Reactions of Metals and Non-metals: Building an Activity Series

Why is gold jewellery still shiny after 3000 years?

An iron nail left out in a snowy Ontario winter is covered in orange rust by spring. A copper roof, like those on the Parliament buildings in Ottawa, slowly turns green over years. But gold artifacts thousands of years old can still be bright and shiny. Metals clearly differ in how readily they react.

In this lesson you will look at how metals and non-metals react with oxygen, water and acids, and use observations to build an activity series: a list of metals ranked from most to least reactive. You will also see how reactivity connects to the ionization energy trend you studied.

Reactions with oxygen

Metals + oxygen → metal oxide (a synthesis reaction)

• 4Na(s) + O₂(g) → 2Na₂O(s): fresh sodium tarnishes within seconds in air, which is why it is stored under oil.
• 2Mg(s) + O₂(g) → 2MgO(s): magnesium ribbon burns with a blinding white flame (never look straight at it).
• 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s): iron reacts slowly, and rusting also needs water.
• Gold does not react with oxygen at all.

Metal oxides that dissolve in water form basic solutions: Na₂O(s) + H₂O(l) → 2NaOH(aq). That is why metal oxides are called basic oxides.

Non-metals + oxygen → non-metal oxide

• C(s) + O₂(g) → CO₂(g)
• S(s) + O₂(g) → SO₂(g), a cause of acid precipitation

Non-metal oxides that dissolve in water form acidic solutions: CO₂(g) + H₂O(l) → H₂CO₃(aq) (carbonic acid in sparkling water), and SO₂ + H₂O → H₂SO₃ (sulfurous acid). So they are called acidic oxides. This difference, basic oxides from metals and acidic oxides from non-metals, is another periodic trend.

Reactions with water

The most reactive metals react with cold water to form a metal hydroxide and hydrogen gas:

• 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) (vigorous)
• 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) (violent; the hydrogen often ignites)
• Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g) (steady bubbling)

Magnesium reacts very slowly with cold water but faster with hot water or steam. Iron, copper, silver and gold do not react with cold water.

Among the non-metals, chlorine reacts with water: Cl₂(g) + H₂O(l) → HCl(aq) + HClO(aq). The hypochlorous acid (HClO) is what kills germs in chlorinated drinking water and pools.

Reactions with acids

Many metals react with dilute acids to produce a salt and hydrogen gas (a single displacement reaction: the metal displaces hydrogen):

• Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
• Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
• Cu(s) + HCl(aq) → no reaction

The rate of bubbling is a good measure of reactivity, as long as the investigation is a fair test: the same acid at the same concentration and temperature, and metal pieces of the same size and surface area. Hydrogen is identified with the burning splint test: a lit splint held at the mouth of the test tube gives a squeaky pop.

Building an activity series

An activity series ranks metals by how readily they react, which means how easily they lose electrons to form positive ions. Chemists build it from evidence:

1. Metals that react with cold water are the most reactive (K, Na, Ca).
2. Metals that react with acids but not cold water come next (Mg, Al, Zn, Fe, Ni, Sn, Pb), ranked by how fast they react.
3. Metals that do not react with dilute acids are least reactive (Cu, Ag, Au).
4. Displacement reactions break ties: a more active metal displaces a less active metal from a solution of its ions. For example, Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s), but Cu(s) + ZnSO₄(aq) → no reaction. So zinc is above copper.

Hydrogen is placed in the series as a reference: metals above it displace hydrogen from acids; metals below it do not.

Connection to periodic trends: reactive metals are those that lose electrons easily, which links to low ionization energy. That is why the group 1 metals are at the top, and why K is above Na. (Other factors also matter, so the series does not exactly follow ionization energies. Calcium, for example, is above sodium in some series.)

Non-metals have a series too

Halogens can also be ranked. A more reactive halogen displaces a less reactive one from a solution of its ions: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq), but I₂ does not displace chloride. The halogen activity series is F₂ > Cl₂ > Br₂ > I₂, which matches the decreasing electronegativity and electron affinity down group 17. Non-metals react by gaining electrons, so the best electron-attractors are the most reactive. You will use both series to predict products in the Chemical Reactions unit.