I would suggest to use banana jacks instead of a 3.5mm jack. The jack itself might be able to handle all the wattage you need , but the wires connected to the jack , going to your speaker would most likely be very thin, so you'd need the proper wires along with the jack. The longer the distance to drive your loudspeaker on thin wires, the worst the result will be. If you're driving the loudspeaker really high and too much current goes through it, a lot can happen. Cheap 3.5mm cables will most likely warm up, solder joints could melt or it could simply change the impedance of your setup so much that the sound is bad, with feedback. You could also just use a barrel connector this way you are sure it can handle all the current/power you need. Quality banana jacks can handle a lot more power than 3.5mm jacks with currents usually up to at least 15A. 6.35mm (1/4) jacks that are heavy duty can handle currents up to 15A.
Just looked a few datasheets on jacks from digikey , and most of the state: max voltage 12v , max current 1A . So that's 12 watts.