Thermal Stability Of Grp 2 and 17 Elements

Grp 2 decomposition eqn

MCO3 (s) → MO (s) + CO2 (g)

Factors affecting thermal stability

• Anionic Size

• Charge density of cation

How does anionic size affect the thermal stability of grp 2 carbonates?

• A greater anionic size would allow for easier distortion of the e- cloud

  • CO3²- VS O²-

• Larger & more unstable anion (CO3²-) would be distorted and broken down into a more stable anion with smaller, less polarisable e- cloud.

How does charge density of cation affect the thermal stability of grp 2 carbonates?

• The higher the charge density, the greater the polarising power, making it easier to distort the e- cloud of anion.

  • C-O bond in CO3²- weakens and breaks easily to form CO2.

• The higher the charge density, the less stable the carbonate (easily broken down)

Trend of thermal stability of grp 2 carbonates down the group

Increases down the group (increase in decomp temperature)

Why does thermal stability of grp 2 carbonates increase down the group?

• Down the group, size of group 2 elements, increase.

• Causes the charge density to decrease, leading to weaker polarising power of cation.

• CO3²- ion is polarised to smaller extent, C-O bonds are weakened to a smaller extent.

• This makes it more difficult for the bonds to break, hence decomposition occurs with greater difficulty.

Grp 17 hydrides decomposition eqn

2HX (g) + heat → H2 (g) + X2 (g)

Trend of grp 17 hydrides down the group

Down the group, bond energy between the H-X grp 17 elements decrease; thermal stability decreases down the group

Why does thermal stability decrease down the group 17 hydrides?

• Down Grp 17, size of halogen atom increases.

• Results in increase in the H-X bond length, weakening the bond energy

• Thus, thermal stability decreases