TESSA Secondary
Resource 4
Background information/subject knowledge for teacher
Reacting metals and acids
You will of course know that metals differ in their reactivity. This is illustrated by the reactivity
series, which is usually displayed as a league table with the most reactive metal at the top.
Usually we regard a metal as reactive if, when it is added to acid, it produces lots of bubbles
and the temperature of the acid solution increases.
Common metals for reacting with acids (in order of reactivity) would be magnesium, zinc,
iron, tin and copper; i.e. one from the top, three from the middle and one from the bottom of
the reactivity series. Magnesium is reactive enough to show significant effervescence
without being dangerous, copper is unreactive but not expensive as are silver and gold.
All of the metals above hydrogen in the reactivity series will, produce hydrogen gas from
acids. However, the reactions become progressively less vigorous as you go down the
reactivity series. The choice of acid is usually hydrochloric acid of concentration 1 mol dm-3
but some metals react better with sulfuric acid; e.g. zinc. Iron reacts but only very slowly. Tin
also reacts so slowly that it is difficult to see the reaction. Concentrated acids should not
be used.
magnesium
+
hydrochloric acid
magnesium chloride
+
hydrogen
Mg (s)
+
2HCl (aq)
MgCl 2(aq)
+
H 2(g)
zinc
+
sulfuric acid
zinc sulfate
+
hydrogen
Zn (s)
+
H 2 SO 4(aq)
ZnSO 4(aq)
+
H 2(g)
iron
+
hydrochloric acid
iron(II) chloride
+
hydrogen
Fe (s)
+
2HCl (aq)
FeCl 2(aq)
+
H 2(g)
tin
+
hydrochloric acid
tin(II) chloride
+
hydrogen
Sn (s)
+
2HCl (aq)
SnCl 2(aq)
+
H 2(g)
Copper is below hydrogen in the reactivity series and will therefore not displace dilute acid
so there is no reaction.
As you can see from the above equations the reactions produce salts. You can introduce
your pupils to the reactions of metals with acids in one lesson to establish the fact that some
displace hydrogen from acids, by collecting and testing for the gas with a lighted splint, and
then, perhaps, extending their understanding by isolating one of the salts in the next lesson.
For example, zinc granules will react with dilute sulfuric acid (0.5 mol dm-3) to produce zinc
sulfate which can then be isolated by crystallisation by evaporating off the excess solvent in
an evaporating basin as shown in the diagram below. If you decide to use zinc and sulfuric
acid to make a salt, then adding a few drops of copper sulfate will speed the reaction up.
TESSA Secondary
Health and safety
Obviously this experiment involves heating so great care needs to be taken when handling
apparatus which should be left for a sufficient time after heating for them to have cooled
down.
Arrange the class so that pupils work in pairs or small groups. No pupils should be seated if
they or members of their group are heating anything.
When heating the evaporating basin the Bunsen flame should be blue but the gas tap
adjusted so that there is a low flame for gentle heating. When the water level in the basin
has been reduced by about a half place a glass stirring rod in the solution and then hold it up
to cool. If small colourless crystals begin to form on the rod stop heating and allow the basin
to cool naturally. If no crystals form continue heating until they do. Do not continue heating
past the point at which crystals are observed at the edge of the solution.
1.0 mol dm-3 HCl is a low hazard at this concentration.
0.5 mol dm-3 H 2 SO 4 is an irritant at this concentration.