RAID5 will have at least one spare, there's no EQL RAID5 policy that allows for no spares.  Yes, when a drive fails, e.g. drive 5.   The spare drive will take its place and when you replace drive 5, that will become the new spare.  Which slot holds the spare is not important. 
Good morning,       I have a PS4000, model 70-0120, with 16 drives. One drive (14) went into alert and said to replace. We replaced the ST3500320NS (500Gb) with a ST31000340NS (1Tb) and all looked good. Once I went into the web console, I see that the drive is showing spare instead of online.      When all the drives were healthy, they all showed online and none was marked spare. Shouldn't the new drive show online? Is there something I need to do? Many thanks.
PS4000 - New drive showing slave, not online