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Txoj Cai Hess: Qhov Tseem Ceeb ntawm Txoj Cai ntawm Kev Txuag Zog hauv Cov Tshuaj Lom Zem
Pengantar
Kev kawm txog tshuaj lom neeg tsis yog hais txog kev hloov xim, cov duab, thiab cov tshuaj tiv thaiv zoo kawg nkaus hauv cov raj kuaj xwb. Tom qab txhua yam tshuaj tiv thaiv muaj cov ntsiab cai tseem ceeb uas tswj hwm tus cwj pwm ntawm cov khoom thiab lub zog. Ib qho ntawm cov ntsiab cai tseem ceeb no yog Hess's Law, uas ua lub luag haujlwm tseem ceeb hauv kev nkag siab txog lub zog koom nrog thiab hloov pauv li cas thaum lub sijhawm tshuaj tiv thaiv.
Kev Txhais Lus ntawm Hess Txoj Cai
Txoj Cai Hess, uas muab lub npe tom qab tus kws tshuaj Lavxias Germain Henri Hess, yog ib qho ntawm cov ntsiab lus tseem ceeb ntawm thermochemistry. Txoj cai no hais tias tag nrho cov enthalpy hloov pauv thaum lub sijhawm tshuaj lom neeg yog tib yam, tsis hais txoj kev lossis cov kauj ruam uas tau ua. Hauv lwm lo lus, yog tias qhov tshuaj tiv thaiv thawj zaug thiab cov khoom kawg tseem zoo ib yam, ces tag nrho cov zog xav tau lossis tso tawm yuav tseem ruaj khov.
Hauv kev lej, Hess Txoj Cai tuaj yeem piav qhia raws li hauv qab no:
\[ \Delta H_{total} = \Delta H_1 + \Delta H_2 + \Delta H_3 + \ldots + \Delta H_n \]
Qhov twg \(\Delta H\) yog qhov kev hloov pauv enthalpy ntawm txhua kauj ruam ntawm cov tshuaj tiv thaiv.
Cov Ntsiab Cai Tseem Ceeb ntawm Hess Txoj Cai
Txoj Cai Hess yog qhov tseeb yog kev siv txoj cai ntawm kev txuag lub zog rau chemistry. Raws li txoj cai ntawm kev txuag lub zog, lub zog tsis tuaj yeem tsim lossis rhuav tshem - nws tsuas yog hloov pauv ntawm ib daim ntawv mus rau lwm daim ntawv. Hauv cov ntsiab lus ntawm cov tshuaj lom neeg, lub zog no feem ntau yog qhia hauv daim ntawv ntawm enthalpy (H), uas ntsuas tag nrho lub zog khaws cia hauv ib lub cev.
Yog tias ib qho tshuaj lom neeg tuaj yeem rov ua dua tshiab rau hauv ntau kauj ruam me me uas txawv ntawm qhov tshuaj ncaj qha, ces peb tuaj yeem suav cov kev hloov pauv enthalpy ntawm txhua kauj ruam me me no kom tau txais tag nrho cov kev hloov pauv enthalpy. Qhov no txhais tau tias peb tuaj yeem xam tag nrho enthalpy ntawm ib qho tshuaj lom neeg nyuaj los ntawm kev rho tawm cov enthalpies ntawm cov tshuaj lom neeg yooj yim uas ua rau nws.
Kev siv Hess txoj cai lij choj
Hess Txoj Cai muaj ntau daim ntawv thov siv tau hauv ntau qhov chaw ntawm chemistry, qee qhov suav nrog:
1. Kev suav ntawm Reaction Enthalpy
Thaum peb ntsib ib qho kev tshuaj tiv thaiv uas nws nyuaj rau ntsuas qhov kev hloov pauv enthalpy ncaj qha, Hess Txoj Cai tso cai rau peb rhuav tshem qhov kev tshuaj tiv thaiv ua cov kauj ruam yooj yim dua, uas qhov kev hloov pauv enthalpy tuaj yeem ntsuas tau yooj yim dua lossis paub los ntawm cov ntaub ntawv thermochemical muaj.
Piv txwv li: Cia peb hais tias peb xav txiav txim siab qhov kev hloov pauv enthalpy (\(\Delta H\)) rau qhov kev tshuaj tiv thaiv:
\[ C(s) + 1/2 \, O_2(g) \rightarrow CO(g) \]
Peb tuaj yeem faib cov tshuaj tiv thaiv no ua:
\[ C(s) + O_2(g) \rightarrow CO_2(g) \quad (\Delta H = -393.5 \; kJ/mol}) \]
\[ CO_2(g) \rightarrow CO(g) + 1/2 \, O_2(g) \quad (\Delta H = +283.0 \; kJ/mol}) \]
Siv Hess Txoj Cai, qhov kev hloov pauv enthalpy tag nrho yog:
\[ \Delta H = -393.5 \; \text{kJ/mol} + 283.0 \; \text{kJ/mol} = -110.5 \; \text{kJ/mol} \]
2. Kev Txheeb Xyuas Lub Zog Sib Koom Tes
Txoj Cai Hess kuj feem ntau siv los txiav txim siab lub zog sib txuas uas tsis tuaj yeem ntsuas ncaj qha. Los ntawm kev siv cov ntaub ntawv enthalpy los ntawm cov tshuaj tiv thaiv paub thiab ntau yam kev xav lossis lwm txoj kev ntsuas, cov kws tshawb fawb tuaj yeem kwv yees lub zog uas koom nrog kev tawg thiab tsim cov tshuaj sib txuas.
