Chemistry 4.1-4.3 - Reactivity and displacement

Building the reactivity series

Reactivity compares how readily substances undergo chemical reactions. For a metal,
the useful particle-level idea is its relative tendency to form cations: the more
reactive the metal, the more readily its atoms form positive ions when a suitable reacting
substance is present.

For this lesson, the required series is:

Carbon and hydrogen are in parentheses because they are not metals. They are included
as useful reference points. Hydrogen gives an immediate prediction about dilute acids:
metals above it usually displace hydrogen from suitable dilute acids such as hydrochloric acid, whereas copper, silver and
gold below it do not. Carbon's position is a reference here; using it to choose metal
extraction methods belongs to later specification points.

The series is not an arbitrary memory list. It summarises evidence. A metal that reacts
more readily with water or dilute acid, or that displaces another metal from its salt
solution, is placed higher. Because reaction conditions affect what is seen, a reliable
order comes from fair comparisons and, when possible, more than one type of reaction.

Evidence from water and dilute acids

Start with the macroscopic evidence: what can actually be observed. A reaction may
produce bubbles, make the metal disappear, release enough energy for a temperature rise,
or move a soft metal across the water surface. Under the same conditions, a more rapid or
vigorous reaction is evidence of greater reactivity. No visible change is also useful
evidence, but only for the conditions and observation time used.

Reactions with water

Potassium and sodium react very vigorously with cold water, while calcium reacts readily.
Magnesium reacts only very slowly with cold water. Aluminium may show little visible
reaction because its surface is protected by a thin aluminium oxide layer. Zinc, iron,
copper, silver and gold show no useful reaction with cold water in an ordinary comparison.

For metals that react with cold water, the general word equation is:

metal + water → metal hydroxide + hydrogen

For example:

$\ce{Ca + 2H2O -> Ca(OH)2 + H2}$

This symbolic equation conserves every atom: there is one calcium atom, four hydrogen
atoms and two oxygen atoms on each side.

Reactions with dilute acids

Metals above hydrogen react with suitable dilute acids to form a salt and hydrogen:

metal + acid → salt + hydrogen

For dilute hydrochloric acid, magnesium gives:

$\ce{Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)}$

The bubbles are hydrogen gas. A small sample gives a squeaky pop with a lit splint, but the
test identifies the gas; the loudness of the pop is not a sound way to rank metals.
Potassium and sodium are not tested with acid because their reactions would be dangerously
violent. Magnesium reacts readily, zinc less vigorously and iron more slowly. Aluminium
can be misleading at first because its oxide coating delays contact with the acid. Copper,
silver and gold do not react with ordinary dilute hydrochloric acid.

At the particle level, each successful comparison shows metal atoms becoming cations
in the products. A metal placed higher in the series has the greater relative tendency to
make those positive ions. This explains why the overall order matters even when an oxide
coating temporarily makes one sample appear slow.

Worked example: deducing an order

Three unknown metals are tested under matched conditions:

1. W is above X because W reacts rapidly with cold water while X does not under the same
   conditions.
2. X is above Y because X reacts with dilute acid and makes hydrogen while Y does not.
3. Combining the two comparisons gives W > X > Y, where > means "is more reactive
   than".

The entry "not tested" is not a negative result. It supplies no comparison for that cell,
so the conclusion must use the observations that were actually made.

Predicting metal displacement

A metal salt solution contains cations of that metal. The classroom rule below applies to suitable metal–salt combinations such as magnesium, zinc, iron and copper in aqueous sulfate solutions. Very reactive metals such as potassium and sodium react with the water itself, so adding them to an aqueous salt is not a clean displacement test. Surface coatings and slow reactions can also hide an expected result. When a different solid metal is
added, a displacement reaction happens only if the solid metal is more reactive than
the metal in the salt.

Use this routine:

1. identify the added solid metal;
2. identify the metal named in the salt solution;
3. compare their positions in the series;
4. if the solid is higher, swap the metals in the word or formula equation;
5. if the solid is lower, write no reaction.

Worked example: magnesium and copper(II) sulfate

Magnesium is above copper, so magnesium displaces copper:

magnesium + copper(II) sulfate → magnesium sulfate + copper

$\ce{Mg(s) + CuSO4(aq) -> MgSO4(aq) + Cu(s)}$

The three levels of chemical explanation agree:

• Macroscopic: the blue copper(II) sulfate solution becomes paler, a reddish-brown
  copper coating appears, and some magnesium is used up.
• Particle: magnesium atoms enter the solution as magnesium cations, while copper(II)
  ions leave the solution and become copper atoms. The sulfate ions remain in solution.
• Symbolic: the balanced formula equation has one Mg, one Cu and one sulfate group on
  each side. Matter is conserved.

Now reverse the solid and solution. Copper is below magnesium, so copper cannot displace
magnesium from magnesium sulfate:

Copper in magnesium sulfate solution: no displacement reaction

"No reaction" is not interchangeable with a reversed equation. Writing magnesium and
copper products would claim a change that the reactivity evidence does not support.