A day at school
A few days ago a colleague and I went to a secondary school to deliver a workshop for 12-year-old pupils as outreach ambassadors, aka “Alchemists“, of the Department of Chemistry at the University of Oxford. It was a practical demonstration of how to assemble a simple Daniell cell, and how to stack a few of them into a battery. It is a battle-tested hands-on activity that never fails to fascinate pupils of this age bracket, and this time it went down particularly well. Moreover, this workshop is a perfect way of “taking apart” that familiar object called battery and break it down into its components, to visualise how chemical reactions, if properly chosen, can generate electricity.
Science laboratories of secondary schools always make me wince as I walk in: being used to the specialised facilities of the technical school I attended, I always have the impression that general science laboratories are under-equipped, or a bit confusing, stacking trays of dissection equipment on top of chemistry glassware, next to models of human organs. Well, maybe I am simply too picky a chemist…anyway, the good thing about this workshop is that it requires very basic glassware and equipment. Instead of the classic Daniell cell, with its metal electrodes, the two jars of copper and zinc sulphates, and the salt bridge, a miniaturised sandwich-like setup is used: the bottom slice of bread is copper, then there is a filling composed of three layers of filter paper, in this order: filter paper impregnated with CuSO4, plain filter paper (aka a makeshift “ion exchange membrane” acting as a salt bridge), filter paper impregnates with ZnSO4, and finally zinc as the top slice of bread. Held together by a peg, this simple sandwich cell will work perfectly, generating a potential of ca. 1 V after being immersed into tap water (compare to the potential of 1.1 V for the original Daniell cell and 1.5 of a typical AAA battery). The cell reaction is:
Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)
The most delicate part of this demonstration, when the pupils’ enthusiasm drops visibly (and dangerously), is when the cell is first assembled and it is still dry…sandwiches are often not as tasty without sauce, and this workshop definitely proves it! Typically, pupils quickly stack the cell components and then, curiously, rush to measure the potential…only to discover that the voltmeter shows three zeroes, and a blinking minus sign in front. The pupils’ expressions often betray bitter disappointment, and this is when the demonstrator must step in and motivate them to move on. By the way, measuring the potential under “dry” conditions is meant to introduce the concept of “blank” measurement, before the actual experiment begins.
Luckily, dipping the sandwich into water makes the scientists-to-be buzz with excitement: the dripping cell becomes a living thing (water is the basis of life after all!), the voltmeter finally showing nonzero potential. To the pupils’ surprise, the voltmeter reading is not stable, slowly climbing over time from 0.97 V to, usually, 1.01 V. I have always been fascinated by the pupils’ bewildered reaction to these changes in the value of potential: normally, they write down the first value they see right after connecting the crocodile clips of the voltmeter to the metal plates; then, despite noticing the increase in potential, they do not add any further notes on their activity sheet. I find it interesting and strange, because today’s young generations should be used to continuous change, being “digital natives” accustomed to the impermanence of the Web, the F5 “refresh syndrome”, and similar things. Could it be that the pupils instinctively regard numbers (in this case the first value of potential) as intrinsically correct, imbued with that aura of exactness typically associated with, for example, arithmetic operations, which admit only one right result?
Whatever the reason, the hands-on activity now goes on: the pupils swap the crocodile clips and write down the new measurement. They know which of the two leads is connected to the positive terminal of the voltmeter, and this second measurement should let them find out which of the two metals, copper or zinc, is the positive electrode. It is interesting to note that half of the pupils seem not to be familiar with the concept of negative numbers, because they fail to notice the “minus” sign in front of the potential shown during one of the two experiments. As a consequence, they struggle to understand which metal is the positive terminal.
Finally, several cells are stacked to make a battery. Thus, the workshop ends in a thrilling finale, with groups of pupils trying to stack as many cells as possible to achieve the highest potential. It is a chaotic but invigorating moment: pupils are so energetic, and focussed on the “race”, that there is absolutely no dip in their attention; rather, it is almost impossible to stop them when time is up. Yet, there is a last surprise. When pupils are asked to dismantle the sandwich, the zinc plate will invariably show a black patina: I always try and seize this opportunity to point out that this is evidence of a chemical reaction, the very process that allowed the cell to function. Zinc has corroded…corrosion, what a beautiful word, so visual: it shares the same etymological root as rodent, and the rough surface of corroded metal makes me picture to myself an horde of invisible, hungry mice gnawing away at the zinc as if it were a tasty piece of stale bread.
What is the structure of the black patina? That is a tricky question: the electrolyte in contact with the zinc surface is a concentrated sulphate solution containing oxygen (it is “aerated”); zinc corrosion under these practical conditions has attracted significant attention from corrosion scientists, collected in a book and countless publications. That said, I have skimmed through the literature and I have found out that the black layer is most likely a non-stoichiometric zinc oxide, i.e. including an excess of zinc (Zn1+xO).
Whatever the exact nature of this oxide layer, at least now I know the answer to one of my questions…
That’s a pupil’s electrifying shriek of surprise.
PS: An upcoming post will touch on Volta and the discovery of the battery – of course my own way!