Is the prayer valid with a chemical spot on the skin that prevents water from reaching it, and is it obligatory to repeat it?

The drop that fell on your hand is pure unless you are certain of its impurity, as the default state of things is purity. Your prayer is valid. If the drop does not prevent water from reaching the skin, the ablution is sufficient.

If it does prevent water from reaching the skin, it must be removed for ablution, unless its removal would cause harm or difficulty, such as removing a piece of skin. In that case, it is sufficient to pass water over it or wipe it, and your prayer is valid, and you are not required to repeat it.