System vs surroundings; open, closed, isolated

2.5.1 · Chemistry › Thermodynamics (Chemical)

Core Intuition

Poori universe = System + Surroundings. Hum system ko is baat ke basis par choose karte hain ki hum kya study karna chahte hain. Reacting gases ke saath ek steel container? Woh hamara system hai. Uske aaspaas ki lab? Surroundings.

Definitions

The Three System Types

1. Open System

YEH KYUN MATTER KARTA HAI: Real-world chemistry zyaadatar open systems mein hoti hai—air ke liye khuli test tubes, lakes, living cells (nutrients aur waste exchange karti hain).

Examples:

Khule pot mein ubalta paani (steam escape hoti hai = matter loss, stove se heat aati hai = energy input)
Jalti hui mombatti (oxygen andar, CO₂ aur H₂O bahar; heat air mein release hoti hai)
Tumhara body (food andar, waste bahar; environment ke saath heat exchange)

Kya cross karta hai:

Mass ✅ (diffusion, evaporation, flow ke zariye)
Energy: ✅ (heat $q$ ya work $w$ ke roop mein)

2. Closed System

YEH KYUN USEFUL HAI: Hum energy changes (heat, work) study kar sakte hain bina mass changes ki chinta kiye. Zyaadatar lab experiments closed systems use karti hain—sealed flasks, bomb calorimeters.

Examples:

Gas ke saath sealed syringe (piston work karta hai $w = - P_{ext} Δ V$ , heat flows, lekin koi gas escape nahi hoti)
Pressure cooker (mass andar rehta hai, flame se heat aati hai)
Earth ka atmosphere (approximation: space ke saath negligible mass exchange, lekin solar energy andar, IR energy bahar)

Kya cross karta hai:

Mass: ❌ ( $Δ m = 0$ )
Energy: ✅ (heat $q$ conduction/radiation ke zariye, work $w$ expansion/compression ke zariye)

3. Isolated System

YEH KYUN FUNDAMENTAL HAI: Ek isolated system ki energy aur mass constant rehti hai. Yeh woh condition hai jiske liye Second Law hai: isolated system ki entropy kabhi nahi ghatti.

$Δ U_{isolated} = 0, Δ m = 0, Δ S_{isolated} \geq 0$

Examples:

Poori universe (by definition, iske bahar kuch nahi)
Ek perfect thermos flask (idealization: koi heat leak nahi, sealed lid)
Ek insulated bomb calorimeter (thode time ke liye approximation)

Kya cross karta hai:

Mass: ❌
Energy: ❌ (na heat, na work)

Derivation: First Law for Different Systems

Thermodynamics ka First Law: $Δ U = q + w$ jahan $q$ = absorbed heat, $w$ = system par kiya gaya work.

Scratch se derivation:

Energy conservation: $Δ U_{system} + Δ U_{surroundings} = 0$ (universe isolated hai)
Energy heat ke roop mein cross karti hai (boundary par random molecular motion) ya work ke roop mein (organized force×displacement)
Sign convention: $q > 0$ jab heat system mein enter kare, $w > 0$ jab system par work kiya jaaye

System types par apply karo:

System Type	Mass Change	Energy Change	First Law Simplification
Open	$Δ m \neq = 0$	$Δ U = q + w$	Koi simplification nahi (mass flow, flow processes mein enthalpy terms add karta hai)
Closed	$Δ m = 0$	$Δ U = q + w$	Standard form; chemistry mein sabse common
Isolated	$Δ m = 0$	$q = 0, w = 0$	$Δ U = 0$ (energy constant)

Closed systems chemistry mein kyun dominate karte hain:

Fixed moles → $Δ U = n C_{V} Δ T$ ya $Δ H = n C_{P} Δ T$ use kar sakte hain
Calorimetry se heat measure kar sakte hain (koi mass corrections nahi)
Gas expansion ke liye $w = - P_{ext} Δ V$ apply kar sakte hain

Common Mistakes

Summary Table

Formula System Classification

Property	Open	Closed	Isolated
Matter transfer	✅ Haan	❌ Nahi	❌ Nahi
Energy transfer	✅ Haan	✅ Haan	❌ Nahi
$Δ m$	$\neq = 0$	$= 0$	$= 0$
$Δ U$	$q + w$	$q + w$	$= 0$
Example	Boiling pot	Sealed flask	Thermos (ideal)

Connections

First Law of Thermodynamics — $Δ U = q + w$ har system type par alag tarike se apply hota hai
Enthalpy and Constant Pressure — Open systems aksar $Δ H$ use karte hain kyunki mass flow hoti hai
Entropy and Second Law — Isolated systems mein $Δ S \geq 0$ hota hai (entropy kabhi nahi ghatti)
Calorimetry — Bomb calorimeter closed & rigid hai (constant volume, $w = 0$ ); coffee-cup calorimeter bhi mass ke respect mein closed hai ( $Δ m \approx 0$ ) lekin constant pressure par operate karta hai (PV work allow karta hai)
State Functions vs Path Functions — $Δ U$ ek state function hai; calculation ke liye system type par depend karta hai
Heat Capacity at Constant Volume — Closed rigid systems ke liye use hota hai ( $w = 0$ )
Adiabatic Processes — Jab boundary adiabatic ho ( $q = 0$ ), lekin phir bhi work allow ho sakta hai

Mnemonic

Feynman Explanation

Recall Ek 12-saal ke bachche ko explain karo

Socho tum apne room mein khel rahe ho. Thermodynamics poochti hai: kya andar ya bahar aa-ja sakta hai?

