5. Identify the oxidation state of Ca in CaF2(aq).
Ca(s) + 2 HF(aq) → CaF2(aq) + H2(g)

+2

Explanation:

Chemical equation:

Ca + 2HF  →   CaF₂ + H₂

The oxidation state of Ca on left  hand side is 0 while on the right hand side when it form CaF₂ its oxidation state becomes +2 because it loses two. These two electrons are accepted by fluorine and complete its octet while Ca loses its two out most electron to complete the octet.

In this reaction oxidation of calcium takes place and calcium itself is reducing agent.

Reducing agents:

Reducing agents reduced the other element are it self gets oxidized.

Oxidizing agents:

Oxidizing agents oxidize the other elements and itself gets reduced.

Consider another examples.

4KI + 2CuCl₂  →   2CuI  + I₂  + 4KCl

the oxidation state of copper is changed from +2 to +1 so copper get reduced.

CO + H₂O   →  CO₂ + H₂

the oxidation state of carbon is +2 on reactant  side and on product side it becomes  +4 so carbon get oxidized.

vanderwaal's forces of attraction are the electrostatic intermolecular forces that exist in a molecule due to the temporary or permanent dipoles that are developed in a molecule. in , the major intermolecular forces are dipole-dipole interactions as the molecule is polar (bent structure). so the vanderwaal's forces of interaction in between molecules are the permanent dipole-dipole interactions that develop due to the dipole that exists in between the polar h - s bond. the positively charged hydrogen atom is attracted towards the negative end of the dipole that is sulfur of the adjacent molecule. therefore, these forces of attraction are relatively stronger than that of london dispersion forces that exist in non polar molecules.