The goal came when Jose Kante played a pass into the area to Schalk, who had only been on the pitch five minutes, and after taking a touch to control with his left foot, he shot with his right past the onrushing keeper Rodolfo Cota.
Leon's chances of a comeback at the Prince Abdullah Al-Faisal Sports City Stadium were dealt a hammer blow six minutes later when captain William Tesillo was shown a second yellow card and the Mexican club, CONCACAF Champions Cup winners, were reduced to 10 men.
This was Leon's first appearance at the Club World Cup, while Japan's Urawa, Asian champions, finished third in the competition in 2007 and also took part in 2017.
They will face Europe's Champions League winners Manchester City on Tuesday.
The other semi-final, on Monday, will see Copa Libertadores champions Fluminense take on the winner between Al Ahly of Egypt and Saudi Arabian club Al-Ittihad, who play later on Friday.
