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Let’s say I have $\ce{AB (aq) + CD (aq) —> AD + CB}$. When AB and CD are dissolved in water, they get dissociated into their component ions. But why don’t they reform as AB and CD, if it is already known they (A and B, C and D) can form a chemical compound with each other? Why must the ions from each compound combine with ion(s) from the other compound?

By nature, dissociation in water causes ions from within an ionic compound to separate. When the ions are separated from their respective compounds, they are surrounded by water molecules to prevent them from recombining. But I’m guessing this blockage somehow doesn’t apply to to the relationship between the ions of different compounds.

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If the four substances AB, CD, AD and CB are all soluble in water, there is no such reaction in water. If you dissolve AB and CD, or independently AD and CB, you will obtain a solution containing the ions A+, B-, C+ and D-. That is all you can say. There are no pure substances any more. And it does not depend on the original choice of the substances, either AB + CD, or AD + CB. The sense of the reaction can be known when evaporating the solution, because AB, CD, AD and BC are not equally soluble. The least soluble of the four will first crystallize, and there is no way of predicting which one it is. The solubilities cannot be calculated. It may be one of the reagents, or one of the products. The result may be a chemical reaction, or not. Is it what you wanted to know ?

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  • $\begingroup$ Goodness my reply comes so late... so if there is a reaction, one of the products will crystallize? $\endgroup$
    – Pineapples
    May 22 '20 at 12:46
  • $\begingroup$ For example, if you dissolve some KBr and NaCl in water, there are no more KCl or NaCl in the solution. There $\endgroup$
    – Maurice
    May 22 '20 at 14:30
  • $\begingroup$ For example, if you dissolve some KBr and NaCl in water, you will obtain a clear solution and there are no KBr and no NaCl any more in solution. There are only Na+, K+, Cl- and Br- ions. And the same solution could have been done by dissolving NaBr and KCl and mixing. But, if one of the four possible compounds is insoluble, it will produce a deposit at the bottom of the flask, and this would be a chemical reaction. This would happen if you replace NaCl by NaClO4. When mixing NaClO4 solution with a KBr solution, a precipitate will happen, because KClO4 is sparingly soluble in water. $\endgroup$
    – Maurice
    May 22 '20 at 14:46
  • $\begingroup$ I didn’t know that crystallization could also refer to precipitation in a solution. Many thanks for your response! $\endgroup$
    – Pineapples
    May 23 '20 at 16:40
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If one dissolves both AB and CD in water, they are split into their ions i.e. A+, B-, C+ and D-. As long as all ions remain in aqueous solution, thus no precipitation, they don't really exist as AD + BC in the solution. The solution is just a homogeneous mixture of ions at this stage. The difference between reactants and products comes after the species are removed from the solution, e.g. by precipitation. Then, why is AD and BC formed on precipitation, rather than the reactants? This is because AD and BC are more stable than their reactant counterparts. There are several reasons why this could be the case. One possible explanation is HSAB, Hard Soft Acid Base Theory. In general, the larger of the anions pairs with the larger of the cations and vice versa in this method. However, bear in mind that reactions very rarely give 100% yield, and there is likely to be at least traces of the original reactants left over at the end.

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  • $\begingroup$ I see, that really helps clarify things a bit. (Again, sorry for the late response). Thank you so much! $\endgroup$
    – Pineapples
    May 22 '20 at 12:48

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