In chemistry, the reactivity series is a series of metals, in order of reactivity from highest to lowest. It is used to determine the products of single displacement reactions, whereby metal A will replace another metal B in a solution if A is higher in the series.
Activity series of some of the more common metals, listed in descending order of reactivity.
|K||K+||reacts with water|
|Mg||Mg2+||reacts with acids|
|H2||H+||included for comparison|
When a metal in elemental form is placed in a solution of another metal salt it may be more energetically feasible for this "elemental metal" to exist as an ion and the "ionic metal" to exist as the element. Therefore the elemental metal will "displace" the ionic metal and the two swap places. Only a metal higher in the reactivity series will displace another.
A metal can displace metal ions listed below it in the activity series, but not above. For example, zinc is more active than copper and is able to displace copper ions from solution
Siver, on the other hand, cannot displace copper ions from solution.
It is important to distinguish between the displacement of hydrogen from an acid and hydrogen from water. Sodium is highly active and is able to displace hydrogen from water:
- 2 Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g)
Less active metals like iron or zinc cannot displace hydrogen from water but do readily react with acids:
- Zn (s) + H2SO4 (aq) ZnSO4 (aq) + H2 (g)
Those metals that can displace H+ ions from acids are easily recognized by their position above H in the activity series. The boundary between the metals that react with water and those that don't is harder to spot. For example, calcium is quite reactive with water, whereas magnesium does not react with cold water but does displace hydrogen from steam. A more sophisticated calculation involving electrode potentials is required to make accurate predictions in this area.
The reactivity of metals is due to the difference in stability of their electron configurations as atoms and as ions. As they are all metals they will form positive ions when they react.
Potassium has a single outer shell electron to lose to obtain a stable "Noble gas" electron configuration; the precious metals which exist in the d-block cannot form structures which are much more stable than their elemental state with the loss of just a few electrons. Metals that require the loss of only one electron to form stable ions are more reactive than similar metals which require the loss of more than one electron. Group 1A metals are the most reactive for that reason.
Metals with a greater total number of electrons tend to be more reactive as their outermost electrons (the ones which will be lost) exist further from the positive nucleus and therefore they are held less strongly.