when chlorine gas is passed through potassium Bromide solution Potassium Chloride and bromine are formed which is a more reactive non metal bromine or chlorine ​

In a displacement reaction, more reactive element displaces less reactive element from its compound and from the above reaction we observed that, Chlorine displaces bromine from its compound (KBr) to form Potassium chloride (KCl), therefore Chlorine is more reactive non - metal.

The equation for above reaction is :

_____________________________

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.