2.5.11
Har chemical change ya toh heat release karta hai ya absorb karta hai. Hum enthalpy change (Δ H specific tarah ki reactions ke liye special names dete hain taaki unhe tabulate karke reuse kar sakein. Ye chaar bas Δ H specially defined conditions mein measure hote hain — reaction ka type hi definition hai. Har ek ke saath attached condition ko samjho aur tum unhe kabhi confuse nahi karoge.
KYUN: Enthalpy ek state function hai, isliye Δ H Δ H unhe combine karke (Hess's law) koi bhi reaction predict kar sakte hain. Naam dene se exact reactants, products aur conditions fix ho jaate hain.
KYA: Sab constant pressure par measure hote hain, isliye Δ H = q p P Δ H ∘
Definition Standard enthalpy of combustionEnthalpy change jab kisi substance ka 1 mole excess oxygen (O 2 completely burn ho jaaye, saare products apni standard states mein. Ye hamesha negative (exothermic) hoti hai.
KYUN negative? O 2 C O 2 H 2 O
HOW use karte hain — Hess's law for formation:
Δ f H ( compound ) = ∑ Δ c H ( reactants ) − ∑ Δ c H ( products )
Definition Enthalpy of neutralizationEnthalpy change jab ==1 mole H 2 O bane, ek acid aur base ki dilute aqueous solution== mein reaction se.
constant − 57.1 completely ions mein dissociate ho jaate hain. Sirf ye reaction actually hoti hai:
H + ( a q ) + O H − ( a q ) → H 2 O ( l ) , Δ H = − 57.1 kJ mol − 1 N a + C l − Δ H
Common mistake Weak acid meinkam heat release hoti hai — kyun?
Galat feeling: "Neutralization hamesha − 57.1 Fix: Ek weak acid (jaise C H 3 C O O H kharach hoti hai (+ Δ H i o ni z a t i o n ∣ Δ n e u t H ∣ < 57.1 Δ n e u t H w e ak = − 57.1 + Δ H i o ni z a t i o n
Definition Enthalpy of solutionEnthalpy change jab 1 mole solute dissolve ho ek specified amount of solvent mein (usually "infinite dilution"). Ye + ya − dono ho sakti hai.
Intuition Dissolving = lattice todna, phir ions ko paani mein wrap karnaDo steps socho:
Ionic solid ko gaseous ions mein alag karo → lattice enthalpy Δ l a tt i ce H
Paani ions ko gherta hai → hydration enthalpy Δ h y d H
Definition Enthalpy of hydrationEnthalpy change jab 1 mole gaseous ions paani mein dissolve hokar dilute solution banaaye:
M n + ( g ) + aq → M n + ( a q ) , Δ h y d H < 0 exothermic — paani ke dipoles ion ki taraf attract hote hain, ion–dipole bonds banate hain (energy release hoti hai).
Worked example Ex 1 — methane kaΔ f H Δ c H C H 4 = − 890 C = − 393.5 H 2 = − 285.8 Δ f H ( C H 4 )
Target: C ( s ) + 2 H 2 ( g ) → C H 4 ( g ) Δ f H = ∑ Δ c H ( reactants ) − ∑ Δ c H ( products ) Δ f H = [( − 393.5 ) + 2 ( − 285.8 )] − ( − 890 ) Ye step kyun? C H 4 C H 4 un-burn karna (iska combustion subtract karo).
= − 965.1 + 890 = − 75.1 kJ mol − 1
Worked example Ex 2 — NaCl ka solution enthalpyΔ l a tt i ce H ( N a C l ) = + 788 Δ h y d H ( N a + ) = − 406 Δ h y d H ( C l − ) = − 378 Δ so l H = 788 + ( − 406 − 378 ) = 788 − 784 = + 4 kJ mol − 1 Ye step kyun? Total hydration = saare ions ka sum. Almost zero, thoda sa endothermic → NaCl asaani se dissolve hota hai (entropy help karti hai).
Worked example Ex 3 — HCl ko NaOH se neutralize karne par released heat100 mL of 1 M HCl + 100 mL of 1 M NaOH. H 2 O = 0.1 q = 0.1 × ( − 57.1 ) = − 5.71 kJ (released) Ye step kyun? Limiting reagent se 0.1 mol paani banta hai; per-mole Δ n e u t H
Recall Feynman: 12-saal ke bachche ko samjhaoSocho inthein (fuel) aur ek jali hui maachis. Combustion = inthein completely jalti hain aur ek fixed amount ki heat deti hain — cheezein hamesha garam hoti hain. Neutralization = jab paani mein ek "khatta" acid aur ek "phisal wala" base milte hain, toh sirf paani banta hai aur ek fixed warm hug ki heat milti hai. Solution = paani mein namak daalna; kabhi paani thanda ho jaata hai (namak todne mein energy chahiye), kabhi garam (paani chote namak ke tukdon ko kaskar huggle karta hai). Hydration = paani ke molecules har chote charged tukde ke aas-paas bheed lagate hain aur use huggle karte hain, jo hamesha heat release karta hai.
"Cats Never Sip Hot" → C ombustion (hamesha −), N eutralization (strong ke liye −57.1), S olution (± = lattice + hydration), H ydration (hamesha −).
Common mistake Combustion meinC O Sahi lagta hai: "Jala toh paani gas hoga." Fix: Standard Δ c H complete combustion chahiye (→C O 2 standard states mein → H 2 O ( l ) H 2 O ( g )
Δ so l H = Δ l a tt i ce H + Δ h y d H Sahi lagta hai: "Subtract karo kyunki ye oppose karte hain." Fix: Bas correct signs ke saath add karo : Δ l a tt i ce H + Δ h y d H −
Definition of enthalpy of combustion Δ H
Sign of every combustion enthalpy Hamesha negative (exothermic).
Why strong-acid–strong-base neutralization is constant − 57.1 Dono fully ionized hain; sirf reaction hai H + + O H − → H 2 O
Why weak acid gives less heat on neutralization Pehle weak acid ko ionize karne mein energy kharach hoti hai (endothermic), isliye net ∣Δ H ∣ < 57.1
Master relation for solution enthalpy Δ so l H = Δ l a tt i ce H + Δ h y d H
Sign of hydration enthalpy Hamesha negative (ion–dipole attraction se energy release hoti hai).
What makes hydration more exothermic Zyada ionic charge aur chhota radius (zyada charge density q / r
Definition of enthalpy of hydration Δ H
Δ f H Δ f H = ∑ Δ c H ( reactants ) − ∑ Δ c H ( products )
Why NH₄NO₃ cold pack works Iska Δ so l H
Measured at constant P q_p
Enthalpy of neutralization
Always negative exothermic
Strong acid+base -57.1 kJ
Lattice + hydration = solution
