, it is known as corrosion. The metal gradually degrades during the corrosion process. The best example of corrosion where the iron gets converted into iron oxide is the Rusting of iron. Goverments of various countries spend millions of dollars annually to prevent rusting of bridges and other monuments.

(b)Through aerial oxidation of the oil and fat present in the food creates an unpleasant taste and foul smell. When the food is kept inside the refrigerator the rancidity is retarded since the low temperature does not promote the oxidation reaction.