In the 3rd plague, Ex 8:19 says "But Pharaoh's heart was hard", in the 4th plague Ex 8:32 says "But this time also Pharoah hardened his heart".
So, for the first 5 plagues, Pharoah's harden his own heart, in plagues 6 - thru - 10, God harden an already hard heart (to accomplish His purposes)
So, for the first 5 plagues, Pharoah's harden his own heart, in plagues 6 - thru - 10, God harden an already hard heart (to accomplish His purposes)