Open door (open system): Tum andar-bahar ja sakte ho (woh matter hai), aur hallway ki garmi andar aati hai (woh energy hai). Dono cheezein cross karti hain.

Closed door (closed system): Tum room mein rehte ho (matter blocked), lekin tum phir bhi khidki ki thandi ya radiator ki garmi mehsoos karte ho (energy cross karti hai). Tum andar fanse ho, lekin heat nahi.

Sealed spaceship (isolated system): Tum perfect walls wali spaceship mein ho. Na koi hawa andar-bahar jaati hai (matter blocked), aur na taaron ki koi heat andar aati hai (energy blocked). Jo bhi energy lekar shuru kiya, wahi hamesha ke liye paas hai.

Scientists decide karte hain ki kaunsa "room" study karna hai is baat ke hisaab se ki woh kya measure karna chahte hain. Zyaadatar chemistry experiments "closed door" rooms hain—sealed flasks jahan chemicals andar rehte hain, lekin hum unhe heat ya cool kar sakte hain.

#flashcards/chemistry

System aur uske surroundings mein kya difference hai? :: System universe ka woh specific hissa hai jo hum study karte hain; surroundings system boundary ke bahar ki baaki sab cheez hai.

Open system define karo.

Ek aisa system jahan matter aur energy dono boundary cross kar sakte hain (e.g., khule pot mein ubalta paani).

Closed system define karo.

Ek aisa system jahan energy boundary cross kar sakti hai lekin matter nahi kar sakta (e.g., sealed flask, pressure cooker).

Δ m = 0

lekin

q

aur

w

possible hain.

Isolated system define karo. :: Ek aisa system jahan na matter aur na energy boundary cross kar sakti hai (e.g., ideal thermos, universe). $Δ U = 0$ , $Δ m = 0$ .

Closed system mein boundary kya cross karta hai?

Energy (heat

q

ya work

w

ke roop mein) cross kar sakti hai, lekin matter nahi kar sakta. Mass constant rehta hai.

"Rigid" aur "impermeable" boundary mein kya difference hai?

Rigid = fixed volume → koi PV work nahi (

w = 0

); ek mechanical property. Impermeable = matter pass nahi kar sakta; ek matter-transport property. Ye independent hain.

Earth ka atmosphere approximately closed system kyun hai?

Space ke saath mass exchange negligible hai, lekin solar energy andar aati hai aur IR energy bahar jaati hai. Approximation:

Δ m \approx 0

, lekin

q \neq = 0

Isolated system ke liye First Law state karo.

Δ U = 0

kyunki

q = 0

(koi heat transfer nahi) aur

w = 0

(koi work transfer nahi). Energy constant hai.

Bomb calorimeter approximately closed system kyun hai?

Rigid steel container matter ko escape hone se rokta hai (

Δ m = 0

), aur short experiment time surroundings mein heat leakage minimize karta hai. Volume constant hai toh

w = 0

, aur

Δ U = q_{V}

Coffee-cup calorimeter open hai ya closed?

Mass ke respect mein closed (

Δ m \approx 0

)—contents bahar nahi jaate. Yeh constant pressure par operate karta hai (pressure equalization ke liye atmosphere ke liye open), toh hum

Δ H = q_{P}

measure karte hain, lekin yeh mass-exchange sense mein open nahi hai.

Sach ya Jhooth: Ek closed system work nahi kar sakta.

Jhooth. Ek closed system work kar sakta hai (e.g., sealed syringe mein gas expansion). "Closed" ka matlab matter ke liye impermeable hai, energy ke liye nahi.

Isolated system ke liye entropy condition kya hai?

Δ S_{isolated} \geq 0

(Second Law). Isolated system mein entropy kabhi nahi ghatti; irreversible processes ke liye badhti hai aur reversible ones ke liye constant rehti hai.

Daily life mein open system ka ek example do.

Ek jalti hui mombatti (oxygen andar aati hai, CO₂ aur H₂O vapor bahar jaati hai; heat release hoti hai), ya tumhara body (food aur oxygen andar, waste aur CO₂ bahar).

Universe ko isolated system kyun maana jaata hai?

By definition, universe ke bahar kuch nahi hai (koi surroundings nahi), toh na matter aur na energy uski boundary cross kar sakti hai.

Δ U_{universe} = 0

Δ S_{universe} \geq 0