# Homework Help: Ionic solids: enthalpy and entropy

1. Jan 14, 2009

### nobahar

Hello!
If ionic solids are to dissolve, then $$\Delta H_{solution}$$ has to be negative, (possibly 0? For equilibrium?). This can occur if $$\Delta H_{hyd}$$ for both the cations and anions is greater than the $$\Delta H_{LE}$$. But the textbook also states that a positive $$\Delta H_{solution}$$ can also result in the ionic solid dissolving if the entropy change, being positive, is favourable for it to do so. However, it also says that on some occasions, even if there is negative $$\Delta H_{solution}$$, the ionic solid will not dissolve because of an entropy decrease. The large attraction between the ions and, for example, water, is enough to reduce the entropy to a point where it is not a spontaneous process.

If $$\Delta H_{Hyd}$$ is large enough that it is favourable for the ions to be 'pulled' from the lattice; won't this also mean that the ions have a stronger charge (or are smaller, or both) and so there will be a reduction in entropy in the process?

An example given in the textbook is of CaCO3, which will not dissolve even though, apparently, the $$\Delta H_{solution}$$ is negative, because the entropy of the resulting solution is reduced, again to a point where there isn't a favourable change..

I don't know if this makes sense, as you may well be able to tell! Basically, can someone point me in the right direction or expand on this for me; that is, the role of entropy and enthalpy in the process? Since the textbook stops the discussion at this point. If the question isn't clear just let me know and I'll condense it!
Thanks.
Nobahar.

2. Jan 14, 2009

### chemisttree

No. $$\Delta G_{solution}$$ must be negative not $$\Delta H_{solution}$$.

Review the definition of gibbs free energy.

3. Jan 15, 2009

### nobahar

The textbook hasn't even mentioned free energy, it just states $$\Delta H$$.
I'll work on it. Thanks for the reply.

4. Jan 15, 2009

### GCT

Gibbs free energy takes into account both entropy and enthalpy , I am not quite certain where your confusion is on either , both of them describe universal tendencies ... you need to review both of the ultimate statements of entropy and enthalpy.