Galvanic (voltaic) cells — anode (oxidation), cathode (reduction)

2.7.1 · Chemistry › Redox & Electrochemistry (Intro)

Core Concept

Yeh kyun hota hai? Alag-alag metals mein oxidize hone ki tendency alag hoti hai. Zinc prefer karta hai ki woh Zn²⁺ ban jaye aur electrons khoe; copper prefer karta hai ki woh Cu metal hi rahe ya Cu²⁺ se electrons gain karke Cu ban jaye. Jab aap unhe connect karte ho, electrons wire ke through us metal se flow karte hain jo zyada easily oxidize hota hai (Zn) us ki taraf jo zyada easily reduce hota hai (Cu²⁺ → Cu).

The Two Electrodes

Memory aid: "An Ox" — Anode par Oxidation hoti hai.

Memory aid: "Red Cat" — Reduction Cathode par hoti hai.

Visual Structure

Key components:

1. Do half-cells: Har ek mein ek metal electrode hota hai jo apne ions ki solution mein daala hota hai
2. Salt bridge: Electrical neutrality maintain karne ke liye ion flow allow karta hai (charge buildup rokta hai)
3. External wire: Anode se cathode tak electron flow ka path
4. Voltmeter (optional): Cell potential (voltage) measure karta hai

First-Principles Derivation: Electrons Flow Kyun Karte Hain

Step 1: Oxidation-Reduction Driving Force

Har metal ka ek standard reduction potential ($E°$) hota hai, jo measure karta hai ki uski electrons gain karne ki (reduce hone ki) tendency kitni hai. Yeh standard hydrogen electrode (SHE) ke against measure kiya jaata hai.

Classic Daniel cell ke liye:

• Zn²⁺ + 2e⁻ → Zn ka $E{°}_{{\text{Zn}}^{2+}/\text{Zn}}=-0.76\text{ V}$ hai
• Cu²⁺ + 2e⁻ → Cu ka $E{°}_{{\text{Cu}}^{2+}/\text{Cu}}=+0.34\text{ V}$ hai

Iska kya matlab hai? Copper ka POSITIVE reduction potential hai — yeh electrons "chahta" hai. Zinc ka NEGATIVE reduction potential hai — yeh electrons KHONA "chahta" hai (oxidize hona chahta hai).

Step 2: Half-Reactions Likhna

Anode par (Zn): $\text{Zn(s)}\to {\text{Zn}}^{2+}(\text{aq})+2e^{-}$

Yeh step kyun? Zinc metal oxidize hota hai kyunki iska reduction potential zyada negative hai. Reverse reaction (oxidation) spontaneous hai. Electrons produce hokar wire ke through flow karte hain.

Cathode par (Cu): ${\text{Cu}}^{2+}(\text{aq})+2e^{-}\to \text{Cu(s)}$

Yeh step kyun? Solution mein copper ions woh electrons gain karte hain jo zinc anode se wire ke zariye aate hain. Copper cathode par plate out ho jaata hai.

Step 3: Net Cell Reaction

Half-reactions add karo (electrons cancel ho jaate hain): $\text{Zn(s)}+{\text{Cu}}^{2+}(\text{aq})\to {\text{Zn}}^{2+}(\text{aq})+\text{Cu(s)}$

Yeh spontaneous kyun hai? Total free energy change negative hoti hai jab zyada easily oxidize hone wala metal (Zn) electrons deta hai zyada easily reduce hone wale ion (Cu²⁺) ko.

Step 4: Cell Potential Calculate Karna

First principles se derivation:

Cell potential woh maximum electrical work represent karta hai jo cell transfer hone wale electrons ke har mole ke liye kar sakta hai. Yeh do electrodes ke beech "electron pressure" ka difference hai.

• Cathode par, reduction hoti hai: $E{°}_{\text{cathode}}$ Cu²⁺ + 2e⁻ → Cu ke liye potential hai = +0.34 V
• Anode par, oxidation hoti hai (reduction ka reverse): $E{°}_{\text{anode}}$ Zn²⁺ + 2e⁻ → Zn ke liye potential hai = -0.76 V

Cell electrons ko anode se khichta hai aur cathode ki taraf push karta hai. Yeh "push" hai:

$E{°}_{\text{cell}}=0.34-(-0.76)=1.10\text{ V}$

Subtract kyun karte hain? Tum potential difference dhundh rahe ho. Anode lower potential par hai (zyada negative), cathode higher potential par. Current external circuit mein high se low potential ki taraf flow karta hai.

Important: Positive $E{°}_{\text{cell}}$ matlab reaction spontaneous hai. Agar $E{°}_{\text{cell}}<0$, tab energy supply karni padegi (electrolytic cell, galvanic nahi).

Salt Bridge ka Role

Jaise Zn oxidize hota hai, anode solution mein Zn²⁺ ions aaate hain (positively charged ho jaata hai). Jaise Cu²⁺ reduce hota hai, cathode solution mein Cu²⁺ ions kam ho jaate hain (negatively charged ho jaata hai). Charge balance karne ka koi zariya na ho toh reaction turant ruk jaayegi.

