These types of reactions are only possible under certain conditions: one of the species must have an oxidation number of zero or all must necessarily be ionized. What does an oxidation number of zero mean? It means that the species is in its natural state.
A very illustrative example of the above approach is the reaction between a copper wire and a silver nitrate solution. Since copper is a metal in its natural state, then its oxidation number is zero; On the other hand, that of silver is +1 (Ag + ), which lies dissolved together with the nitrate ions (NO 3 – ).
Metals give up electrons, but some are more active than others; This means that not all metals rust so easily. Because copper is more active than silver, it donates its electrons to it, reducing it to its natural state, reflected as a silver surface that covers the copper wire (image above).
Displacement of hydrogen and metals
The top image shows a column in decreasing order of activity, highlighting the hydrogen molecule. Those metals that are above this will be able to displace it in the non-oxidizing acids (HCl, HF, H 2 SO 4 , etc.), and those that are below will not react at all.
The simple displacement reaction can be described by the following general equation:
A + BC => AB + C
A displaces C, which can be the H 2 molecule or another metal. If H 2 is formed by the reduction of the H + ions (2H + + 2e – => H 2 ), then the species A must — due to the conservation of mass and energy — must supply the electrons: it must be oxidized.
On the other hand, if A and C are metallic species, but C is in the ionic form (M + ) and A in its natural state, then the displacement reaction will occur only if A is more active than C, forcing the latter to accept electrons to reduce to their metallic state (M).
Displacement with halogens
In the same way, halogens (F, Cl, Br, I, At) can move with each other but following another series of activities. For these, the activity decreases as one descends through group 7A (or 17): I <Br <Cl <F. This means that fluorine is capable of displacing any of the other halogens, but it is not the same with iodine.
For example, the following reaction occurs naturally:
F 2 (g) + 2NaI (ac) => 2NaF (ac) + I 2 (s)
However, this other does not produce any products for the reasons just explained:
I 2 (s) + NaF (ac) => X
In the above equation X means that there is no reaction.
With this knowledge it can be predicted which mixture of halogen salts with the pure elements produces products. As a mnemonic, iodine (a volatile purple solid) does not displace any of the other halogens, but the others do displace it when in ionic form (Na + I – ).
The double displacement reaction, also known as the metathesis reaction, is represented as follows:
AB + CD => AD + CB
This time, not only A displaces C, but also B displaces D. This type of displacement occurs only when solutions of soluble salts are mixed and a precipitate forms; that is, AD or CB must be insoluble and have strong electrostatic interactions.
For example, when mixing KBr and AgNO 3 solutions , the four ions move through the medium to form the corresponding pairs of the equation:
KBr (aq) + AgNO 3 (aq) => AgBr (s) + KNO 3 (aq)
Ag + and Br – ions form the silver bromide precipitate, while K + and NO 3 – cannot be arranged to form a potassium nitrate crystal.
Acid-base neutralization reaction
When an acid is neutralized with a base, a double displacement reaction occurs:
HCl (aq) + NaOH (aq) => NaCl (aq) + H 2 O (l)
No precipitate is formed here, since sodium chloride is a very soluble salt in water, but a pH change occurs, which adjusts to a value close to 7.
However, in the following reaction, a change in pH and the formation of a precipitate occur simultaneously:
H 3 PO 4 (aq) + 3Ca (OH) 2 => Ca 3 (PO 4 ) 2 (s) + 3H 2 O (l)
Calcium phosphate is insoluble, precipitating as a white solid, while phosphoric acid is neutralized with calcium hydroxide.
Cu (s) + 2AgNO 3 (aq) => Cu (NO 3 ) 2 (aq) + 2Ag (s)
This is the image reaction of the copper wire. If you look at the series of chemical activities for metals, you will find that copper is above silver so that it can displace it.
Zn (s) + CuSO 4 (aq) => ZnSO 4 (aq) + Cu (s)
With this other reaction the opposite occurs: now the bluish solution of CuSO 4 becomes transparent as the copper precipitates as metal and, at the same time, the metallic zinc disintegrates into the soluble zinc sulfate salt.
2Al (s) + 3NiBr 2 (ac) => 2AlBr 3 (aq) + 3Ni (s)
Again, this reaction occurs because aluminum is above nickel in the series of chemical activities.
Sn (s) + H 2 SO 4 (aq) => SnSO 4 (aq) + H 2 (g)
Here tin displaces hydrogen, although it is very close to it in the series.
2K (s) + 2H 2 O (l) => 2KOH (aq) + H 2 (g)
Finally, those metals that are in the highest part of the series are so reactive that they displace even the hydrogen in the water molecules, generating a very exothermic (and explosive) reaction.
Zn (NO 3 ) 2 (aq) + 2NaOH (aq) => Zn (OH) 2 (s) + 2NaNO 3 (aq)
Although the base does not neutralize any acid, the OH – ions have more affinity for Zn 2+ than the NO 3 – ions ; for this reason double displacement occurs.
Cu (NO 3 ) 2 (aq) + Na 2 S (aq) => CuS (s) + 2NaNO 3 (aq)
This reaction is very similar to the previous one, with the difference that both compounds are salts dissolved in water.
- Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 145-150.
- Toby Hudson. (April 3, 2012). Precipitation of silver on copper. [Figure]. Taken from: commons.wikimedia.org
- Helmenstine, Anne Marie, Ph.D. (May 3, 2018). What Is a Displacement Reaction in Chemistry? Taken from: thoughtco.com
- amrita.olabs.edu.in ,. (2011). Single Displacement Reaction. Taken from: amrita.olabs.edu.in
- Byju’s. (September 15, 2017). Displacement Reactions. Taken from: byjus.com
- Types of Chemical Reactions: Single- and Double- Displacement Reactions. Taken from: jsmith.cis.byuh.edu