Heat Effects of Chemical Reactions

Transcription

1 * eat Effect f hemical Reactin

2 Enthalpy change fr reactin invlving cmpund Enthalpy f frmatin f a cmpund at tandard cnditin i btained frm the literature a tandard enthalpy f frmatin Δ O g = J/mle at K a + bo K, atm co Element are aigned 0 enthalpy f frmatin Enthalpy f a reactin invlving cmpund i calculated uing e law: Rxn = prduct reactant he enthalpy change are calculated frm tandard enthalpie f frmatin if the reactin ccur at tandard cnditin prduct = prduct reactant = reactant

3 nider the chemical reactin: + O K, atm O + O Rxn = = O + O + O O = O + + O ince O = O + O Similarly, O = O + + O = + + S Rxn = = O + O = prduct reactant

4 Mlecular interpretatin f enthalpy change Enthalpy change f ytem invlving gaeu cmpund may be explained uing bnd enthalpie nider the reactin g O g O g O g Bnd cnfiguratin fr abve reactin: + O==O O====O + O Each bnd ha certain Δ* O==O - kj, ==O -98 kj, O -7 kj, -6 kj Δ = Δ*bnd made Δ*bnd brken Δ = * = -686 kj

5 e Law and it applicatin Enthalpy change accmpanying a chemical reactin i the ame whether it take place in ne r everal tage ince enthalpy i a tate functin A Δ B X Y Z Δ = Δ +Δ +Δ +Δ Reactin AX XY YZ ZB AB Enthalpy change Δ Δ Δ Δ Δ e law i ueful fr calculating the unknwn enthalpy change f a reactin uing knwn reactin

6 e Law and it applicatin Example alculate the tandard enthalpy f frmatin f lid Fe O frm the fllwing enthalpy data Δ = J Fe / O FeO Fe O Main Reactin Δ = J Δ = J Δ =? Add the reactin in the fllwing rder t btain the net reactin Net reactin: / O / O FeO Fe O Fe O 6Fe 6FeO Fe O O 6Fe O Fe / O O Fe 6FeO O Fe O Fe Fe Fe Fe O O O / O O Δ= 6 Δ + Δ - Δ Δ/= -70 J/mle

7 Ithermal enthalpy change fr reactin nt ccuring at K nider a general reactin with cmpnent at the ame temperature different than K: aa bb c dd Δ are tabulated fr cmpnent d d c d d Δ= cδ + dδ D - aδ A - bδ B D d d d D A a d d B A d b d B prd. react.

8 Ithermal enthalpy change fr reactin nt ccuring at K Δ value fr chemical reactin can be determined frm Δ value: d d d Kirchff equatin Δ can al be calculated frm the enthalpy increment equatin: prd. react.

9 Example Find the net heat available r required when the fllwing reactin take place at 800 K Subtance Δ kj/mle J/mle K ao *0 - *-6.95*0 5 * - O g *0 - *-8.5*0 5 * - ao *0 - *-5.9*0 5 * - ao O g ao O ao ao = = kj ao ao O = *0 - *-0.5*0 5 * - J/K * * d 800 J

10 Alternatively Δ can be calculated frm e law ao800 O ao 800 ao O ao ao d 800 d O d ao

11 Ithermal enthalpy change invlving phae change nider a general reactin with cmpnent at the ame temperature different than K where liquid B and phae are lid at K: aa bb l c l dd Kirchff equatin can be ued t calculate Δ : d Δ m Δ mb Δ mpnent tranfrm t liquid at m mpnent B tranfrm t liquid at mb Δ eperated int range

12 Ithermal enthalpy change invlving phae change aa bb l c l dd Δ m Δ mb Δ I II III c c c l l d d d D D D a a a A A A b b b B B B l herefre m I d c m m B m II d b m B m B III d Since - enthalpy increment equatin incrprate phae change, prd. react.

13 Ithermal enthalpy change invlving phae change - cδ m bδ mb Σ - prd. Σ - react. Δ Δ Δ Δ m mb Σ - prd. -Σ - react.

14 Anther alternative methd that can be ued in enthalpy cmputatin fr chemical reactin that invlve phae tranfrmatin i the applicatin f e law aa bb l c l dd 5 aa bb c dd 5 a b c d m B m A B l D d d d d m B m a a 5 m B m B B l d d

15 Nn-ithermal chemical prcee dd l c l bb aa dd c bb aa D D l l m react prd l B B B m l B A A d d d c d a d c b d a d b a d a m m B m B m

16 Example A furnace that i deigned t melt ilver/cpper crap i t be fired with prpane and air. he prpane vapr mixe with dry air at K. Flue gae are expected t exit the furnace at 505 K under teady tate cnditin. w lng will a 5.5 kg cntainer f prpane maintain the furnace temperature if heat i cnducted thrugh the brickwrk at the rate f 0000 kj/hur? O 8 g 5O g g 8.8N O g 8.8N g O g g 505 O g 8.8N Subtance - J/mle Δ kj/mle 8 g J/mle K O g *+0.00* +8.6* *0 - *-8.5*0 5 * - Ol Og * * -0.* *0 - *+0.*0 5 * - N g *+0.00* *0 - * Δ v O = 0897 J/mle g Air: % O + 79% N

17 Flame temperature he maximum temperature the gaeu prduct can reach upn prceeding f an exthermic reactin i called the flame temperature he furnace i cnidered a adiabatic fr n heat l t the urrunding and maximum flame temperature aa bb g c g dd g 5 aa bb g c l dd g 5 c m l d v a flame m g d c flame g 5 d flame D g d d flame D g