One of the final steps of copper refining is electroplating copper onto a copper electrode. When the deposited copper comes from a copper sulfate solution, such as that produced from sulfuric acid leaching, the process is called electrowinning. In electrowinning, a lead anode and copper cathode are immersed in a bath of copper(II) sulfate, and a potential difference is applied. What is the half-equation of the reaction that occurs at the cathode?
Before we answer this question, let’s review the process of electrowinning and the steps that lead up to it. Copper is first mind as copper oxide ore, which doesn’t have a very high percentage of copper in it. But we want pure copper so we can use it to make things. So, we need to somehow remove this copper from the copper oxide ore. In a process known as sulfuric acid leaching, sulfuric acid is then added to the copper ore. The sulfuric acid causes the copper in the copper oxide ore to be dissolved, which can then be collected in a copper(II) sulfate solution.
Now that the copper from the ore is in the copper(II) sulfate solution, we can move on to the final step of the copper refining process, which is electrowinning. In electrowinning, the copper(II) sulfate solution is added to an electrolytic cell. In an electrolytic cell, we use a battery to apply an electrical current, which makes electrons flow, which powers a reaction in the nonspontaneous direction.
In the electrolytic cell used for electrowinning, we have a lead anode and a copper cathode. As a reminder, oxidation occurs at the anode, that is chemical species lose electrons there. And reduction occurs at the cathode, that is chemical species gain electrons there. During the process of electrowinning, copper two plus, from the copper sulfate solution, is plated onto the copper cathode. So we gain pure copper, which is the ultimate goal of the copper refining process.
Now that we’ve reviewed the process of electrowinning, let’s address the question. We want to know the half-equation of the reaction that occurs at the cathode. We know that at the cathode, reduction is occurring. And we also know that copper two plus, from the copper(II) sulfate solution, is being plated onto the copper cathode as pure copper. The chemical equation that describes this process is copper two plus aqueous plus two electrons forming copper solid. We can confirm that this is the correct equation because copper is gaining electrons and its oxidation state is being reduced from two plus to zero.
What is the half-equation of the reaction that occurs at the anode?
We know that at the anode, oxidation is occurring. And that some chemical species is being oxidized to provide a source of electrons for plating the copper at the cathode. Our lead anode is inert, so lead is not the chemical species that’s being oxidized. This means that the chemical reaction that must be occurring at the anode is the electrolysis of water, or H₂O liquid. Forming 2H⁺ aqueous plus one-half O₂ gas plus two electrons.