The following setup is designed to make potassium and fluorine gas according to the following equation. 2KF liquid reacts to form 2K liquid plus F₂ gas. What is missing in this setup? A) A battery, B) an ammeter, C) a salt bridge, D) a voltmeter, or E) a porous membrane.
In the diagram, we can see a pool of liquid potassium fluoride with a couple of electrodes, the anode and cathode, and a few wires that terminate in media. The two target products, liquid potassium metal and fluorine gas, are highly reactive. In fact, fluorine gas can be considered one of the most reactive substances known to exist. So these two substances would naturally react together, meaning that the breaking down of potassium fluoride salt into potassium and fluorine isn’t nonspontaneous process. In order to drive a nonspontaneous process, we need to put energy in.
Looking at the diagram, we can’t see any particular sources of energy. The anode is where oxidation would occur, and the cathode is where reduction will occur. But we don’t know the composition of these electrodes. So we can assume they’re not chemically active. And wires dangling in the air are going to have no effect on the reaction whatsoever. Where we see an anode and a cathode and an ionic liquid like this, we’re dealing with electrolysis, which means we need a power source to drive the reaction forward. So the thing missing in this setup is most likely a power source.
The only power source we’re given is a battery. The chemical potential stored in a battery can be converted into electrical energy to help drive our reaction forwards. An ammeter is a useful tool to have for a practical setup like this, but it only measures the current. It doesn’t provide power. So while useful, it isn’t necessary. Meanwhile, a salt bridge is a useful tool for completing the circuit, connecting two half-cells. In this case, we already have a complete cell where both anode and cathode are in the same medium. So we don’t need a salt bridge. Like an ammeter, a voltmeter is a useful tool to have for a practical setup like this. But it’s not necessary. It measures the voltage and doesn’t provide the energy we need to drive the reaction forwards.
Lastly, in electrochemical cells, a porous membrane might be used to separate reactive media, while allowing certain ions to move from one side to another. A porous membrane wouldn’t be necessary in this case, since we’re dealing with an ionic liquid. We’ll generate liquid potassium at one electrode and gaseous fluorine at the other. And the fact the electrodes are separated is all the separation we need. And, of course, a porous membrane is not going to provide the energy we need to drive the reaction in the first place. So this is our complete setup, where we could insert a battery that will drive the reaction.
The natural sound potential for this nonspontaneous process is highly negative at negative 5.8 volts. So we would need a battery, which could deliver more than 5.8 volts to drive it forward. At the cathode, we would see potassium ions absorbing electrons turning in to liquid potassium. And at the anode, we’d see fluorine gas being generated as fluoride ions sacrifice their electrons. And if we were using pure potassium fluoride salt to begin with, all this would have to be done in excess of 858 degrees Celsius to make sure the salt was molten. So all in all, not an experiment to be done at home. Nonetheless, we’ve identified the item missing in this setup is a battery.