some one help me plz (1 Viewer)

[abu-mahmoud]

1. Explain the importance of the first ionisation energy in determining the reactivity of a metal such as sodium

2.water is less volatile than hexane, but has a greater viscosity. account for these differences in terms of their intermolecular forces.

3. explain why many ionic substances dissolve in water but most macromolecular substances do not..

4. explain why silicon and phosphorus, which are so close to each other, have such a marked difference in their melting points.

if u could help me out in these questions it will be much appricated

 

[Roobs]

what is the context of the questions?

 

[airie]

1. The less the first ionisation energy, the more reactive a metal is, as the less the energy required to remove one electron from the outmost shell of an atom, the easier it is to let the metal react (as metals are usually electron donors).

2. I don't quite see th reason of using "but", anyways... Water is less volatile than hexane due to the stronger intercular forces that exist in water, including hydrogen bonding apart from dispersion forces. Since H-bonds are much stronger than dispersion forces, which is the only intermolecular force that exists in hexane (if the strength of H-bond is assigned a value of 10, that of dispersion forces would be about 0.1 to 0.3), the intermolecular forces between water molecules are much harder to break (ie. more energy input is required) than those between hexane molecules, thus water is more volatile. Its relatively higher viscosity is also due to the presence of strong H-bonds, which keeps individual molecules close to one another and therefore the liquid flows around more slowly.

3. Remember, "like dissolves like". Dissolution of most ionic substances occurs in water, where, by electrostatic forces, the hydrogen ends of water molecules would attract the anions of the substance while oxygen ends attract the cations, forming ion-dipole attractions. This is because water is a polar substance, and thus most of the time it is more energetically economic to form ion-dipole attractions, rather than the anions and cations of the substance remaining in its lattice form. There are exceptions, of course, BaSO₄ for example, where the ionic bonding between anions and cations of the substance requires more energy to break than that released when ion-dipole bondings are established. This is the case for macromolecular substances: as bonds between individual atoms in a network is stronger and requires more energy to break (and it is NOT more economic in energy terms to break them), they generally dissolve little or do not dissolve. However, if there are hydroxyl ends (ie. OH^-) on the network, they will form some weak bonds with the hydrogen ends of water molecules, thus the substance will dissolve slightly in water.

4. Silicon is a macromolecular substance, with a structure similar to that of carbon in the form of diamonds, whereas phosphorus exists in simple molecules. Since covalent bonding in a macromolecular network between individual atoms is much, much stronger than dispersion forces that exist between molecular substances (using the same scale as above with H-bond assigned a value of 10, covalent bonds would be 100, compared to, as stated above, only about 0.1 to 0.3 for dispersion forces), they require much more energy to break. The MP and BP of these two substances are therefore very different.

Hope that helped