Silver reacts with moist air to form black Ag2S, but your reactions don't show H2O as a reactant, but as a product. Why?
Silver tarnish is the discoloration that occurs on silver. Silver is not appreciably affected by dry or moist air that is free from ozone, halogens, ammonia, and sulphur compounds. The presence of hydrogen sulphide in any material that silver comes into contact with is one of the prime reasons for silver tarnish. The hydrogen sulphide reacts with the silver to form silver sulphide.
2Ag + H2S --> Ag2S + H2
Silver sulphide is black. When a thin coating of silver sulphide forms on the surface of silver, it darkens the silver. Rubber contains sulphur, which will cause silver to tarnish. Certain foods like eggs, mayonnaise, mustard, table salt, olives, salad dressing, vinegar, fruit juices and onions also hasten the silver tarnish process. The sulphur in these foods will corrode silver. Flowers and fruits can etch the silver containers due to the acid produced as they decay.