State which Alkaline Earth Metal cation forms the fewest insoluble salts and whi

Question

State which Alkaline Earth Metal cation forms the fewest insoluble salts and which forms the most insoluble salts. Describe how the relative solubilities vary according to the cation's position in the Periodic Table. - Do you expect RaCrO, to be soluble or insoluble in water? Why? Describe how the relative oxidizing powers of the Halogens according to their position in the Periodic Table. Explain your reasoning - Do you think At2 (astatine) could oxidize l? Could C reduce F2? Write out all appropriate equations and explain your reasoning

Explanation / Answer

a) The solubility of salts of alkaline earth metals increases down the group.

The metal cation which forms fewest insoluble salts is Ra and the one which forms most insoluble salts is Be. The order of solubility of alkaline earth metal salts is (decreasing order),

Ra salts > Ba salts > Sr salts > Ca salts > Mg salts > Be salts

With Ra cation forming the fewest insoluble salts and Be forms the most insoluble salts.

b) As discussed above, the solubility of salts of alkaline earth metal cations increases as we go down the group. the order of solubility is,

order of solubility of alkaline earth metal salts is (decreasing order),

Ra salts > Ba salts > Sr salts > Ca salts > Mg salts > Be salts

With Ra cation forming the fewest insoluble salts and Be forms the most insoluble salts. This is atrributed the increase in size of the cation as we go down the group which inturn reduces the electronegativity. Barium salts are the most ionic form of alkaline eath metal salts. The larger the metal ion, the more polarizable is molecule and more soluble are the salts formed from it.

c) Halogens act as oxidizing agents. Halogens are highly electronegative atoms. They can easily take up an electron to complete their octet and attain the stable next inert gas configuration. The ability to take up an electron is known as electron affinity. The extent of electron affinity depends on the distance between the valence shell and the nucleus. Smaller atoms have strong attraction for the electrons added with the nucleus. In other words how easily the element forms its ions depends on how strongly the new electrons are attracted. As the atoms get bigger (from F to I), the new electrons find themselves further from the nucleus, and more and more screened from it by the inner electrons (offsetting the effect of the greater nuclear charge). The bigger atoms are therefore less good at attracting new electrons and forming ions.

Thus the halogens easily accept electrons gets reduced and oxidizes the other species. Hence acting as an oxidizing agent. The oxidizing stregth of halogens decreases as we go down the group, such that,

F (strongest oxidizing agent) > Cl > Br > I (weakest ozidizing agent).

d) RaCrO4 will be a soluble salt. As we have seen earlier, the solubility of salts of alkaline earth metals increases as we go down the group. Hence keeping this point in mind, we predict this salt to be soluble.

e) At2 (astatine) cannot oxidize I-. This is due to the fact that to act as oxidising agent the At2 must be able to attract the newer electrons stronger then I-. As discussed earlier, as the size of element increases the valence shell becomes farther from the nucleus which in turn reduces the electron affinity. Thus, the electron affinity is lowest for At2 in the halogen series and will not act as an oxidant for I-. Infact I2 will be able to oxidize At- easily.

Cl- can reduce F2. The equation is as given below.

2Cl-(aq) + F2(g) ------> Cl2(g) + 2F-(aq)

Note we have already shown how F2 is a better oxidizing agent then Cl2. Cl- reduces F2 and F2 forms anions which gets solvated in water. Thus, F2 which acts as the oxidizing agent in a reaction with reducing agent Cl- gets itself oxidised in solution, the F2 ends up as F- ions with water molecules attached to them. As you go down the Group, the ease with which these hydrated ions of halogens are formed decreases, and so the halogens become less good as oxidising agents i.e. less ready to take electrons from something else. Inturn they become better reducing agents.

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