Why should we believe in the reactivity series? Teacher notes

Education in Chemistry September 2020 rsc.li/2ZJsXq6

Experiment Observations What hypothesis is How much does this evidence confirmed by the support the overall theoretical evidence? idea of a ‘reactivity series’? Is the experiment ‘good’ evidence for the hypothesis?

Add Cu to a solution Example: The copper Copper is more reactive Not very much: only two metals of conc. AgNO3 gets smaller and silver than silver. were used and this is only one way forms on the surface. The experiment has only to compare their reactivity. The colourless solution been completed once, so turns blue. might not be reliable.

Add Li, Na and K to The reactions become The reactivity of K is more It shows the order of reactivity for water and repeat more vigorous down than Na, which is more than three elements, so some support is each experiment the group. Students Li. This experiment provided because it allows for three times should be able to give supports the hypothesis direct comparisons. accurate experimental that reactivity increases details. down Group 1. This would be good evidence for the former (the experiment was carried out reliably) and provides some evidence for the broader hypothesis.

Burn Li, Mg, Fe, and Li: red flame, brighter The trend in reactivity is not Very limited evidence: the trend is Cu in air and in in O2 very clear from these not clear, and the brightness of the oxygen Mg: bright white light, experiments because Mg magnesium flame challenges the very intense in O2 appears to give out more proposed order of reactivity. Fe: glows in air, bright energy than Li. At best this You can get students to discuss sparks in O2 confirms the hypothesis ways of making this experiment Cu: no reaction that Cu is not very reactive. more useful, eg using a calorimeter The comparison of air and or spectroscope. oxygen is a red herring in this case because the difference is not relevant to the reactivity series.

Count the bubbles Order from most to That the order of reactivity The experiment shows the order of produced in the fewest bubbles: for these four metals is Mg, reactivity for four of the elements, reaction of metals Mg, Zn, Fe, Cu (no Zn, Fe, Cu. so it does provide some support (Fe, Mg, Zn, Cu) with bubbles) This experiment would because it allows for direct dil. hydrochloric acid provide some quantitative comparison. But this experiment is data, which can make it less reliable than the Li, Na and K easier to compare the experiment above. vigour of the reaction more objectively. However, the reliability is limited.

 Royal Society of Chemistry Page 1 of 2 Registered charity number 207890 Experiment Observations What hypothesis is How much does this evidence confirmed by the support the overall theoretical evidence? idea of a ‘reactivity series’? Is the experiment ‘good’ evidence for the hypothesis?

Add Mg, Zn and Fe Copper will form on It confirms that Mg, Zn and Fairly limited evidence: the to a solution of the surface of the Fe are all more reactive experiment only confirms that Mg, CuSO4 metal in all three than Cu, but not the order Zn and Fe are more reactive than cases. The blue of reactivity. [All this Cu, but doesn’t allow for any solution will become experiment really does is comparison between them. colourless for Mg and confirm that these metals This is another experiment in Zn, but yellow for Fe. displace copper, the idea which students could be pushed to that a more reactive metal give ideas about how the displaces a less reactive experiment could provide better metal is another theoretical evidence of the idea of a ‘reactivity idea.] For this limited series’, eg by measuring the hypothesis the experiment temperature changes in each provides fairly good reaction and comparing how much evidence but it needs to be heat energy is given out. repeated.

1. Are there any aspects of the reactivity series that are not supported by any of the experiments?

Yes! Firstly, Ca, Al, Au and Pt all appear in the reactivity series but not in any of the experiments, so assigning them a position cannot be achieved on the basis of this evidence. Secondly, there is no way to compare the reactivity of the Group 1 metals with Mg, Zn, Fe, Cu and Ag, so from this evidence you cannot work out if they are all more reactive than Mg, if there is overlap, or if they are much less reactive. Comparing the reactivity of water and acid draws on another theoretical idea – that metals react more vigorously with acid than with water – which has not been established in these experiments.

4. Do you think you should ‘believe’ in the reactivity series? Why, or why not?

Some examples of ideas students may have about whether or not to believe in the reactivity series:

For Against Ambivalent

The use of a range of different The reactivity series cannot be The reactivity series is useful, it experiments to confirm the conclusively proved because no doesn’t matter whether it is ‘true’ hypothesis makes it more likely to one experiment can be used to or not, what matters is being able be ‘true’. compare all of the different metals to use it to solve real world Scientists have carried these directly. problems. experiments out thousands of Working out the reactivity series times, we don’t need to question required chemists to use different their reliability. theoretical ideas, which haven’t been proved either.

 Royal Society of Chemistry Page 2 of 2 Registered charity number 207890