Salt bridge mein ek inert electrolyte hota hai (jaise KNO₃ ya Na₂SO₄) jo ions ko migrate karne deta hai:

• Anions (NO₃⁻, SO₄²⁻) anode ki taraf move karte hain taaki Zn²⁺ ke buildup ko neutralize kar sakein
• Cations (K⁺, Na⁺) cathode ki taraf move karte hain taaki remove ho rahe Cu²⁺ ki jagah le sakein

Seedha solutions kyun connect nahi karte? Isse Zn aur Cu²⁺ ke beech direct mixing aur reaction ho jaati external circuit mein electron flow ke bina — koi electricity generate nahi hoti.

Worked Examples

Setup:

• Anode: 1 M ZnSO₄ mein Zn electrode
• Cathode: 1 M CuSO₄ mein Cu electrode
• Salt bridge: KNO₃

Dhundho: Cell potential, electron flow ki direction, 2 moles electrons flow hone par deposit hone wale Cu ki mass.

Solution:

Step 1: Half-reactions likho

• Anode (oxidation): Zn(s) → Zn²⁺(aq) + 2e⁻
• Cathode (reduction): Cu²⁺(aq) + 2e⁻ → Cu(s)

Kyun yeh? Zn ka $E°$ zyada negative hai, isliye woh oxidize hota hai.

Step 2: $E{°}_{\text{cell}}$ calculate karo $E{°}_{\text{cell}}=E{°}_{{\text{Cu}}^{2+}/\text{Cu}}-E{°}_{{\text{Zn}}^{2+}/\text{Zn}}=0.34-(-0.76)=1.10\text{ V}$

Positive kyun? Reaction is direction mein spontaneous hai.

Step 3: Electron flow ki direction Electrons Zn (anode) → Cu (cathode) external wire ke through flow karte hain.

Yeh direction kyun? Electrons anode par produce hote hain aur cathode par consume hote hain.

Step 4: Deposit hone wale Cu ki mass Cathode half-reaction se: 2 moles e⁻ → 1 mole Cu

Agar 2 moles e⁻ flow hon: $n_{\text{Cu}}=1$ mole

$m_{\text{Cu}}=1\text{ mol}\times 63.5\text{ g/mol}=63.5\text{ g}$

Yeh calculation kyun? Half-reaction ki stoichiometry directly electrons ko copper atoms se relate karti hai.

Given: Ag|Ag⁺ (E° = +0.80 V) aur Ni|Ni²⁺ (E° = -0.25 V) se bana cell

Dhundho: Kaun anode hai? Kaun cathode hai? Cell voltage?

Solution:

Step 1: Reduction potentials compare karo

• Ag⁺ + e⁻ → Ag: E° = +0.80 V (zyada positive — reduce hona chahta hai)
• Ni²⁺ + 2e⁻ → Ni: E° = -0.25 V (zyada negative — oxidize hona chahta hai)

Compare kyun? Zyada positive reduction potential zyada strong oxidizing agent indicate karta hai (reduce hota hai). Zyada negative wala oxidize hota hai.

Step 2: Electrodes assign karo

• Cathode (reduction): Ag⁺ + e⁻ → Ag (E° = +0.80 V)
• Anode (oxidation): Ni → Ni²⁺ + 2e⁻ (E° = -0.25 V)

Kyun? Silver nickel se zyada electrons gain karna chahta hai, isliye silver reduce hota hai (cathode).

Step 3: Cell potential calculate karo $E{°}_{\text{cell}}=0.80-(-0.25)=1.05\text{ V}$

Positive kyun? Galvanic cell mein spontaneous reaction confirm hoti hai.

Question: Kya Pb|Pb²⁺ (E° = -0.13 V) aur Sn|Sn²⁺ (E° = -0.14 V) se bana cell electricity generate karega? Anode kaun hai?

Solution:

Step 1: Identify karo kaun zyada easily oxidize hota hai Sn ka E° = -0.14 V hai (zyada negative) → zyada easily oxidize hota hai Pb ka E° = -0.13 V hai → (relatively) zyada easily reduce hota hai

Step 2: Electrodes assign karo

• Anode: Sn → Sn²⁺ + 2e⁻
• Cathode: Pb²⁺ + 2e⁻ → Pb

Step 3: Calculate karo $E{°}_{\text{cell}}=(-0.13)-(-0.14)=+0.01\text{ V}$

Itna chhota kyun? Dono metals ke reduction potentials BAHUT similar hain. Cell kaam karega lekin sirf bahut thodi voltage produce karega.

Answer: Haan, electricity generate karega (positive $E°$), lekin bahut weak. Sn anode hai.

Common Mistakes & Misconceptions

Yeh sahi kyun lagta hai: Roz ke battery labels mein hum sochte hain ki jis terminal se current "aata" hai woh positive hai.

Sach: Galvanic cell mein, anode negative aur cathode positive hota hai. Electrons anode (negative) se OUT flow karte hain aur cathode (positive) mein INTO external circuit ke through. Conventional current (electron flow ke opposite) + se − ki taraf jaata hai.

