Cov Lus Nug Piv Txwv Sib Tham Txog Enthalpy thiab Kev Hloov Pauv Enthalpy
Enthalpy yog lub tswv yim tseem ceeb hauv kev siv tshuaj thermodynamics, uas feem ntau pom muaj nyob rau hauv ntau yam kev kawm txog chemistry, txij li cov tshuaj tiv thaiv mus rau kev hloov pauv theem. Hauv tsab xov xwm no, peb yuav tshuaj xyuas ntau qhov teeb meem piv txwv thiab tham txog enthalpy thiab kev hloov pauv enthalpy los pab peb nkag siab zoo dua txog lub tswv yim.
Nkag Siab Txog Enthalpy
Enthalpy (H) yog tag nrho cov zog hauv lub thermodynamic system. Nws tsis yog tsuas yog suav nrog lub zog sab hauv uas khaws cia rau hauv cov khoom me me xwb tab sis kuj suav nrog lub zog uas xav tau los tsim qhov chaw rau cov khoom me me hauv ib puag ncig siab. Enthalpy ntsuas hauv joules (J) hauv International System (SI).
Mathematically, enthalpy yog txhais li:
\[ H = U + PV \]
Qhov twg:
- \( H \) yog lub enthalpy
- \(U \) yog lub zog sab hauv
- \( P \) yog qhov siab
- \( V \) yog qhov ntim
Kev Hloov Pauv Enthalpy
Kev hloov pauv ntawm enthalpy (\( \Delta H \)) tshwm sim thaum muaj kev tshuaj lom neeg lossis cov txheej txheem lub cev tshwm sim. Qhov kev hloov pauv enthalpy no tuaj yeem txhais tau tias yog qhov kub uas tso tawm lossis nqus los ntawm lub kaw lus ntawm qhov siab tas li. Hauv lej:
\[ \Delta H = H_{\text{khoom}} – H_{\text{reactant}} \]
Cov tshuaj tiv thaiv exothermic yog cov tshuaj tiv thaiv uas tso cua sov rau hauv ib puag ncig, thiab hauv cov tshuaj tiv thaiv no, \( \Delta H \) yog qhov tsis zoo. Lub caij no, cov tshuaj tiv thaiv endothermic yog cov tshuaj tiv thaiv uas nqus cua sov los ntawm ib puag ncig, thiab hauv cov tshuaj tiv thaiv no, \( \Delta H \) muaj tus nqi zoo.
Cov Lus Nug Piv Txwv thiab Kev Sib Tham
Piv txwv lus nug 1: Kev hloov pauv ntawm Enthalpy ntawm Combustion
Lo lus nug:
Nws paub tias qhov kev sib xyaw ua ke ntawm 1 mole ntawm methane (\(CH_4\)) tsim cov pa roj carbon dioxide (\(CO_2\)) thiab dej (\(H_2O\)). Cov ntaub ntawv enthalpy ntawm kev tsim yog raws li nram no:
– \( \Delta H_{{f, H_2O (l)}} = -285.8 \text{kJ/mol} \)
– \( \Delta H_{{f, CO_2 (g)}} = -393.5 kJ/mol} \)
– \( \Delta H_{{f, CH_4 (g)}} = -74.8 \text{kJ/mol} \)
Xam qhov kev hloov pauv enthalpy (\( \Delta H \)) ntawm qhov kev sib xyaw ua ke.
Kev Sib Tham:
Cov tshuaj tiv thaiv combustion ntawm methane yog:
\[ CH_4 (g) + 2 O_2 (g) \rightarrow CO_2 (g) + 2 H_2O (l) \]
Qhov kev hloov pauv enthalpy ntawm cov tshuaj tiv thaiv (\( \Delta H \)) tuaj yeem suav nrog siv enthalpy ntawm kev tsim:
\[ \Delta H = \Sigma \Delta H_f \text{cov khoom} – \Sigma \Delta H_f \text{cov tshuaj tiv thaiv} \]
Khoom:
\[ \Delta H_f (CO_2 (g)) = -393.5 kJ/mol} \]
\[ \Delta H_f (H_2O (l)) = -285.8 kJ/mol} \]
Tag nrho cov enthalpy ntawm cov khoom:
(-393.5 kJ/mol) + 2 zaug (-285.8 kJ/mol) = -393.5 – 571.6 = -965.1 kJ/mol)
Cov tshuaj tiv thaiv:
\[ \Delta H_f (CH_4 (g)) = -74.8 \text{kJ/mol} \]
\[ \Delta H_f (O_2 (g)) = 0 \text{kJ/mol} \]
(Cov pa oxygen nyob rau hauv nws lub xeev txheem muaj ib qho enthalpy ntawm kev tsim ntawm xoom.)
Tag nrho cov enthalpy ntawm cov reactants:
(-74.8 kJ/mol) + 2 lub sijhawm (0 kJ/mol) = -74.8 kJ/mol
Yog li, qhov kev hloov pauv hauv enthalpy (\( \Delta H \)) yog:
\[ \Delta H = -965.1 \text{kJ/mol} – (-74.8 \text{kJ/mol}) \]
\[ \Delta H = -965.1 + 74.8 \]
\[ \Delta H = -890.3 kJ/mol} \]
Yog li, qhov kev hloov pauv enthalpy rau qhov combustion ntawm 1 mole ntawm methane yog \(-890.3 \text{kJ/mol}\).
