Why doesn’t the computer like my odd queen trade?

Question

I played a game as Black some months ago on chess.com in which I made a rather amusing, in my opinion at the least, queen trade. I won the game in the end, but only after being a bastard and promoting to a bishop twice.

[Title "kassabian-Rewan Demontay, Friendly June U1200, chess.com, June 10-July 1, 2019"]
[FEN ""]
[startply "33"]

1. e4 e5 2. d4 exd4 3. Qxd4 Nc6 4. Qd1 d6 5. b3 Be6 6. Bb2 Nf6 7. Bxf6 Qxf6 8.  h4 Be7 9. Nd2 O-O-O 10. g3 d5 11. Rb1 dxe4 12. f4 exf3 13. Rh2 Bg4 14. Rf2 Bc5  15. Rxf3 Bxf3 16. Ngxf3 Rhe8+ 17. Be2 Qxf3 18. Nxf3 Rxd1+ 19. Kxd1 Nd4 20. Nxd4  Bxd4 21. Bg4+ Kd8 22. Kd2 Bf2 23. Rd1 Bxg3 24. Kc3+ Ke7 25. Rd3 Be5+ 26. Kc4 Kf6  27. b4 Re7 28. Rf3+ Kg6 29. Kd3 h5 30. Bf5+ Kh6 31. Be4 b6 32. a4 f6 33. Rf2 Bg3  34. Rg2 Bxh4 35. Rg6+ Kh7 36. Rxf6+ Kg8 37. Bd5+ Kh8 38. Rf8+ Kh7 39. Be4+ Kh6  40. c4 Bf6 41. Ra8 g5 42. Rxa7 g4 43. b5 g3 44. c5 h4 45. cxb6 cxb6 46. Ra6 Rd7+  47. Ke3 Bg5+ 48. Kf3 Rf7+ 49. Ke2 Rf2+ 50. Ke1 Bd2+ 51. Kd1 Ba5 52. Ra8 Kg5 53.  Rg8+ Kf4 54. Rf8+ Ke3 55. Rxf2 Kxf2 56. Bd5 g2 57. Bxg2 Kxg2 58. Ke2 h3 59. Kd3  h2 60. Ke4 h1=B 61. Ke5 Kf2 62. Kd6 Ke3 63. Kc7 Bd5 64. Kb8 Bb3 65. Ka7 Bxa4 66.  Ka6 Kd4 67. Kb7 Bxb5 68. Ka7 Kc5 69. Kb7 Be8 70. Ka6 Be1 71. Kb7 b5 72. Ka6 b4 73. Ka5 b3+ 74. Ka6 b2 75. Ka7 b1=B 76. Kb7 Bc6+ 77. Ka7 Kd6 78. Kb8 Kd7 79. Ka7 Bf2+ 80. Ka6 Bd3+ 81. Ka5 Be1+ 82. Kb6 Bb4 83. Ka7 Bcb5 84. Kb6 Kc8 85. Ka7 Bc5+ 86. Ka8 Be4#

The computer gives a question mark to my move ‘17... Qxf3?’. Why is this so? The only downside to it is that I lose an exchange, but on the other hand the game becomes more simplified and easier to play. Is the loss of an exchange the reason, or are there other factors at play?

Additionally, why doesn’t the engine suggest 36. Rg4+ to pick up Black’s dark sqaure bishop? And why is 36... Rex4 the best move versus Kg8 or Kh8?

Answer 1

Losing an exchange is itself enough to make the move weak. You go from being up R+P for N, which is a winning advantage, to up just one pawn with opposite color Bishops, which could often be drawn.

The move is also weak because you let White off the hook: you were on the attack against White's exposed King, so the trade of Queens made the position "easier to play" -- for White. Instead of giving up material just to trade your active Qf6 for White's passive Qd1 (it has only one move!), try involving the in the attack your Nc6 (your only piece not already aiming at the Ke1), or trading it for one of White's defenders. Either 17...Ne5 or 17...Nd4 should win quickly:

17 ... Ne5 actually threatens the Nf3: after Nxf3+, both defenders (Nd2,Be2) will be pinned. What's more, the Nf3 itself is "pinned" in that if it moves (say 18 Nxe5) then 18 ... Qf2 is mate! White has no real defense; if 18 Kf1 then simply Nxf3 19 Bxf3 Rxd2 20 Qxd2 Qxf3+ and mate.

17 ... Nd4 adds the threat against the Be2; now White can defend for a move with 18 Nxd4, but 18 ... Qxd4 restores the threat Qf2# and White might as well resign (19 Qc1 Qf2+ 20 Kd1 Qxe2#).

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After 36 Rxf6+? Rxe4 simply wins a piece: if 37 Kxe4 then Black takes on f6, and if the Rf6 moves then Rxb4. The alternatives "Kg8 or Kh8" are not equivalent: 36 ... Kh8?? would have allowed 37 Rf8#!

Answer 2

The computer says you're ahead after the move you played. But you're only up a little, and you ended up with only a rook and opposite-colored bishops along with the pawns. You still have to work for the win; it's not like the extra pawn ices the win for you in the simplified position.

The reason that the computer considers it a mistake is that you had much better moves you could have played. Most of White's pieces are either on the back rank or pinned, and tactics are all over the place.

Look at 17...Bb4, for example. It hit the pinned knight (and pins it again!) and Qxf3 is a real threat now because that knight can't recapture. If White tries Kf1, the d2 knight falls because the f3 knight becomes pinned. Or, did you consider 17...Rxd2? If Nxd2, Qf2#. If Qxd2, Bb4 pins and wins the queen.

Additionally, why doesn’t the engine suggest 36. Rg4+ to pick up Black’s dark sqaure bishop? And why is 36... Rex4 the best move versus Kg8 or Kh8?

After 36.Rg4+ Rxe4 Rxe4, White wins an exchange rather than a bishop. And Black has three connected passed pawns, which will be hard to stop. It still might be the move I'd play, though.

After 36.Rxf6+ Rxe4 Black wins the bishop outright. (Obviously not 38...Kh8?? Rf8#.) White can either save his rook or capture Black's; he can't do both.