Midfielder Capela netted the decisive goal after 12 minutes to secure all three points for Rui Quinta's side, his first for the club since arriving from Academico Viseu.
Arouca pushed for an equaliser but, even after the visitors were reduced to 10 men following Tony's red card, they were unable to find a way through.
The win lifted Penafiel to within two points of Arouca, while Gil Vicente dropped to the foot of the table.
Pacos de Ferreira missed the chance to move into the top six as they fell to a 2-0 defeat against Moreirense.
Andre Simoes gave the hosts the lead from the penalty spot in first-half stoppage time, before Rodrigo Battaglia secured the win with nine minutes to play.
Meanwhile, Vitoria Guimaraes were made to rue missed chances, as they failed to overtake second-placed Porto, after a goalless draw with Sporting Braga.
An ill-tempered game for both teams have men sent off, with Bruno Gaspar and Hernani seeing red for Guimaraes and Salvador Agra following them from Braga.