Piv txwv lus nug 2: Kev hloov pauv ntawm Enthalpy hauv cov txheej txheem lub cev
Lo lus nug:
Xam qhov kev hloov pauv enthalpy thaum 50 grams ntawm dej khov (\(H_2O_{(s)}\)) ntawm 0°C yaj mus ua dej (\(H_2O_{(l)}\)) ntawm 0°C. Nws paub tias qhov kub ntawm kev yaj ntawm dej khov (\( \Delta H_{\text{fus}} \)) yog 6.01 kJ/mol thiab qhov hnyav molar ntawm dej yog 18 g/mol.
Kev Sib Tham:
Kauj ruam thawj zaug yog xam tus naj npawb ntawm moles ntawm dej khov.
\[ \text{Moles ntawm dej khov} = \frac{50 \text{g}}{18 \text{g/mol}} \approx 2.78 \text{mol} \]
Tom ntej no peb xam qhov kev hloov pauv enthalpy rau kev yaj cov dej khov:
\[ \Delta H = n \cdot \Delta H_{\text{fus}} \]
\[ \Delta H = 2.78 \text{mol} \cdot 6.01 \text{kJ/mol} \]
\[ \Delta H \approx 16.7 \text{kJ} \]
Yog li, qhov kev hloov pauv enthalpy thaum yaj 50 grams ntawm dej khov ntawm 0 ° C yog li 16.7 kJ. Qhov no yog ib qho txheej txheem endothermic vim tias dej khov nqus cua sov kom tig mus ua dej.
Piv txwv 3: Hess Reaction
Lo lus nug:
Siv Hess txoj cai los txiav txim siab qhov kev hloov pauv enthalpy rau cov tshuaj tiv thaiv hauv qab no:
\[ 2 C(graphite) + 3 H_2(g) \rightarrow C_2H_6(g) \]
Muaj ntau yam kev tshuaj tiv thaiv paub nrog lawv cov kev hloov pauv enthalpy:
1. \( C(graphite) + O_2(g) \rightarrow CO_2(g), \Delta H = -393.5 \text{kJ} \)
2. \( H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(l), \Delta H = -285.8 \text{kJ} \)
3. \( 2 C_2H_6(g) + 7 O_2(g) \rightarrow 4 CO_2(g) + 6 H_2O(l), \Delta H = -3119.6 \text{kJ} \)
Kev Sib Tham:
Yuav kom xam tau qhov kev hloov pauv enthalpy (\( \Delta H \)) ntawm cov tshuaj tiv thaiv, peb yuav tsum thim rov qab thiab muab qee qhov kev tiv thaiv sib npaug rau qhov kev tiv thaiv uas xav tau.
Cov Kauj Ruam:
1. Hloov qhov kev tshuaj tiv thaiv \(C_2H_6 \rightarrow 2 CO_2 + 3 H_2O\):
\[2C_2H_6(g) + 7O_2(g) \rightarrow 4 CO_2(g) + 6H_2O(l), \Delta H = -3119.6 \text{kJ}\]
Rov qab thiab faib cov tshuaj tiv thaiv:
\[ 4CO_2(g) + 6H_2O(l) \rightarrow 2C_2H_6(g) + 7O_2(g), \Delta H = 3119.6/2 = 1559.8 \text{kJ} \]
2. Kev tshuaj tiv thaiv \(C \rightarrow CO_2\):
\[ 4C(graphite) + 4O_2(g) \rightarrow 4CO_2(g), \Delta H = 4 \times -393.5 = -1574 \text{kJ} \]
3. Kev tshuaj tiv thaiv \(H_2\rightarrow H_2O\):
\[ 6H_2(g) + 3O_2(g) \rightarrow 6H_2O(l), \Delta H = 6 \times -285.8 = -1714.8 \text{kJ} \]
Ua ke txhua yam, peb tau txais:
\[2C(graphite) + 3H_2(g) \rightarrow C_2H_6\]
\[ \Delta H = 1559.8 – 1574 – 1714.8 = -1729 \text{kJ}\]
Yog li, qhov kev hloov pauv enthalpy ntawm cov tshuaj tiv thaiv ntawm 2 moles ntawm graphite thiab 3 moles ntawm \(H_2\) rau \(C_2H_6(g)\) yog -1729 kJ.
Yog li ntawd, kev nkag siab txog lub tswv yim ntawm enthalpy thiab nws daim ntawv thov rau ntau hom kev tshuaj tiv thaiv muab kev nkag siab txog seb lub zog koom nrog li cas rau hauv kev hloov pauv tshuaj lom neeg thiab lub cev. Cov teeb meem saum toj no yog qee qhov piv txwv uas siv ntau zaus hauv cov ntawv kawm los txhawb kev nkag siab txog cov tswv yim no.
