That is, from the first law of thermodynamics a positive energy change occurs when is energy added to the system endothermic and a negative energy change occurs when is is released exothermic.
This may be easier to see by looking at the back reaction, where you need to add energy to remove the hydrated ion and place it in the gas phase, since adding energy is endothermic, the reaction as drawn must be exothermic. Since lithium is smaller, it would take more energy to remove it, so the formation of the hydrated ion is more exothermic negative than the large Cesium. You must always correlate the sign of an energy to its process, and recognize that you can "form" or "break" any bond or intermolecular force.
As written, this is the formation, which is the exothermic process. If you had written the reverse reaction, all the values would be positive. Hydrated Salts : Ion-dipole forces also explain why many salts will trap water when they crystallize and form hydrated salts.
This is common for small cations like sodium and lithium, which form hydrates like sodium carbonate decahydrate Na 2 CO 3 H 2 O, while larger salts like rubidium and cesium do not tend to form hydrates, as they have weaker ion-dipole interactions.
Robert E. The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, rebelford ualr. You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to:. Ion-Dipole Interactions Ion-Dipole Forces are involved in solutions where an ionic compound is dissolved into a polar solvent, like that of a solution of table salt NaCl in water.
Answer One is an inverse distance relationship, the other is an inverse distance square relationship. Ion-ion interactions fall off slower than ion-dipole. There are two components to every solution. The solute is dissolved. The solvent is doing the dissolving. A solute is soluble if the intermolecular interactions are favorable. Insolubility is the result of unfavorable intermolecular interactions.
What causes favorable or unfavorable intermolecular interactions? The solvent's ability to handle the charge of the solute. Let's say we add a polar solute to a non-polar solvent. The solute will attract itself. The solvent cannot balance the polarity of the solute. Because the solute is polar it has a positive and negative side.
The positive and negative sides will interact to balance the partial charges. For example, the difference in polarity is why water and oil separate. Water is a polar molecule. Oil is non-polar. Water can hydrogen bond to itself to balance its polarity.
The oil is neutral and does not help reduce the charge of the water. It is more stable for water to interact with itself. When we add an ionic compound to a polar solvent it dissociates. Water is the most common polar solvent because it hydrogen bonds. The solvent needs to balance the charge of both the cation and anion. But, the charge of the ion is greater than the partial charges of the solvent. Remember, the charges on ions are always whole numbers.
Dipole charges are much smaller fractions. It will take a lot of solvent molecules to balance one solute molecule. This is why solutions reach saturation. Beyond saturation, there is too much solute for the solvent to balance the solute's charge. To balance the charge of the ion, the solvent will form a shell around the ion.
By forming a shell the solvent molecules disperse the ion's charge. For the cation, the solvent uses the negative side of its dipole. For the anion, it uses the positive side of its dipole. Let's imagine dissolving sodium chloride NaCl in water. The temporary partially charged dipole and the ion are attracted to each other and form a fleeting interaction. Boundless vets and curates high-quality, openly licensed content from around the Internet.
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Ion-Dipole Force.
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