Chapter 1 – Chemical Reactions And Equations
CBSE Class 10 Science – NCERT Notes with Important Questions
📚 Complete Study Material Includes:
- Detailed Chapter Notes with Examples
- Step-by-step Equation Balancing Methods
- MCQs with Answers (Multiple Choice Questions)
- Short Answer Questions (2-3 Marks)
- Long Answer Questions (5 Marks)
- Assertion-Reason Questions
- CBSE Previous Year Questions
Ways of Representing a Chemical Reaction
Word Equation
(Reactant)
(Product)
Chemical Equation
(Reactant)
(Product)
Chemical Equation
- A chemical reaction can be represented by chemical equation. It involves uses of symbol of elements or chemical formula of reactant and product with mention of physical state.
- The necessary conditions such as temperature, pressure or any catalyst should be written on arrow between reactant and products.
e.g., Magnesium is burnt in air to form Magnesium oxide.
Balancing Chemical Equation
- Law of conservation of Mass: Matter can neither be created nor be destroyed in a chemical reaction.
- So number of atoms of the elements involved in chemical reaction should remain same at reactant and product side.
STEPWISE BALANCING (Hit and Trial)
Step 1. Write a chemical equation and draw boxes around each formula.
H2O →
Fe2O3 +
H2
Step 2. Count the number of atoms of each element on both the sides of chemical equation.
| Element | No. of atoms at reactant side | No. of atoms at product side |
|---|---|---|
| 1. Fe | 1 | 3 |
| 2. H | 2 | 2 |
| 3. O | 1 | 4 |
Step 3. Equalise the number of atoms of element which has maximum number by putting in front of them.
Step 4. Try to equalize all the atoms of elements on reactant and product side by adding coefficient in front of them.
* Now all the atoms of elements are equal on both sides.
Step 5. Write the physical states of reactants and products.
Solid state = (s)
Liquid state = (l)
Gaseous state = (g)
Aqueous state = (aq)
TYPES OF CHEMICAL REACTIONS
I. COMBINATION REACTION:
The reaction in which two or more reactants combine to form a single product. A+B→C
e.g.
(i) Burning of coal
C (s) + O2 (g) → CO2 (g)
(ii) Formation of water
2H2 (g) + O2 (g) → 2H2O (l)
(iii) CaO (s) + H2O (l) → Ca(OH)2 (aq)
Quick lime Slaked lime
Exothermic Reactions:
Reaction in which heat is released along with formation of products.
e.g.
(i) Burning of natural gas
CH4 (g) + O2 (g) → CO2 (g) + 2H2O (g) + Heat
(ii) Respiration is also an exothermic reaction.
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + energy
II. DECOMPOSITION REACTION:
The reaction in which a compound splits into two or more simple substances is called decomposition reaction.
A→B + C
• Thermal decomposition:
When decomposition is carried out by heating.
e.g.
(i) 2FeSO4 (s) Heat → Fe2O3 (s) + SO2 (g) + SO3 (g)
(Ferrous sulphate) (Ferric oxide)
Green colour Red-brown colour
(ii) CaCO3 (s) Heat → CaO (s) + CO2 (g)
(Lime stone) (Quick lime)
(iii) 2Pb (NO3)2 (s) Heat → 2PbO(s) + 4NO2(g)+O2
(lead nitrate) (lead oxide) (Nitrogen dioxide)
• Electrolytic Decomposition:
When decomposition is carried out by passing electricity.
e.g.
2H2O(l) Electric current → 2H2(g) + O2(g)
↓
Oxygen ← → Hydrogen
↓
Test tube
↓
Water
Water + dilute H2SO4
↓
Graphite rod ← → Rubber Stopper
↓
Anode ← → Cathode
↓
Switch → 6 V Battery
• Photolytic Decomposition:
When decomposition is carried out in presence of sunlight.
The decomposition reactions are mostly endothermic in nature. Energy in the form of heat, light or electricity is generally absorbed in these reactions.
e.g.
