Both sides entered Sunday's match at Ballymore knowing the winner would secure second spot and the loser would finish fourth.
The Force took an early 3-0 lead, but the Reds ran in seven tries to one from then on to secure the bonus-point victory.
Karpani was the star of the show with three tries and a series of bustling runs, while Lori Cramer nailed 6-of-7 conversions.
Queensland will host Fijiana Drua in a semi-final, with the Force to travel to Sydney to take on the ladder-leading Waratahs in the other semi.
The Brumbies were the only team to miss the finals after finishing fifth of five teams.
Sunday's only try for the Force came in the 29th minute when Adi Vani Buleki beat two tacklers down the right to touch down.
The scoreboard read 21-10 at that stage, but Karpari scored three of the next four tries to put the contest to bed.
"For me to get over the try-line, it's definitely a team effort. So hats off to the girls," Karpani told Stan when asked about her hat-trick.
"We (forwards) have really been trying to step up each week to give our backs a better, cleaner platform. Today was the day to step up."
The Force's results this season leading up to the final round hadn't been overly alarming.
They started their campaign with a 29-29 draw with the Brumbies, followed with a fighting 26-21 away loss to the Waratahs, and beat Fijiana Drua 47-31 last week.
But the way they capitulated against the Reds is a major concern heading to their semi-final showdown with the Waratahs.
"We'll take our learnings from that," Force captain Trilleen Pomare said.
"You can't win the game without the ball. We need to be better next week.
"If we don't take our learnings, it's going to be the same result.
"We don't have a home semi, that's something we've got to live by. We've got to put our head down and grit our teeth - that's what wins games."
15°C