A plate-shaped heat exchanger (20, 120) for a heat exchange unit (40) of a chemical reactor (60), that advantageously presents a thermal insulation obtained in an unusually simple and reliable manner, has a substantially flattened box-like structure (22), with a substantially parallelepiped, rectangular configuration, defining an internal chamber (24), and comprises an inlet connection (28) and an outlet connection (29) of a heat exchange operating fluid into and from said chamber (24), and a distributor pipe (10, 110) of said operating fluid inside said chamber (24), extended in said structure (22) at a long side (22a) of it, said distributor pipe (10, 110) comprising a first tube (30, 130) and a second tube (32, 132), positioned one inside the other, between said tubes, respectively external tube (30) and internal tube (32, 132), an interspace (30a) being defined in fluid communication, on one side, with said chamber (24) through a plurality of openings (26) provided in the external tube (30, 130) of said distributor pipe (10, 110), and, on the other side, with the internal tube (32, 132) of the same distributor pipe (10, 110), said internal tube (32, 132) being hydraulically connected to said inlet connection (28) for the heat exchange operating fluid.