So, if we have a single jar of sulfuric acid and dip a zinc wire into the acid, the acid that comes into contact with the zinc will react directly. But most of the acid is not in contact with the zinc. But if we pass a wire from the copper to the zinc, the electrons can pass from the zinc through the wire, through the copper, to the acid. The sulfate ions left over from the acid will diffuse toward the zinc electrode where they will balance the positive charge of the zinc ions as they come off the electrode. As long as the wire is a good conductor it will represent the path of least resistance for electrons to get from the acid to the zinc.
Some reactions have a large driving force.
They want to go really bad and will release a lot of energy when they do. For example charcoal, sulfur, and saltpeter release a lot of energy when they react as burning gunpowder. Other reactions are more timid, for example, the oxidation of glucose to ethanol. We can quantify this driving force the electromotive force , measured in Volts . The EMF for each half-reaction is called the standard reduction potential .