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The thermal stability order of hydroxides is said to be increasing down the group. But my question is, hydroxide is a small anion so it should be stabilized by a small cation. Hence shouldn`t the stability decrease down the group?

The thermal stability order of hydroxides is said to be increasing down the group. But my question is, hydroxide is a small anion so it should be stabilized by a small cation. Hence shouldn`t the stability decrease down the group?

Grade:12

2 Answers

Ravleen Kaur
askIITians Faculty 1452 Points
6 years ago
Hello Student,

Thermal stability increases down the group due to the decrease in the charge density of the metal cation. All group 2 metal ions have the same charge (2+) but the smaller ones at the top of the group packed into a smaller space so they polarise the O-H bond more (pull electrons towards the oxygen). This means the O-H bond is already partly broken so it requires less heat to break fully. As you go down the group the O-H is less polarised by the larger group 2 cations with a lower charge density so the thermal stability increases.

Regards
IHE
Ravleen Kaur
askIITians Faculty 1452 Points
6 years ago
Hello Student,

Thermal stability increases down the group due to the decrease in the charge density of the metal cation. All group 2 metal ions have the same charge (2+) but the smaller ones at the top of the group packed into a smaller space so they polarise the O-H bond more (pull electrons towards the oxygen). This means the O-H bond is already partly broken so it requires less heat to break fully. As you go down the group the O-H is less polarised by the larger group 2 cations with a lower charge density so the thermal stability increases.

Regards
IHE

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