In exchange for Kempny, the Blackhawks received a conditional 2018 third-round pick from the Capitals, who made the deal shortly before their game at Buffalo and a week before the NHL trade deadline.
The Capitals will part with the higher selection between their own third-rounder and the Toronto pick they acquired from New Jersey as part of the Marcus Johansson trade.
"We see as the season goes on and the games get more intense, you need depth," Trotz said. "And no one can have enough depth on defence."
The Capitals began the day in second place in the Metropolitan Division standings, and are injury-free in concluding a four-game road swing.
The 27-year-old Kempny had a goal and six assists in 31 games in his second NHL season, both with Chicago. He can become an unrestricted free agent after this season.
The Capitals got a firsthand look at Kempny, who had an assist and was credited with two blocked shots in a 7-1 loss at Chicago on Saturday.
"He’s a real good skater and he’s a little bit more of a veteran player," Trotz said.