The displacement reactions are all those in which a chemical species displaces another within a compound. This displacement can be simple or double, differing in that in the first one it is an element that is displaced, while in the second there is a change of "pairs" between two compounds..
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 nitrate ions (NO3-).
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, reducing it to its natural state, reflected as a silver surface that covers the copper wire (image above).
Article index
The top image shows a column in decreasing order of activity, highlighting the hydrogen molecule. Those metals that are above this can displace it in the non-oxidizing acids (HCl, HF, HtwoSW4, etc.), and those 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 may be the H moleculetwo or other metal. Yes Htwo is formed by the reduction of H ions+ (2H+ + 2e- => Htwo), then species A must - due to the conservation of mass and energy - provide 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 its metallic state (M).
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 For example, the following reaction occurs naturally: Ftwo(g) + 2NaI (ac) => 2NaF (ac) + Itwo(s) However, this other does not produce any products for the reasons just explained: Itwo(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 rule of thumb, iodine (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; i.e. AD or CB must be insoluble and have strong electrostatic interactions. For example, when mixing KBr and AgNO solutions3, the four ions move through the medium until they form the corresponding pairs of the equation: KBr (ac) + AgNO3(ac) => AgBr (s) + KNO3(ac) Ag ions+ and Br- form the silver bromide precipitate, while K+ and no3- cannot be arranged to form a crystal of potassium nitrate. When an acid is neutralized with a base, a double displacement reaction occurs: HCl (aq) + NaOH (aq) => NaCl (aq) + HtwoO (l) Here no precipitate is formed, since sodium chloride is a very soluble salt in water, but a pH change occurs, which is adjusted to a value close to 7. However, in the following reaction, a change in pH and the formation of a precipitate occur simultaneously: H3PO4(ac) + 3Ca (OH)two => Ca3(PO4)two(s) + 3HtwoO (l) Calcium phosphate is insoluble, precipitating as a white solid, while phosphoric acid is neutralized with calcium hydroxide. Cu (s) + 2AgNO3(ac) => Cu (NO3)two(ac) + 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 you can displace it. Zn (s) + CuSO4(ac) => ZnSO4(ac) + Cu (s) With this other reaction, the opposite occurs: now the bluish solution of CuSO4 becomes transparent as copper precipitates as metal and at the same time metallic zinc disintegrates into soluble zinc sulfate salt. 2Al (s) + 3NiBrtwo(ac) => 2AlBr3(ac) + 3Ni (s) Again, this reaction occurs because aluminum is above nickel in the series of chemical activities. Sn (s) + HtwoSW4(ac) => SnSO4(ac) + Htwo(g) Here tin displaces hydrogen, although it is very close to it in the series. 2K (s) + 2HtwoO (l) => 2KOH (aq) + Htwo(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 (NO3)two(aq) + 2NaOH (aq) => Zn (OH)two(s) + 2NaNO3(ac) Although the base does not neutralize any acids, the OH ions- feel more affinity for Zntwo+ that ions are NOT3-; for this reason the double displacement occurs. Stamp3)two(ac) + NatwoS (ac) => CuS (s) + 2NaNO3(ac) This reaction is very similar to the previous one, with the difference that both compounds are salts dissolved in water..Double
Acid-base neutralization reaction
Examples
Simple
Double
References
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