Single and double replacement reactions usually involve ionic compounds (and sometimes water, which we treat as the ionic compound H+OH−).
In a single replacement reaction, atoms of an element react with a compound, replacing the atom of the same type. Metals replace metals; non-metals replace non-metals. For example:
Na + CaCl2 à NaCl + Ca
(Na replaces Ca in the compound.)
KBr + Cl2 à KCl + Br2
(Cl
replaces Br in the compound.)
If an element reacts with a compound, you can predict what the products are
going to be, because the element simply replaces the other element of the same
type. For example, if you were given the problem:
Ca + NaCl à ?
Calcium is a metal, so it will replace sodium. This means calcium will end up with chloride (CaCl2), and sodium will end up by itself (Na). The reaction is therefore:
Ca + NaCl à CaCl2 + Na
Notice that we have to balance the charges every time we put two ions together. Na needed only one Cl atom to be balanced, but Cl needs two.
In a double replacement reaction, the two ions of the same type switch places, as in:
KCl + MgO à MgCl2 + K2O
(K
and Mg are trading places;
K is now with O and Mg is now with Cl.)
Notice again that we had to balance the charges. We needed only one K+ ion with Cl−, but we need 2 K+ ions with O2−. Similarly, Mg2+ needs only one O2− ion, but it needs two Cl− ions.
If we had the problem:
NH4OH + Ca3(PO4)2 à ?
we would swap NH4+ with Ca2+. Balancing the charges, NH4+ would now go with PO43− to form (NH4)3PO4, and Ca2+ would now go with OH− to form Ca(OH)2. This gives the equation:
NH4 OH + Ca3(PO4)2 à (NH4)3 PO4 + Ca(OH)2
Watch out for H+ and OH− getting together to form HOH (which we write as H2O), e.g.:
HCl + Ca(OH)2 à CaCl2 + H2O