12 no, MgCl2 is not purely ionic. It has some covalent character (Mg is a small ion with a relatively high charge, therefore polarizing)
14 What has dissolving got to do with anything? The reaction 2 is strongly exothermic as the CaO reacts with the water
17 sulphur burns in excess oxygen to form SO2. It can't be oxidised further without catalytic help
37 burning is exothermic, yes. But that's not the question. They are asking why nitrogen and oxygen don't react at RT