What makes Jem so sure that Tom will be acquitted in To Kill a Mockingbird?

Jem thinks that Atticus has adequately proven that Tom Robinson did not attack Mayella Ewell. 

The trial of Tom Robinson is very divisive in Maycomb.  Many people automatically feel that Robinson is guilty, because he is a black man and she is a white woman.  A lot of people are angry at Atticus for defending a black man.  

Jem is old enough that he understands much of what is happening at the trial.  His father has taught him to respect all people, so he is not a racist.  He watches the trial with interest because he wants to be a lawyer, and because he respects his father.  He believes that his father is doing a fine job defending Robinson, even though it is an uphill battle.

Scout watches too, but she is not as convinced as Jem.  She believes he is “counting chickens” to assume that Robinson will be acquitted. 

Atticus was trying to show, it seemed to me, that Mr. Ewell could have beaten up Mayella. That much I could follow. If her right eye was blacked and she was beaten mostly on the right side of the face, it would tend to show that a left-handed person did it. (Ch. 17) 

The crux of Atticus’s case is that Mayella was attacked by a left-handed person and Tom Robinson is not left handed.  When we learn that Tom Robinson is a cripple, and can’t even use his left hand, that seems to be enough for Jem.  He believes that his father has won.  He has proven that Tom Robinson could not physically have attacked Mayella Ewell, because he is not left-handed (and also doesn’t have the use of his left arm). 

Scout comes in halfway through Atticus’s speech to the jury, and asks Jem what he said.  Again, Jem is convinced that Atticus has won. 

“He’s just gone over the evidence,” Jem whispered, “and we’re gonna win, Scout.

I don’t see how we can’t. He’s been at it ‘bout five minutes. He made it as plain and easy as—well, as I’da explained it to you. You could’ve understood it, even.” (Ch. 20) 

Jem asks Atticus if they won, and he says he doesn’t know.  When discussing the case with Mr. Sykes, Jem is completely confident.  Sykes tells him not to be so sure, because he has never seen “any jury decide in favor of a colored man over a white man” (Ch. 21).  Although the jury does deliberate, they return a guilty verdict.  

Jem is horrified by the verdict, feeling it almost physically.  As far as he was concerned, there was no way they could have lost.  It was a terrible miscarriage of justice.  Unfortunately, in Maycomb justice is still divided by black and white.