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

pH of Period 3 metal chlorides pH trend

The pH of these aqueous solutions are related to the polarising power of the metal cations.

Polarising Power and Charge Density

pH of Period 3 metal chlorides polarising power trend

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

pH of Period 3 metal chlorides 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.

pH of Period 3 metal chlorides comparison and explanation

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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