[iseeu]

2 CCl₄ (g) + O₂ (g) --> 2COCl₂(g) + 2Cl₂(g)

Predict the sign of ΔS, as the reaction proceeds to the right: the products have a higher ΔS because there are more mols of gaseous products than reactants. ΔG and ΔH are also positive at room temp. (it's not spontaneous).

So does this reaction have a critical temperature? For some reason I know it doesn't, but I have no idea why!

Take this equation for example- why can't I just solve this for T to get the critical temp.?ΔG+TΔS=ΔH. I'm thinking it might be because entropy and enthalpy are both positive. I don't know though, please explain this to me! I want to make sense out of this. thanks!

[Corribus]

You can't just solve for T because ΔH and ΔS are not temperature independent.  Over small temperature ranges this is not a bad approximation, but you can't make it generally.

[iseeu]

this doesnt answer my question though. how do i know if the reaction has a critical temperature or not?

[Corribus]

By critical temperature, do you mean a temperature at which the reaction goes from endergonic to exergonic?  Without analytical expressions for ΔH° and ΔS° (as a function of temperature), you really can't.  You can assume they are invariant to temperature and then project at what temperature ΔG° = 0.  The accuracy of this assessment depends on whether ΔH° and ΔS° at the crossing point are reasonably close to what they are at the temperature for which you have data.  That is, if you have ΔH° and ΔS° values at 298.15 K and the crossing point happens at, say 310 K, then you will probably calculate something close to this because the ΔH° and ΔS° values are likely to be almost the same at 310 K.  If the crossing point is at 2500 K, however then your projected crossing temperature is likely to be far off, because ΔH° and ΔS° are likely to change substantially over that wide temprerature interval.  There's no general way that I know of to determine this ahead of time - you just need to be aware of the assumption you are making.  Otherwise, you need real data or an analytical expression for ΔH° and ΔS°, so you know how these values vary specifically as a function of temperature.  This would involve the heat capacities and so forth.