Chapter 1: Chemical Reactions and Equations
Class X Science
Characteristics of Chemical Reactions
A chemical reaction is characterized by certain observable changes:
- Formation of a precipitate
- Evolution of gas
- Change in colour
- Change in state
- Change in temperature
Balancing of a Chemical Equation
Balancing of a chemical equation means making the number of atoms of each element equal on both sides of the equation. This follows the Law of Conservation of Mass.
Steps involved in balancing:
- Write the word equation.
- Write the skeletal chemical equation.
- Enclose the formulae in boxes.
- List the number of atoms of different elements on LHS (Reactants) and RHS (Products).
- Select the biggest formula to start balancing.
- Start balancing different elements which have the highest number of atoms.
- Check the correctness of the balanced equation.
Types of Chemical Reactions
1. Combination Reaction
Reactions in which two or more reactants combine to form one product.
General Formula: \( A + B \rightarrow AB \)
Examples:
- \( 2Mg(s) + O_2(g) \rightarrow 2MgO(s) \)
- \( C(s) + O_2(g) \rightarrow CO_2(g) \)
2. Decomposition Reaction
Reactions in which one compound decomposes into two or more compounds or elements. It is the opposite of a combination reaction.
General Formula: \( AB \rightarrow A + B \)
Types of Decomposition:
- Thermal Decomposition: Decomposition on heating.
Example: \( CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g) \) - Electrolytic Decomposition (Electrolysis): Decomposition by passing electricity.
Example: \( 2H_2O(l) \xrightarrow{\text{Electricity}} 2H_2(g) + O_2(g) \) - Photochemical Decomposition: Decomposition by sunlight.
Example: \( 2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g) \)
Note: Silver chloride turns grey in sunlight. This reaction is used in black and white photography.
3. Displacement Reaction
A chemical reaction in which a more reactive element displaces a less reactive element from a compound.
General Formula: \( A + BC \rightarrow AC + B \) (where A is more reactive than B)
Examples:
- \( Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g) \)
- \( Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s) \)
4. Double Displacement Reaction
Reactions in which ions are exchanged between two reactants forming new compounds.
General Formula: \( AB + CD \rightarrow AC + BD \)
Example: \( BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq) \)
Precipitation Reaction: Any reaction that produces a precipitate (an insoluble solid) is called a precipitation reaction. In the above example, \( BaSO_4 \) is a white precipitate.
5. Neutralization Reaction
The reaction in which an acid reacts with a base to form salt and water by an exchange of ions.
Example: \( NaOH + HCl \rightarrow NaCl + H_2O \)
6. Redox Reactions
Reactions where oxidation and reduction take place simultaneously.
- Oxidation: Addition of oxygen or removal of hydrogen.
- Reduction: Addition of hydrogen or removal of oxygen.
- Oxidizing Agent: Substance that gives oxygen or removes hydrogen (gets reduced).,
- Reducing Agent: Substance that gives hydrogen or removes oxygen (gets oxidized).,
Example: \( CuO + H_2 \rightarrow Cu + H_2O \)
- \( CuO \) is reduced to \( Cu \) (Removal of Oxygen).
- \( H_2 \) is oxidized to \( H_2O \) (Addition of Oxygen).
7. Exothermic and Endothermic Reactions
- Exothermic Reaction: Reaction in which energy (heat) is released. Example: Respiration, Burning of natural gas, Reaction of quick lime with water.,
\( CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat} \) - Endothermic Reaction: Reaction in which energy is absorbed. Example: Decomposition of calcium carbonate.
Question Bank
Section A: Multiple Choice Questions
- Identify the substance oxidized in the equation: \( MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2 \)
Options: (a) \( MnCl_2 \) (b) \( HCl \) (c) \( H_2O \) (d) \( MnO_2 \)
Answer: (b) \( HCl \) - Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated is:
Options: (a) 1:1 (b) 2:1 (c) 4:1 (d) 1:2
Answer: (b) 2:1 - Reaction \( Pb + CuCl_2 \rightarrow PbCl_2 + Cu \) is an example of:
Options: (a) combination (b) double displacement (c) decomposition (d) displacement
Answer: (d) displacement - Green coloured ferrous sulphate crystals change colour on heating because:
Answer: (b) it loses water of crystallisation - Reaction \( 2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O \) is an example of:
Answer: (b) (ii) double displacement and (iii) neutralisation - Reaction between potassium iodide and silver nitrate produces:
Answer: (c) yellow precipitate of AgI
Section B: Assertion-Reason Type Questions
Options: (a) Both A & R true, R is correct explanation of A. (b) Both A & R true, R is not correct explanation. (c) A is true, R is false. (d) A is false, R is true.
- Assertion: Photosynthesis is considered an endothermic reaction.
Reason: Energy gets released in the process of photosynthesis.
Answer: (c) Assertion is correct but Reason is incorrect (Energy is absorbed, not released)., - Assertion: Decomposition of vegetable matter into compost is an exothermic reaction.
Reason: Exothermic reactions are those in which heat is evolved.
Answer: (a) Both correct and R explains A., - Assertion: White silver chloride turns grey in sunlight.
Reason: Decomposition of silver chloride in presence of sunlight forms silver metal and chlorine gas.
Answer: (a) Both correct and R explains A.
Section C: Case Study Questions
Context: Displacement reactions involve a more reactive element displacing a less reactive element. Iron (III) oxide and aluminium powder react to produce molten iron (Thermite reaction).
- Copper displaces which metal from its salt solution?
Answer: (c) \( AgNO_3 \) (Silver is less reactive than Copper). - When zinc reacts with dilute sulphuric acid, the gas evolved is:
Answer: (c) colourless, odourless and burns with a pop sound (Hydrogen). - Which reaction is a displacement reaction?
(a) \( CaO + H_2O \rightarrow Ca(OH)_2 \)
(b) \( MgCO_3 \rightarrow Mg + CO_2 \)
(c) \( Mg + CuSO_4 \rightarrow MgSO_4 + Cu \)
Answer: (c)
Section D: Short Answer Type Questions
Q1. List four observations that determine if a chemical reaction has taken place.
Ans: (i) Evolution of gas (ii) Change in temperature (iii) Change in state (iv) Change in colour.
Q2. What happens when Carbon dioxide is passed through lime water?,
Ans:
(i) Short duration: Lime water turns milky due to insoluble \( CaCO_3 \).
(ii) Long duration: Solution clears due to formation of soluble \( Ca(HCO_3)_2 \).
Q4. Why do we store silver chloride in dark coloured bottles?
Ans: To prevent photolytic decomposition caused by exposure to sunlight.
Q6. What happens when an iron nail is put inside copper sulphate solution?
Ans: The iron nail turns brown and the blue colour of the solution fades. Reaction: \( Fe + CuSO_4 \rightarrow FeSO_4 + Cu \).
Section E: Long Answer Questions
Q1. (a) 2g of ferrous sulphate crystals are heated in a dry boiling tube. List two observations. (b) Write chemical equations for: (i) Change in colour (ii) Change in temperature (iii) Formation of precipitate.,,
Ans:
(a) (i) Green colour of \( FeSO_4 \) disappears and reddish-brown solid is formed. (ii) Smell of burning sulphur.
(b)
(i) Displacement: \( Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s) \) (Solution turns blue).
(ii) Exothermic: \( NaOH + HCl \rightarrow NaCl + H_2O + \text{Heat} \).
(iii) Precipitation: \( Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) \text{ (Yellow ppt)} + 2KNO_3(aq) \).