Polar compounds tend to dissolve in water, and we can prolong that generality come the most polar compound of all—ionic compounds. Table salt, or sodium chloride (NaCl), the most common ionic compound, is soluble in water (360 g/L). Recall the NaCl is a salt crystal composed not the discrete NaCl molecules, yet rather of an extensive array of Na+ and Cl- ion bound together in three dimensions through electrostatic interactions. Once NaCl disappear in water, the electrostatic interactions within the crystal must be broken. By contrast, once molecular link dissolve in water, it is the intermolecular forces between separate molecules that space disrupted. One might imagine that the breaking of ionic interaction would call for a an extremely high- energy input (we have currently seen the diamonds do not dissolve in water due to the fact that actual covalent bonds have to be broken). That would certainly be true if every we considered was the energy required to break the ionic interactions, as shown by the fact that NaCl melts in ~ 801 oC and boils at 1413 oC. However we understand that substances prefer NaCl dissolve easily in water, so clearly there is something else going on. The cheat is to think about the entirety system once NaCl dissolves, just like we did because that molecular species. We need to think about the interactions that room broken and also those that space formed. These changes in interactions are reflected in the ΔH ax (from ΔG = ΔH – TΔS).


When a crystal of NaCl come into contact with water, the water molecules connect with the Na+ and also Cl- ion on the crystal’s surface, as shown in the figure. The hopeful ends of water molecule (the hydrogens) connect with the chloride ions, if the negative end the the water molecule (the oxygen) interacts through the salt ions. For this reason the ion on the surface ar of the heavy interacts with water molecule from the solution; these water Vismmsanotherstage2019.com computer animation from molecules kind a dynamic cluster roughly the ion. Thermal motion showing the hydration that a Na (which reflects the kinetic power of the molecules, that is the + ion ~ above a NaCl surface.motion moved by collisions with other molecules in the system) climate moves the ion and also its water shell right into solution.116 The water covering is very dynamic—molecules space entering and also leaving it. The ion–dipole interaction in between ions and water molecules deserve to be an extremely strongly stability (- ΔH). The process by i beg your pardon solvent molecules interact with and stabilize solute molecules in equipment is called solvation. Once water is the solvent, the procedure is well-known as hydration.

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Questions come Answer

Draw a molecular-level picture of a systems of NaCl. Present all the kinds of particles and also interactions existing in the solution.

When we calculate and measure thermodynamic amounts (such together ΔH, ΔS and also ΔG), why is it necessary to specify the system and the surroundings?

When a problem dissolves in water, what is the system and what space the surroundings? Why? What criteria would you use to specify the system and surroundings?

For a solution made indigenous NaCl and also water, what interactions need to be get over as the NaCl goes right into solution? What new interactions are developed in the solution?

If the temperature goes up as soon as the equipment is formed, what deserve to we conclude about the family member strengths the the interactions that space broken and those that space formed? What can we finish if the temperature walk down?

When you measure up the temperature of a solution, are you measure up the device or the surroundings?

Questions to Ponder

Why is the water shell roughly an ion not stable?

What space the boundaries of a biological system?

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