Fix: Yaad rakho: electrons negative hote hain. Woh negative terminal (anode) se repel hote hain aur positive terminal (cathode) ki taraf attract hote hain.

Note: Electrolytic cells mein yeh ULTA hota hai (jahan aap energy supply karte ho). Dono ko confuse mat karo.

Yeh sahi kyun lagta hai: Hamesha |bada − chhota| karne ki temptation hoti hai taaki positive number aaye.

Sach: Formula HAMESHA $E{°}_{\text{cell}}=E{°}_{\text{cathode}}-E{°}_{\text{anode}}$ hai, jahan dono reduction potentials as written hain. Pehle identify karo kaun electrode cathode hai (higher/more positive reduction potential).

Fix:

1. Dhundho kaun sa metal/ion pair zyada positive E° rakhta hai → woh cathode hai (reduction)
2. Doosra anode hai (oxidation)
3. Subtract karo: cathode E° minus anode E°

Example: Zn|Cu cell ke liye: $E{°}_{\text{cell}}=0.34-(-0.76)$, NOT $0.34-0.76$.

Yeh sahi kyun lagta hai: Yeh do solutions ko connect kar raha hai, aur hume circuit complete karna hai.

Sach: Salt bridge ions (charged atoms/molecules) conduct karta hai, electrons NAHI. Electrons metal wire ke through flow karte hain. Salt bridge anions aur cations ko half-cells ke beech migrate karke electrical neutrality maintain karta hai.

Fix: Electrons metals (electrodes connect karne wali wire) se conduct hote hain. Ions electrolytes (salt bridge, solutions) se conduct hote hain.

Yeh sahi kyun lagta hai: Yeh mix up karna aasaan hai ki kaun se ions kahan jaate hain.

Sach:

• Salt bridge se anions (negative ions) anode ki taraf move karte hain (jahan Zn²⁺ build up ho raha hai, positive charge create kar raha hai)
• Salt bridge se cations (positive ions) cathode ki taraf move karte hain (jahan Cu²⁺ deplete ho raha hai, negative charge create kar raha hai)

Fix: Opposite charges attract karte hain. Anode par positive buildup salt bridge se negative ions attract karta hai. Cathode par negative buildup positive ions attract karta hai.

Active Recall Practice

Jab tum zinc ka ek tukda ek solution mein rakhte ho zinc ions ke saath, aur copper ka ek tukda ek solution mein copper ions ke saath, phir unhe ek wire se connect karte ho, toh kuch cool hota hai. Zinc electrons dena shuru kar deta hai (kyunki woh generous hai), aur woh electrons wire ke through copper side tak travel karte hain, jahan copper ions unhe pakad lete hain (kyunki copper greedy hai).

Wire ke through electrons ka yeh flow HI electricity hai! Aise hi battery kaam karti hai. Zinc ka tukda chhota hota jaata hai kyunki woh zinc ions mein convert ho raha hai, aur copper build up hota hai copper ke tukde par kyunki copper ions wapas solid copper metal mein convert ho rahe hain.

Hume ek salt bridge chahiye (ise charged particles ke liye ek hallway samjho) kyunki jaise zinc electrons khota hai, solution bahut positive ho jaata hai, aur jaise copper electrons gain karta hai, solution bahut negative ho jaata hai. Salt bridge kuch negative aur positive particles (electrons nahi, balki ions jo paani mein ghule hain) ko dono sides ke beech move karne deta hai taaki sab kuch balanced rahe aur reaction chalti rahe.

Jis side par zinc electrons khota hai (deta hai) use anode kehte hain, aur jis side par copper electrons gain karta hai (leta hai) use cathode kehte hain. Electrons anode se cathode ki taraf wire ke through flow karte hain, electric current create karte hain jo bulb ya phone power kar sakta hai!

Flashcards

#flashcards/chemistry

Galvanic (voltaic) cell kya hota hai? :: Ek aisa device jo chemical energy ko electrical energy mein convert karta hai spontaneous redox reactions ke zariye, jismein do half-cells ek wire aur salt bridge se connected hoti hain.

Negative E°_cell kya indicate karta hai? :: Reaction forward direction mein non-spontaneous hai; ise hone ke liye energy supply karni padegi (electrolytic cell).

Galvanic cell mein anode positive hota hai ya negative? :: Negative (yeh electrons ka source hai)

Connections

• Reduction Potentials — E° values jo determine karte hain kaun electrode anode/cathode hai
• Nernst Equation — Non-standard conditions mein cell potential calculate karna
• Electrolytic Cells — Ulta: non-spontaneous reactions drive karne ke liye electricity use karna
• Faraday's Laws of Electrolysis — Charge aur mass ke beech quantitative relationship
• Standard Hydrogen Electrode — Saare reduction potentials ka reference
• Oxidation Numbers — Redox reactions mein electron transfer track karna
• Gibs Free Energy and Cell Potential — ΔG° = -nFE°_cell relationship
• Battery Technologies — Galvanic cells ke practical applications

Last updated: 2026-06-30

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