That is normal behaviour. Wireless charging is not 100% efficient and the coupling losses are radiated as heat. Optimal charging occurs when the antennas are parallel to each other, so you will get less heat if the phone is optimally oriented. And some brands are more efficient than others.
But it sounds like your figures are normal. Heat is detrimental to Li-Ion battery life. But this isn't an unusual or particularly excessive amount of heat.. so unless you want to charge your phone inside your refrigerator, there isn't much to do other than orient your phone squarely on the charger.
Also FYI, your temp app does not measure your phone's temperature per se. It measures the processor temperature. The S5 does not have an ambient temperature sensor.