For the first part, I would say on any molecule such as CH2ClCOOH, if the halogen Cl is substituted by any other halogen, then the relative acidity depends upon the -I effect (where I stands for inductive). Say, if Cl is replaced by F, acidity increases, while it decreases if Br replaces Cl (F exerts a more powerful -I effect than Cl, whereas Br exerts a less prominent -I effect than Cl).
Sorry, I have no idea about the second part!