Video: Calculating the Amount of Ag⁺ That Can Be Reduced to Ag Using the Same Charge Used to Reduce a Given Amount of Cu²⁺ to Cu

What is the number of moles of Ag⁺ that can be reduced to Ag metal by the same amount of charge used to reduce 1.0 mol of Cu²⁺ to Cu? [A] 0.5 mol [B] 1.0 mol [C] 2.0 mol [D] 4.0 mol [E] 8.0 mol


Video Transcript

What is the number of moles of Ag+ that can be reduced to Ag metal by the same amount of charge used to reduce 1.0 mole of Cu2+ to Cu? (A) 0.5 mole, (B) 1.0 mole, (C) 2.0 mole, (D) 4.0 mole, (E) 8.0 mole.

In the first part of this question, we’re being asked about silver ions that undergo a reduction process to produce silver metal. In chemistry, oxidation and reduction processes refer to reactions where electrons are either lost or gained. These two processes are best remembered by using the mnemonic “OILRIG.” Oxidation and reduction processes always happen together in a chemical reaction. A redox reaction is therefore a chemical reaction, where one species is reduced, whilst another species is simultaneously oxidized.

If a silver ion is to turn into a silver atom, it needs to gain one electron. This is indeed a reduction process as reduction is defined as the gain of electrons. Where the charges come from makes more sense if we compare the number of protons and electrons in a silver ion and a silver atom. The atomic number of silver is 47. This means that both the silver ion and the silver atom have 47 protons in their nucleus. Both of these species contain 47 units of positive charge associated with these protons. Since a silver atom contains an equal number of electrons, each with a negative charge, the atom has zero net charge overall.

However, since a silver ion is a silver atom that has lost one electron, it only has 46 electrons versus 47 protons. There is therefore a net charge of positive one overall on a silver ion. If we scale this reduction process up to mole quantities, we can see from our balanced equation that one mole of silver ions will require one mole of electrons to form one mole of silver atoms. The reduction of silver ions to silver metal is frequently used in an electrolytic cell where objects are to be silver plated. A piece of solid silver is collected to the positive terminal of a DC or direct-current power supply.

Silver atoms leave this solid silver anode, and electrons flow around the external circuit to the negative electrode where the object to be plated is placed. The silver atoms become silver ions. These enter the solution. And this is the site of oxidation. The silver ions migrate through the solution to the negative electrodes or the cathode. Here, the silver ions pick up electrons from the external circuit, and they are reduced to form silver metal. The object becomes plated. Exactly how much silver gets plated onto the cathode depends on the amount of charge that is passed through the circuit. Charge is carried by the moving electrons in the circuit.

If we allowed one mole of electrons to pass through the circuit, we would in fact deposit one mole of silver atoms. One mole of electrons carries a charge of exactly one faraday. This is approximately 96485 coulombs, which is the unit for charge. If we want to reduce copper two plus ions to copper metal, we need each copper two plus ion to gain two electrons to become a neutral copper atom. Again, if we scale this up to more quantities, we see that one mole of copper two plus ions will need to gain two moles of electrons to become a mole of copper atoms. Per mole of copper ions, double the amount of charge is required for this process compared with one mole of silver ions.

So for a fixed number of electrons or the same amount of charge as specified in the question, double the moles of silver plus ions will be reduced compared with the moles of copper two plus ions. So if one mole of copper two plus ions were to be reduced, as in the question, this would require a charge of two faradays or two moles of electrons. Two faradays of charge, or the charge equivalent to two moles of electrons, has the potential to reduce two moles of silver one plus ions to silver metal.

Two moles of Ag+ ions is therefore the correct answer.

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