Gibbs free energy of the thermal hydrogenation reaction of silicon tetrachloride
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Thermal hydrogenation of silicon tetrachloride is a method currently used to deal with silicon tetrachloride, which is a byproduct in the production of polysilicon: at 1250 degrees Celsius and under low pressure, silicon tetrachloride reacts directly with hydrogen gas inside a reactor, resulting in the formation of trichlorosilane as one of the products. The equation is: SiCl4 + H2 → SiHCl3 + HCl. I have a question: has anyone calculated the Gibbs free energy for this reaction? Can this reaction take place at 1250 degrees Celsius? If not, what is the problem? This post was last edited by LHY8771 on 2009-4-15 10:06.]
If the reaction Gibbs free energy is greater than 0, the reaction equilibrium constant is very small
Then wouldn’t it mean that this reaction is impossible to occur under these conditions? Then why do those people still achieve yields, and why is it still used as an important method for processing silicon tetrachloride?
That’s right. Is it thermodynamically possible at all? I’m concerned too. Is the driving force strong? Is it an endothermic or exothermic reaction? :lol
May I ask how you can prove that it definitely can respond?
This reaction should be endothermic; I’ve done the calculations, and it is only when the temperature reaches around 2500 degrees that the Gibbs free energy change becomes negative. Moreover, pressure has little effect on driving this reaction forward. If it is a reversible reaction, then its Gibbs free energy change should be around 0, but from my analysis, that is not the case! I’m not sure if what I said is correct. I wonder if any expert can provide accurate information? This question is really confusing.
Why are so few people answering this question? Is my question too stupid? It shouldn’t! Are there still very few people in this field? Those who know please share their opinions! Thank you!
The response is positive; it’s just that the basis and process for your calculation are unclear. Could you provide them or have them checked?
This post was last edited by zq1981110719 on 2009-5-5 11:41. For sesame, the range of temperatures for which my empirical formula is applicable is 400K–1600K; at 1250 degrees (1523K), the free energy is greater than 0. But I still calculated it using that empirical method, and the free energy is around 0 at 2500 K, not at 2500 degrees (100 kPa) ； Additionally, I disagree with the view that pressure has little effect on free energy – given dG = -SdT + Vdp, it seems that pressure does have an impact; I’m not sure why you say it has no effect I’ll take another look to find out what the real reason is!
SiCl4(g) + H2(g) = SiHCl3(g) + HCl(g)
T ΔH ΔS ΔG K Log(K) K kJ J/K kJ
973.150 63.194 26.185 37.712 9.454E-003 -2.024
1073.150 62.745 25.746 35.116 1.953E-002 -1.709
1173.150 62.386 25.425 32.558 3.550E-002 -1.450
1273.150 62.099 25.190 30.028 5.860E-002 -1.232
1373.150 61.871 25.017 27.518 8.977E-002 -1.047
1473.150 61.692 24.892 25.023 1.296E-001 -0.887
1573.150 61.557 24.802 22.539 1.785E-001 -0.748
1673.150 61.455 24.740 20.062 2.364E-001 -0.626
1773.150 61.381 24.697 17.590 3.032E-001 -0.518
1873.150 61.328 24.667 15.122 3.787E-001 -0.422
1973.150 61.290 24.648 12.656 4.623E-001 -0.335
2073.150 61.264 24.635 10.192 5.536E-001 -0.257
2173.150 61.244 24.625 7.729 6.519E-001 -0.186
2273.150 61.227 24.618 5.267 7.568E-001 -0.121
2373.150 61.210 24.610 2.806 8.674E-001 -0.062
2473.150 61.190 24.602 0.345 9.833E-001 -0.007
2500.150 61.184 24.600 -0.319 1.015E+000 0.007
The calculated values are as shown above for ΔG0. However, if external assistance is available, the reaction can still proceed; in such cases, the criterion becomes: the additional work done on the system by external forces must be greater than the increase in free energy, so that the reaction can take place! (There is an explanation in physical chemistry for the criterion of free energy.)
T ΔH ΔS ΔG K Log(K) K kJ J/K kJ
973.150 63.194 26.185 37.712 9.454E-003 -2.024
1073.150 62.745 25.746 35.116 1.953E-002 -1.709
1173.150 62.386 25.425 32.558 3.550E-002 -1.450
1273.150 62.099 25.190 30.028 5.860E-002 -1.232
1373.150 61.871 25.017 27.518 8.977E-002 -1.047
1473.150 61.692 24.892 25.023 1.296E-001 -0.887
1573.150 61.557 24.802 22.539 1.785E-001 -0.748
1673.150 61.455 24.740 20.062 2.364E-001 -0.626
1773.150 61.381 24.697 17.590 3.032E-001 -0.518
1873.150 61.328 24.667 15.122 3.787E-001 -0.422
1973.150 61.290 24.648 12.656 4.623E-001 -0.335
2073.150 61.264 24.635 10.192 5.536E-001 -0.257
2173.150 61.244 24.625 7.729 6.519E-001 -0.186
2273.150 61.227 24.618 5.267 7.568E-001 -0.121
2373.150 61.210 24.610 2.806 8.674E-001 -0.062
2473.150 61.190 24.602 0.345 9.833E-001 -0.007
2500.150 61.184 24.600 -0.319 1.015E+000 0.007
The calculated values are as shown above for ΔG0. However, if external assistance is available, the reaction can still proceed; in such cases, the criterion becomes: the additional work done on the system by external forces must be greater than the increase in free energy, so that the reaction can take place! (There is an explanation in physical chemistry for the criterion of free energy.)
Our laboratory has also calculated this; between 1000 and 1250 degrees, the equilibrium constant is around 0.3
Want to know how to get this set of data? Please give me some advice!