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Experiment 10 - Single Replacement Reactions
A single replacement reaction is a type of oxidation-reduction reaction. In a single
replacement reaction (also called a single displacement reaction), an element reacts with
an ionic compound to give a different free element and a different ionic compound. The
general form of a single replacement reaction looks like this:
A + BC à B + AC
Where A is an element, BC is an ionic compound consisting of positively charged B ions
and negatively charged C ions, B is an element, and AC consists of positively charged A
ions and negatively charged C ions. It is called a “single replacement” reaction because it
appears as though the element A is replacing B in its compound. (In actuality, BC is
usually a soluble compound, so it consists of separated Bx+ ions and Cx- ions. Since they
are already separated, it isn’t really accurate to say that any replacement is occurring.) If
this reaction occurs as written, it is said that A is more active than B. If it does not occur,
B must be more active than A. (We are assuming, of course, that A and B do not have the
same exact activity.)
Recall that oxidation-reduction reactions involve a transfer of electrons. The
substance that gets oxidized loses electrons and its oxidation number increases. On the
other hand, the substance that is getting reduced is gaining electrons and its oxidation
number is therefore decreasing. In a reaction of this type, the element A is going from a
zero oxidation number to a positive oxidation number. Its oxidation number is therefore
increasing, so it is being oxidized. The substance B, however, is going from a positive to
a zero oxidation number, and is therefore being reduced.
Therefore, the more active element is the one that is oxidized the most easily.
To use a specific example, if a strip of copper metal is immersed in a solution of
mercury (II) chloride, a reaction occurs:
Cu (s) + HgCl2 (aq) à CuCl2 (aq) + Hg (l)
Orange colorless pale blue silvery or black
Two electrons are taken from each copper atom, and transferred to a mercury (II) ion,
converting it to a neutral mercury atom. The process simultaneously converts Cu to Cu2+,
the copper (II) ion. Copper is thus more active than mercury.
The reverse is not the case. That is, Cu2+ cannot oxidize Hg (it cannot take
electrons from Hg). This can be proven by adding Hg (l) to a solution of CuCl2 (aq):
