[SOS]

BCl3(g) + NH3(g) --> Cl3BNH3(s)

I'm pretty sure that the entropy change is -Delta S because a solid was created from gases. But heres the question that got me stuck, it asks:

If the reaction spontaneously proceeds to the right, predict the sign of enthalpy change, Delta H. Explain your prediction.

Do I use this formula to assume enthalpy change?: Delta G = Delta H - T x Delta S?

Heres what I tried to do: (-G) = (Delta H) - (+T)(-S). Would the sign have to be a negative then? Why.

[DevaDevil]

you are in the correct way!

spontaneous process forward: Delta G = negative

"disorder" lost: delta S= negative; and -TdS = positive!


dG = dH + ( - TdS)

negative = dH + positive

negative - positive = dH

so dH has to be negative.

[SOS]

Thanks.

and another question to confirm things for me:

When questions that ask you to find Delta ΔG from partial pressures, do you always use the formula: ΔG = ΔG° + RTlnQ?

And when a question asks you to find the equilibrium constant, K, you use ΔG° = -RTlnK, which turns into K = e^-ΔG°/RT right?