Chemical Reactions and Equations — NCERT Solutions, AI Tutor & Practice

Understanding: A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different chemical properties.

Observable indications of a chemical reaction:

1. Change in colour — e.g., iron filings turn brownish when they rust.
2. Evolution of a gas — e.g., bubbles form when zinc reacts with dilute sulphuric acid.

(Other valid signs: change in temperature, change in state, formation of a precipitate.)

Conclusion: Whenever a new substance with new properties is formed, a chemical reaction has occurred.

Step 1 — Write the skeletal equation: $Fe + H_2O \rightarrow Fe_3O_4 + H_2$

Step 2 — Balance Fe: There are 3 Fe on the right, so put 3 before Fe on the left. $3Fe + H_2O \rightarrow Fe_3O_4 + H_2$

Step 3 — Balance O: 4 O atoms on the right, so put 4 before $H_2O$. $3Fe + 4H_2O \rightarrow Fe_3O_4 + H_2$

Step 4 — Balance H: Left has $4 \times 2 = 8$ H atoms; put 4 before $H_2$ on the right. $3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$

Check: Fe = 3 = 3, O = 4 = 4, H = 8 = 8. ✓

Type of reaction: Displacement reaction (iron displaces hydrogen from steam).

Understanding: In a chemical reaction atoms are only rearranged; they are neither created nor destroyed.

Reason for balancing: A balanced equation has an equal number of atoms of each element on both sides. This ensures that the total mass of reactants equals the total mass of products.

Law involved: This follows the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.

Conclusion: An unbalanced equation would violate this law, so all chemical equations must be balanced.

(a) $CaO + H_2O \rightarrow Ca(OH)_2$ — two reactants combine to form a single product → Combination reaction (also exothermic).

(b) $2KClO_3 \xrightarrow{\Delta} 2KCl + 3O_2$ — a single compound breaks into simpler substances on heating → Decomposition reaction (thermal decomposition).

(c) $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ — a more reactive element (Zn) displaces a less reactive one (Cu) → Displacement reaction.

(d) $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$ — ions are exchanged and an insoluble precipitate ($BaSO_4$) forms → Double displacement (precipitation) reaction.

Oxidation: A reaction in which a substance gains oxygen or loses hydrogen.

Reduction: A reaction in which a substance loses oxygen or gains hydrogen.

Example: $CuO + H_2 \xrightarrow{\Delta} Cu + H_2O$

• $CuO$ loses oxygen → it is reduced to $Cu$.
• $H_2$ gains oxygen → it is oxidised to $H_2O$.

Redox reaction: A reaction in which oxidation and reduction occur simultaneously is called an oxidation–reduction (redox) reaction. In the above example, oxidation and reduction happen together, so it is a redox reaction.

Definition: A precipitation reaction is a reaction in which two soluble salt solutions react to form an insoluble solid called a precipitate.

Example: When sodium sulphate solution is mixed with barium chloride solution, a white precipitate of barium sulphate is formed: $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$

Here $BaSO_4$ is the white precipitate (insoluble in water).

Conclusion: Precipitation reactions are a type of double displacement reaction where the exchange of ions produces an insoluble product.

Understanding: Iron is more reactive than copper (higher in the reactivity series).

What happens: When an iron nail is dipped in blue copper sulphate solution, iron displaces copper from the solution: $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$

Observations:

• The blue colour fades because $CuSO_4$ (blue) is converted to $FeSO_4$ (pale green/light green).
• A brownish deposit of copper forms on the iron nail.

Type of reaction: Displacement reaction.

Conclusion: The colour changes because the more reactive iron replaces copper, forming a differently coloured iron sulphate solution.

Combination reaction: A reaction in which two or more substances combine to form a single new substance.

Burning of coal: $C + O_2 \rightarrow CO_2 + \text{heat}$ Carbon combines with oxygen to form carbon dioxide.

Quick lime with water: $CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$ Calcium oxide combines with water to form calcium hydroxide (slaked lime).

Energy comparison: Both are combination reactions and both are exothermic (release heat). The amount of heat released differs, but both involve joining of reactants into a single product accompanied by heat evolution.

Conclusion: Both are combination reactions; they are also exothermic reactions.

Cause of rancidity: When fats and oils in food are oxidised (react with oxygen in air), they become spoilt, producing an unpleasant smell and taste. This oxidation of food is called rancidity.

Ways to prevent rancidity:

1. Adding antioxidants to food, which slow down oxidation.
2. Packaging in nitrogen gas (flushing out oxygen), e.g., in chips packets.

(Other valid methods: keeping food in airtight containers, refrigeration, storing away from light.)

Conclusion: Since rancidity is caused by oxidation, preventing contact with oxygen or slowing oxidation prevents rancidity.

Understanding: Silver salts decompose when light falls on them.

Silver chloride: $2AgCl \xrightarrow{\text{sunlight}} 2Ag + Cl_2$ White silver chloride turns grey as silver metal is formed.

Silver bromide: $2AgBr \xrightarrow{\text{sunlight}} 2Ag + Br_2$

Type of reaction: These are decomposition reactions. Since light energy is used to break the compound, they are specifically called photolytic decomposition (photochemical) reactions.

Use: This light-sensitive decomposition is used in black-and-white photography and photographic films.

Conclusion: Light energy causes the breakdown of silver halides into silver and the halogen.

Identifying the gas: A gas that burns with a 'pop' sound is hydrogen ($H_2$).

Balanced equation: $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)\uparrow$

Check atoms: Zn = 1 = 1; H = 2 = 2; Cl = 2 = 2. ✓

Type of reaction: Zinc, being more reactive than hydrogen, displaces hydrogen from the acid → Displacement reaction.

Conclusion: Hydrogen gas is evolved through a displacement reaction between zinc and hydrochloric acid.

Exothermic reaction: A reaction in which heat energy is released (given out) along with the products.

Example: Respiration / combustion of natural gas: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$

Endothermic reaction: A reaction in which heat energy is absorbed from the surroundings.

Example: Decomposition of calcium carbonate on heating: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2 \;\; (\text{heat absorbed})$

| Feature | Exothermic | Endothermic | |---|---|---| | Heat | Released | Absorbed | | Surroundings | Get warmer | Get cooler |

Conclusion: The key difference is whether heat is given out (exothermic) or taken in (endothermic).