[GroceryStore]

At school(high school), to calculate the voltage needed for electrolysis, we calculate the voltage produce if the elements were in a galvanic cell, preceded by a minus. So if a galvanic cell with Cu and Cr would produce 1,14V, we would say that an electrolysis with Cu and Cr will need -1.14V.

But why do we do this and is this correct?

Thank you in advance.

[Enthalpy]

Welcome, GroceryStore!

This computation is grossly idealised. And whether the sign changes is a matter of convention, here they aren't uniform. Maybe they're within electrochemistry, but they don't fit electrical engineering for instance.

Cr is an extremely bad choice. It makes an insulating oxide layer.

You need a voltage drop in the electrolyte to let a current flow. Not wasting too much voltage in this loss is seriously difficult in industrial applications, because most batteries or electrolysis cells need a big current density.

Among metals of good will (Cu, Au, Mn... not Ta, Nb, Ti, Al, Cr) the redox potentials apply more or less, but only careful experiments can mimic what the theory claims. Beware if a metal has varied ions too. And it needs the proper electrolyte so the expected reactions happen.