Hey guys. I just need help with waves and galvanic cells.

Physics:

1) What causes the phases change of a wave when it reaches a fixed end?

2) What causes the reflection of a wave when it reaches a free end and why is there no phase change?

Chemistry:

1) Why is Na⁺ and NO₃ and other substances like that considered to be inert?

2) Why do we need to keep the electrode in an electrolyte of the same element?