Two electrodes are immersed in a 1 M HBr solution. One of the electrodes is a silver wire coated with a deposit of AgBr(s). Th e second electrode is a platinum wire in electrical contact with a mixture of metallic mercury and Hg₂Br₂(s). Voltage measurements show that the Pt electrode is positive.

(a) Write balanced half-reactions for the two electrodes.

(b) Write the equation for the spontaneous cell reaction.

(c) In which direction do electrons flow in the external circuit?

(d) Why is a salt bridge unnecessary in this cell?