In 2014, there were 19 races and 16 of them were won by Mercedes. Has there ever been a season where one team won all the races?
No. This has never happened. The closest any team has come to winning every race in a season was 1988, when McLaren won 15 of the 16 races, only failing to win the Italian Grand Prix when Jean-Louis Schlesser's lapped Williams collided with Ayrton Senna with two laps to go. Ferrari in 2002, and Mercedes in 2016, were the next-closest, winning all but two of the races in those seasons.