pH of Period 3 Metal Chlorides in Aqueous Medium
Under the topic of Periodicity, we need to explain the reactions of Period 3 chlorides with water and their corresponding pH values.
In this discussion we want to focus on the pH of Period 3 metal chlorides.
We notice that NaCl(aq), MgCl2(aq) and AlCl3(aq) have the following pH values:
NaCl(aq) - pH7
MgCl2(aq) - pH6
AlCl3(aq) - pH3
The pH of these aqueous solutions are related to the polarising power of the metal cations.
Polarising Power and Charge Density
As we move across Period 3, there is an increase in charge from Na+ to Al3+.
Hence charge density and polarising power increase across Period 3.
Polarising Power and Distortion of Electron Cloud
Using Al3+ as an example, a metal cation with higher charge density and polarising power will distort the electron cloud of a neighboring molecule or anion to a greater extent.
In this case the electron cloud of water molecules will be distorted to a greater extent which weakens the O-H bonds in water.
This makes it easier to break the O-H bonds and release more H+, which makes the solution more acidic.
Finally we can put this all together to compare the polarising power and pH trends of the Period 3 metal chlorides.
For the detailed step-by-step discussion on how to apply polarising power to pH of Period 3 metal chlorides, check out this video!
Topic: Periodicity, Inorganic Chemistry, A Level Chemistry, Singapore
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