Original question.
When a plate of copper, and a plate of amalgamated zinc are placed in a vessel containing dilute sulphuric acid, state what takes place (1) when the plates are not connected, (2) when they are connected by a wire.
With the electrodes disconnected, the copper has no sustained external route through which to receive electrons from the zinc. It is not appreciably attacked by a clean, dilute nonoxidizing sulfuric acid solution. Zinc can in principle reduce acid while dissolving, but amalgamation suppresses much of the local action associated with impurities and promotes a high kinetic barrier for hydrogen evolution on the amalgamated surface. Thus the intended historical observation is little visible action with the circuit open, not a thermodynamic assertion that zinc cannot react with acid.
Closing the external wire supplies an electron path. At the zinc anode,at the copper cathode,The net redox is thereforeThe zinc dissolves, and bubbles of hydrogen appear chiefly at the copper. Electrons flow through the wire from zinc to copper; conventional electric current flows oppositely there. Ionic transport through the electrolyte completes the circuit and prevents indefinite charge accumulation in the solution. The copper primarily provides a conducting surface: there is no stipulated copper-ion solution from which metallic copper must be deposited, so this is not the Daniell cell reaction.
The chemical energy supplies the electrical work. Accumulation of hydrogen and changing interfacial conditions can polarize the cell, reducing the current. A complete description therefore distinguishes the initial closed-circuit response from an indefinitely constant-current idealization. Electrochemical overpotential explains why a favorable reaction and its observable rate are different questions.
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