3. Kev Kawm Txog Thermodynamics ntawm Cov Kev Daws Teeb Meem
Hauv kev tshuaj thiab kev tshuaj xyuas tshuaj, Hess Txoj Cai pab nkag siab txog kev yaj (kev yaj) thiab kev sib sau ua ke (kev khi) ntawm cov tshuaj. Piv txwv li, nws pab sib txawv ntawm entropy (ΔS) thiab enthalpy (ΔH) ntawm cov tshuaj yaj hauv ntau yam kuab tshuaj.
Keeb Kwm thiab Kev Loj Hlob
Txoj Cai Hess tau tsim thawj zaug los ntawm Germain Henri Hess hauv xyoo 1840. Lub sijhawm ntawd, Hess tau pom tias qhov kub uas tso tawm lossis nqus tau hauv cov tshuaj lom neeg ntawm qhov siab tas li yog tib yam, tsis hais seb qhov tshuaj tiv thaiv tshwm sim ncaj qha lossis los ntawm ntau kauj ruam.
Nws qhov kev tshawb pom tau qhib txoj hauv kev rau kev txhim kho ntxiv hauv kev kawm txog thermochemistry, suav nrog lwm cov ntsiab lus tseem ceeb xws li lub xeev cov haujlwm, qhov twg qhov ntau tsis nyob ntawm seb lub kaw lus tau mus txog lub xeev ntawd li cas tab sis tsuas yog nyob rau ntawm thawj thiab kawg xeev.
Piv txwv lej ntawm kev siv Hess txoj cai
Ua ib qho piv txwv ntawm kev siv Hess's Law, peb yuav xam qhov kev hloov pauv enthalpy los ntawm kev hlawv methane mus rau carbon dioxide thiab dej:
\[ CH_4(g) + 2 \, O_2(g) \rightarrow CO_2(g) + 2 \, H_2O(g) \]
Cov theem yog raws li nram no:
1. Kev hlawv methane mus rau hauv carbon dioxide thiab dej:
\[ CH_4(g) + 2 \, O_2(g) \rightarrow CO_2(g) + 2 \, H_2O(g) \quad (\Delta H_{reaction?}) \]
2. Peb paub:
\[ CH_4(g) + 2 \, O_2(g) \rightarrow CO_2(g) + 2 \, H_2O(l) \quad (\Delta H = -890.3 \; kJ/mol}) \]
Ces:
\[ H_2O(l) \rightarrow H_2O(g) \quad (\Delta H_{vap} = +44.0 \; \text{kJ/mol}) \]
Siv Hess Txoj Cai:
\[ \Delta H = -890.3 \; kJ/mol} + 2 \cdot 44.0 \; kJ/mol} = -802.3 \; kJ/mol} \]
Yog li, qhov kev hloov pauv enthalpy rau kev hlawv ntawm methane rau carbon dioxide thiab dej nyob rau hauv ntau yam kev mob sib txawv tuaj yeem suav tau yooj yim siv Hess Txoj Cai.
Xaus
Txoj Cai Hess muab kev nkag siab tob txog kev txuag lub zog hauv cov tshuaj lom neeg thiab pab cov kws tshawb fawb tshuaj lom neeg thiab cov kws kho mob nkag siab thiab kwv yees tus cwj pwm thermochemical ntawm ntau yam tshuaj lom neeg. Nrog kev pab los ntawm cov ntsiab cai ntawm Txoj Cai Hess, kev hloov pauv zog uas tshwm sim ntawm qib microscopic tuaj yeem suav thiab tshuaj xyuas nrog qhov tseeb siab, ua rau muaj kev txhim kho ntxiv hauv ntau ceg ntawm chemistry, physics, thiab cov khoom siv thev naus laus zis.
Yog tias koj nkag siab zoo txog Hess's Law, tsis yog cov kws tshuaj xwb, tab sis kuj cov kws tshawb fawb los ntawm ntau yam kev kawm tuaj yeem tshawb nrhiav thiab tswj cov tshuaj lom neeg rau ntau yam kev siv, txij li kev tsim tshuaj mus rau kev tsim kho tshiab hauv kev siv tshuab hluav taws xob.
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Tsab xov xwm no piav qhia txog cov lus txhais, cov ntsiab cai yooj yim, kev siv tau, keeb kwm, thiab cov piv txwv ntawm Hess Txoj Cai hauv thermochemistry. Peb vam tias qhov no yuav pab koj nkag siab thiab siv lub tswv yim tseem ceeb no hauv chemistry.
