Why Does My Copper-Zinc System Show Unexpected EMF and Precipitation Results?

[ngkamsengpeter]

I conduct an experiment to find the emf of the copper-zinc system . I connect the copper to the positive
terminal of the batery and a potentiometer and the zinc plate to the galvanometer.The galvanometer is connected
to a jockey . The battery I use is 3V .I touch the jockey to the potentiometer so that the galvanometer show
no current but suprisingly , no matter where I move the jockey , the galvanometer show only a litte change(smaller
than the smallest scale of the galvanometer) . Is it true ?

I also try to connect a voltmeter across the copper-zinc system to check the emf of the system . The voltmeter show
only about 0.5V but from the calculation it should be around 1.1 V . What wrong with my experiment .

The solution I use is 2.0 mol/dm3 copper(II) sulphate and 1.0 mol/dm3 zinc(II) sulphate .

Because of unsuccessful conducting the experiment , I tried to dip the zinc plate to the copper(II)sulphate with the
copper plate still inside the copper(II) sulphate . Something black solid precipitates at the zinc plate . What is
this solid .

 

[Astronuc]

Please write the anodic and cathodic reactions one would expect.

Usually, if one does not get the expected voltage, it can mean that another reaction is taking place.

Something black could be CuS or Cu₂O.