Whoops, didn't see this before! Here's what I wrote on the question thread for sake of completeness:
When writing ionic equations for the dissociation of a compound, are we to always use a reversible arrow?
For example, for the reaction FeCl2 (s) ⇌ Fe2+ (aq) + 2Cl- (aq)
Why is an equilibrium arrow used and not just a forward arrow? How do we know when to use which arrow?
Depends on whether the reaction is reversible or not. If the reaction is reversible, the arrow goes both forwards and backwards. Reactions like combustion or many precipitation reactions will not be reversible, as the enthalpy/ entropy is too varied for ΔG to = 0 (which happens when a reaction is in equilirbium). I'm assuming that everything is in a closed system, by the way.
Sometimes, the single forwards arrow also denotes an equilibrium reaction that has gone to completion, but it isn't used too often. Let me explain what I mean...
Iron (II) chloride, like most compounds, are soluble to some degree. At one point, though, the solution will become saturated (no more water molecules can attach to the dissociated ions) and the stuff will precipitate as solid. How much you need to add in to achieve this effect is determined by each compunds' solubility/ ksp value. FeCl2 is pretty soluble given it's a halide salt. In a large amount of water, a small amount of it will be easily dissolved and the equation above will achieve completion. Of course, we can't guarantee that this situation will always be the case, so the double arrow covers all possibilities of amounts of water/ compound, etc. This also includes other factors like increasing the temperature. In short, all dissolution reactions will be reversible and should be written with a double arrow
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