Chemistry A level revision resource: Measuring electrochemical cells
Electrochemistry is sometimes considered one of the more challenging areas of chemistry, but it doesn’t have to be confusing. Just think of the reaction as completing an electric circuit. It is a really important subject to study as we can use electricity to split chemicals to make more useful products, and there is a lot of current research that uses electrochemistry to sense diseases.
What is standard electrode potential?
As part of your A Level course you are expected to know what the standard electrode potential is and how to measure it using the hydrogen electrode. Now that may all sound like scientific jargon, but we’ll take it one step at a time.
The standard electrode potential is the force needed to push electrons in order for an electrochemical reaction to take place. In other words, it is how negative or positive an electrode has to be for the electrons to move between it and the solution to complete the circuit. The standard electrode potential is measured in volts and is written like this:
For reasons you do not need to know at A Level you cannot directly measure standard electrode potential. It is very difficult to get an exact value for the real potential, so we measure it against a known standard. It’s a bit like looking at a tower which has a tree next to it, unless you’re a genius you would not be able to tell exactly how tall the tower was, but you could say how many times bigger the tower was than the tree next to it. Our tower is the reaction electrode, for example zinc in zinc sulphate, and our tree is the standard hydrogen electrode.
The standard hydrogen electrode is quite complicated to set up but basically consists of a very clean platinum wire in hydrochloric acid (normally but it can be others). The half-cell reaction is:
We treat the potential of this reaction as if it was 0 V at room temperature and pressure. If we measure the standard potential of the reaction we are interested in at the same time as we measure the standard hydrogen electrode system, we obtain the difference between those two reactions. This can be done by attaching a voltmeter between the two electrodes. This difference in potential is then what we record as the standard potential electrode for the reaction we are interested in.
How do I measure this in the laboratory?
How was hydrolysis discovered?
It had been discovered in the late 1700s that water was made up of hydrogen and oxygen. This had been done by exploding samples of the gases in close proximity to each other and noticing that drops of water were formed. Now this was pretty dangerous but it did suggest that there must be a way to make the pure gases from water which, of course, is highly abundant.
In 1799 Alessandro Volta (the originator of the ‘volt’) designed and tested the first simple electrochemical cell which worked in a very similar way to the one described in the video above. Two scientists, Sir Anthony Carlisle and William Nicholson, who were excited by Volta’s new system, decided to build their own and see what it could do. They connected it to two platinum wires, acting as electrodes, which were dipped in tub of water spiked with a few drops of sulphuric acid. They collected the gases produced at each electrode and found they had separated water into its simplest components: hydrogen and oxygen.
The acid acts as an electrolyte. An electrolyte is a species that can carry charge and the acid does this by separating into hydrogen and sulphate ions. This facilitates the electrical current and acts like a liquid wire, completing the circuit.
The two platinum wires act as the cathode and anode. Negative charge builds up at the cathode and positive charge at the anode. One way to remember this is PANIC (not literally): Positive Anode, Negative Is Cathode. Opposites attract so the positive hydrogen ions in the water move to the cathode which allows the charges to cancel out and form hydrogen gases. The same process occurs at the anode to produce oxygen.
Laboratory Confessions
In the Laboratory Confessions podcast researchers talk about their laboratory experiences in the context of A Level practical assessments. In this episode we look at the safe handling of solids and liquids and setting up an electrochemical cell.
What are the applications of electrochemistry?
Use of electrolysis to separate water is now rarely used in the production of hydrogen, since extracting hydrogen from methane is much more cost effective. However, this process relies heavily on fossil fuels so as we look to use our planet's resources more responsibly, industrial scale electrolysis for hydrogen production might become much more widely used. Despite this, the general use of electrochemical methods, largely based on Volta’s original cell, are wide ranging. We can plate metals, detect diseases and provide power using electrochemistry. Even our bodies and plants function based in its principles so it’s essential we understand the basics then we can apply this information to other areas of research.