Ch-1: Chemical Reactions 🔥
1. Introduction (परिचय)
- Whenever a chemical change occurs, a chemical reaction has taken place.
- जब कभी रासायनिक परिवर्तन होता है, हम कह सकते हैं कि एक रासायनिक अभिक्रिया हुई है।
- Daily life examples: Rusting of iron, Milk turning into curd, Digestion of food.
2. Observations of Reaction
- How to identify a reaction?
(i) Change in State (e.g. Mg ribbon burning)
(ii) Change in Colour (e.g. Iron nail in $CuSO_4$)
(iii) Evolution of Gas (e.g. $Zn$ + Acid $\to$ $H_2 \uparrow$)
(iv) Change in Temperature
3. Chemical Equations & Balancing
- A shorthand representation of a reaction using symbols.
- Law of Conservation of Mass: Mass can neither be created nor destroyed.
अभिक्रिया के पहले और बाद में प्रत्येक तत्व के परमाणुओं की संख्या समान होनी चाहिए। - Hit-and-Trial Method: Stepwise balancing using whole numbers.
Balanced Equation Example
$3Fe(s) + 4H_2O(g) \to Fe_3O_4(s) + 4H_2(g)$
(s): solid | (l): liquid | (g): gas/steam | (aq): aqueous
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4. Combination Reaction (संयोजन अभिक्रिया)
- Two or more reactants combine to form a single product.
- Quick Lime + Water: $CaO + H_2O \to Ca(OH)_2 + Heat$.
- बिना बुझा हुआ चूना (CaO) जल के साथ अभिक्रिया करके बुझा हुआ चूना $Ca(OH)_2$ बनाता है।
5. Exothermic Reactions (ऊष्माक्षेपी अभिक्रियाएं)
- Reactions in which heat is released along with the product.
- Examples: Burning of Natural Gas, Respiration (digested glucose reacts with oxygen).
Fact: Whitewashing uses $Ca(OH)_2$. It reacts with $CO_2$ in air to form $CaCO_3$ (Marble) which gives a shiny finish!
6. Decomposition Reaction (वियोजन अभिक्रिया)
- A single reactant breaks down to give simpler products.
- Types of Decomposition:
1. Thermal: Heat is used.
Ex: $2Pb(NO_3)_2 \xrightarrow{Heat} 2PbO + 4NO_2 \uparrow$ (Brown Fumes!)
2. Electrolytic: Electricity used.
Ex: Electrolysis of Water. $H_2$ gas is double the volume of $O_2$.
3. Photolytic: Sunlight used.
Ex: $2AgCl \xrightarrow{Sunlight} 2Ag + Cl_2$. Used in Black & White Photography.
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7. Displacement Reaction (विस्थापन अभिक्रिया)
- A more reactive element displaces a less reactive one from its compound.
- अधिक क्रियाशील तत्व कम क्रियाशील तत्व को उसके यौगिक से विस्थापित कर देता है।
Activity 1.9: Iron Nail in $CuSO_4$
→
$Fe(s) + CuSO_4(aq) \to FeSO_4(aq) + Cu(s)$
8. Double Displacement Reaction
- Mutual exchange of ions between the reactants.
- अभिकारकों के बीच आयनों का आदान-प्रदान होता है।
- Precipitation Reaction: Any reaction that produces an insoluble solid (precipitate).
- Example: $Na_2SO_4 + BaCl_2 \to BaSO_4 \downarrow$ (White ppt) + $2NaCl$.
Reactivity Secret: Zinc and Lead are more reactive than Copper. That’s why they easily displace it from its chloride or sulphate!
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9. Oxidation and Reduction (उपचयन एवं अपचयन)
- Oxidation: Gain of Oxygen OR Loss of Hydrogen.
- Reduction: Loss of Oxygen OR Gain of Hydrogen.
- किसी पदार्थ में ऑक्सीजन की वृद्धि या हाइड्रोजन का ह्रास ‘उपचयन’ कहलाता है।
10. Redox Reactions
- Reactions where both Oxidation and Reduction occur simultaneously.
Example: CuO + $H_2$
$CuO + H_2 \xrightarrow{Heat} Cu + H_2O$
Reduction (loss of O)
Oxidation (gain of O)
- Reducing Agent: The substance that gets oxidized (it gives H or takes O).
- Oxidising Agent: The substance that gets reduced (it gives O or takes H).
11. Other Redox Examples
- $ZnO + C \to Zn + CO$ (Carbon is oxidized, Zinc oxide is reduced).
- $MnO_2 + 4HCl \to MnCl_2 + 2H_2O + Cl_2$.
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12. Effects of Oxidation in Daily Life
- A. Corrosion (संक्षारण): When a metal is attacked by moisture, acids, etc.
- Rusting of Iron: Reddish-brown powder.
- Green coating on Copper.
- Black coating on Silver.
- B. Rancidity (विकृतगंधिता): Oxidation of fats and oils in food.
- Results in bad smell and change in taste.
- वसा एवं तेलों का उपचयन होने से उनका स्वाद और गंध बदल जाते हैं।
13. Prevention Methods
- Anti-oxidants: Added to food items to prevent oxidation.
- Inert Gas: Flushing chip packets with Nitrogen to prevent rancidity.
- Coating: Painting or galvanizing iron articles to stop corrosion.
14. Chapter Summary (सारांश)
1. Balancing is key (Conservation of Mass).
2. Combination (+) vs Decomposition (-).
3. Reactivity determines Displacement.
4. Exothermic = Heat OUT | Endothermic = Heat IN.
Practice Tip! ✍️
Always check the skeletal equation before solving. If it’s already balanced, don’t waste time on Hit-and-Trial!
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