2AgCl (s) Sunlight → 2Ag (s) + Cl2 (g)
2AgBr (s) Sunlight → 2Ag (s) + Br2 (g)
Silver chloride turns grey on exposure to sunlight
* Above reaction is used in black and white photography.
Endothermic Reactions:
The reactions which require energy in the form of heat, light or electricity to break reactants are called endothermic reactions.
III. DISPLACEMENT REACTION:
The chemical reaction in which more reactive element displaces less reactive element from its salt solution.
(a) Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)
↓
Test tube
Thread
↓
Copper sulphate
solution
Iron nail
The iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO4 changes to dirty green colour due to formation of FeSO4.
(b) Zn + CuSO4 → ZnSO4 + Cu
Zn is more reactive than copper.
IV. DOUBLE DISPLACEMENT REACTION:
A reaction in which new compounds are formed by mutual exchange of ions between two compounds.
(i) Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
(Sodium sulphate) (Barium chloride) (Barium sulphate) (Sodium chloride)
white precipitate of BaSO4 is formed, so it is also called precipitation reaction.
Note: All double displacement reactions are not precipitation reactions.
(ii) 2KI + Pb(NO3)2 → +PbI2 + 2KNO3
Potassium iodide Lead nitrate Lead iodide Potassium nitrate
(Yellow ppt)
(iii) 2KBr + BaI2 → 2KI + BaBr2
Potassium bromide Barium iodide Potassium iodide Barium bromide
V. OXIDATION AND REDUCTION:
Oxidation:
(i) The addition of oxygen to reactant.
(ii) The removal of hydrogen from a reactant.
C + O2 → CO2
2Cu + O2 Heat → 2CuO
CuO + H2 Heat → Cu + H2O
Reduction:
(i) The addition of hydrogen to reactant.
(ii) The removal of oxygen from a reactant.
In this reaction CuO is reduced to Cu and H2 is oxidized to H2O. So, oxidation and reduction taking place together is redox reaction.
Effects of Oxidation in Daily Life
1) Corrosion
- When a metal is exposed to moisture, air, acid etc. for some time, a layer of hydrated oxide is formed which weakens the metal and hence metal is said to be corroded.
- Rusting of iron, black coating on silver and green coating on copper are examples of corrosion.
- Corrosion can be prevented by galvanization, electroplating or by applying paint.
2) Rancidity:
The oxidation of fats and oils when exposed to air is known as rancidity. It leads to bad smell and bad taste of food.
Methods to Prevent Rancidity
- (i) By adding antioxidants
- (ii) Keeping food in air tight containers
📝 IMPORTANT QUESTIONS WITH ANSWERS
🔘 MULTIPLE CHOICE QUESTIONS (1 Mark)
Q.1 The shiny finish of wall after white wash is because of.
a) Calcium oxide b) Calcium hydroxide
c) Calcium Carbonate d) Calcium phosphate
Q.2 Electrolysis of water is decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is
a) 1:1 b) 2:1 c) 4:1 d) 1:2
Q.3 Which the following statements about the given reaction are correct:
3Fe(s) + 4H2O(g) → Fe2O3(s) + 4H2(g)
i) Iron metal is getting oxidized
ii) Water is getting reduced
iii) Water is acting as reducing agent
iv) Water is acting as oxidizing agent
a) (i), (ii) and (iii) b) (iii) and (iv)
c) (i), (ii) and (iv) d) (ii) and (iv)
Q.4 In order to prevent the spoilage of potato chips, they are packed in plastic bags containing the gas
a) Cl2 b) O2 c) N2 d) H2
Q.5 The process of respiration is –
a) an oxidation reaction which is endothermic
b) a reduction reaction which is exothermic
c) a combination reaction which is endothermic
d) an oxidation reaction which is exothermic
Q.6 Burning of Methane is an example of
I. Exothermic reaction
II. Combustion reaction
III. Decomposition reaction
a. I only b. II only c. I and II d. I and III
📋 SHORT ANSWER QUESTIONS (2-3 Marks)
1. Define combination reaction. Give two examples of combination reaction, which are exothermic in nature.
2. What is decomposition reaction? Explain with the help of an example.
3. Name and state the law which is kept in mind when we balance a chemical equation.
4. Give one example of each:
(a) Chemical reaction showing evolution of gas.
