how come maleic acid is more acidic than fumaric acid?

Answer 1

Since in maleic acid both carboxyl groups are at the same side of the molecule, while in fumaric acid they are in opposite sizes. In maleic acid, after the first H+ dissociate, the negative ion is much stable due to both oxygen atoms may have strong inetraction with the left H atom. In another word, the first H+ is much easier to dissociate. With the same reason, the second H+ would be more difficult to dissociate in maleic acid than in fumaric acid. Therefore, since the first H+ in maleic acid is much easier to dissociate, maleic acid is more acidic than fumaric acid.

Answer 2

Fumaric Acid Pka