Hantel vs. Langhantel
While both words relate to weightlifting equipment, Hantel is the generic term for a free weight and typically refers to a dumbbell, whereas Langhantel specifically denotes a long bar, or barbell, that requires two hands.
Hantel
B1This word functions as the general term for a handheld weight, most commonly translating to dumbbell.
Ich habe mir ein neues Paar Hanteln gekauft.
(I bought myself a new pair of dumbbells.)
Für diese Übung brauchst du nur eine Hantel.
(For this exercise, you only need one dumbbell.)
Die Hantel ist mir zu schwer für den Bizeps.
(The dumbbell is too heavy for me for biceps.)
Achte darauf, die Hanteln langsam abzulegen.
(Make sure to put the dumbbells down slowly.)
Langhantel
B2This term specifically describes a barbell, which is a long metal bar intended for two-handed lifting exercises.
Er macht Kniebeugen mit der Langhantel.
(He is doing squats with the barbell.)
Bitte lade nicht so viel Gewicht auf die Langhantel.
(Please do not load so much weight onto the barbell.)
Beim Bankdrücken benutzt man normalerweise eine Langhantel.
(For bench pressing, one usually uses a barbell.)
Die Langhantel hat einen Durchmesser von 50 Millimetern.
(The barbell has a diameter of 50 millimeters.)
Summary
In short, use Hantel when referring to single-hand weights or dumbbells, and use Langhantel exclusively when talking about the long bar used for heavy compound lifts.