(b) Change in colour of a substance during a chemical reaction.
(c) Chemical reaction showing change in temperature.
5. What is rancidity? Write two ways by which it can be prevented.
6. What are two conditions which promote corrosion?
7. A small amount of Ferrous sulphate is heated in hard glass tube.
(a) Write the chemical equation.
(b) What type of reaction is taking place or Name the type of reaction.
8. What happens when Zn strip is dipped in CuSO4 solution?
Give equation and identify the type of reaction.
9. What is redox reaction? Write down a chemical reaction representing it.
📝 LONG ANSWER QUESTIONS (5 Marks)
1. White wash was being done at Mukesh’s house. Mukesh saw that the painter added quick lime to drum having water. Mukesh touched outer surface of drum, it was unbelievably hot.
(a) Write the chemical equation for above reaction.
(b) What type of reaction is it?
(c) This reaction is exothermic or endothermic. Justify your answer.
2. Write down the balanced chemical equation for the following:
(a) Silver chloride is decomposed in presence of sunlight to give silver and chlorine gas.
(b) Calcium oxide reacts with water to give lime water.
(c) Sodium hydroxide reacts with hydrochloric acid to give sodium chloride and water.
(d) Dil. hydrochloric acid is added to copper oxide to give green coloured copper chloride and water.
(e) Solution of barium chloride and sodium sulphate in water reacts to give insoluble Barium sulphate and solution of Sodium chloride.
🔍 ASSERTION AND REASON QUESTIONS
Instructions:
In the following questions a statement of Assertion is followed by a statement of Reason. Mark the correct choice as two statements are given, one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
(a) Both A and R are true, and R is correct explanation of the assertion.
(b) Both A and R true, but R is not the correct explanation of the assertion.
(c) A is true, but R is false.
(d) A is false, but R is true.
1. Assertion (R):
Chemical reaction changes the physical and chemical state of substance.
Reason (R):
When electric current is passed through water (liquid), it decomposes to produce hydrogen and oxygen gases.
2. Assertion (R):
In a balanced chemical equation, total mass of the each element towards reactant side is equal to the total mass of the same element towards product side.
Reason (R):
Mass can neither be created nor destroyed during a chemical change.
3. Assertion (R):
When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.
Reason (R):
The decomposition reaction takes place on application of heat, therefore its an endothermic reaction.
💡 CASE STUDY QUESTIONS
Case Study: Lime Water and Quick Lime
In most pollution control application lime is used as calcium hydroxide. To manufacture calcium hydroxide the limestone must be converted to calcium oxide then calcium oxide then converted to calcium hydroxide. The following is a brief chemical reaction of this process:
Limestone + Heat → Calcium oxide + Carbon dioxide
However, calcium oxide is unstable in the presence of moisture and carbon dioxide. A more stable form of lime is calcium hydroxide.
Calcium oxide + Water → Calcium hydroxide + Heat
The process of adding water to calcium oxide to produce calcium hydroxide is referred to as hydration process or lime slaking. The hydration of Calcium oxide; commercially referred to as quick lime, is an exothermic process releasing a great quantity of heat. The hydration takes place quickly, releasing a lot of heat energy. This heat will boil the water and generate steam, which makes the particles burst, exposing the inner surfaces to water for further slaking. This process will continue until hydration is complete.
(i) Lime is used as calcium hydroxide. The formula of lime is calcium hydroxide that absorb CO2 from air and become white, the compound formed is
(a) CaO (b) Ca(HCO3)2
(c) CaCO3 (d) CaO.2H2O
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✅ Previous Year Questions
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