hello sir my question is dat there are 4 statemens dat are confusing me...

1.the reactivity of the alkali metals twards a particular halogen increwases down d group and thermal stability

2.it is again said dat the -ve value of the enthalpy decreases down the group ...i.e the thermal stability decreases...

so my question does reactivity has nuthin to do wid thermal stability..

another thing is the thermal stability has the order f>cl>br>i

but the thermal stability of cl-
is der ne relation???

shriya sarangi, 9 years ago

Grade:

1 Answers

ruchi yadav

askIITians Faculty 27 Points

8 years ago

You have to know that electronegativity has nothing got to do with ionic bond strength. Electronegativity only determines the ionic character (To what extent is a compound ionic). You also have to know that ionic bond strength and reactivity of metals has little/nothing to do with thermal stability of compounds.

The fact that calcium carbonate decomposes while sodium carbonate do not is due to the lower charge density of sodium ion. Charge density is proportional to charge/ionic radius (q/r). Both Calcium and Sodium have relatively similar ionic radius but Calcium has a greater charge, which results in Calcium ion having a greater charge density.
Calcium ion thus attracts (polarises) the electron cloud of the carbonate ion to a greater extent, weakening the C-O bond to a greater extent, resulting in less energy required to break the C-O bond.
On the other hand, sodium ion has lesser charge density and thus attracts the electron cloud to a small extent, not weakening the C-O bond enough, resulting in a lot of energy required to break the C-O bond (or you can say it does not break).

Finally, in your "Additional Details", you again messed up ionic bond strength with thermal stability.
Perhaps, it's due to the previous part where "sodium carbonate is more thermally stable than calcium carbonate" that you concluded that "sodium carbonate is stronger than aluminum carbonate". This is true because aluminium ion has a very small ionic radius and a very high charge, giving it a much higher charge density than sodium ion, thus it attracts (polarises) the electron cloud of the carbonate ion to a very large extent, weakening the C-O bond to a very large extent, resulting in a little energy required to break the C-O bond.

However, the reason for Sodium oxide being weaker than aluminum oxide is no longer due to thermal stability because there is no such thing as decomposition for oxides. The actual reason is because of lattice energy, which can be considered as similar to ionic bond strength. Lattice energy/ionic bond strength is defined as the heat released when 1 mole of solid ionic compound is formed from its constituent elements under room temperature and pressure. Lattice energy of a ionic compound is proportional to [(cation charge)*(anion charge)]/[(cationic radius) + (anionic radius)]. Aluminium ion has a greater charge and also a smaller cationic radius, which upon substitution into the equation, gives a very large lattice energy value for aluminum oxide compared to sodium oxide. Thus aluminum oxide is "stronger" than sodium oxide.

*In summary:
for thermal stability, we are comparing the charge density because the bond that breaks is the C-O bond not the ionic bond.
e.g. CaCO3 --> CaO + CO2 (C-O bond break => correct answer)
CaCO3 --> Ca2+ + CO3 2- (ionic bond break => wrong concept)

for ionic bond strength, we are comparing the lattice energy because the bond that breaks is the ionic bond
e.g. Na2O --> 2Na+ + O2- (ionic bond break => correct answer)