Precipitation reactions: In these reactions, you mix solutions of two ionic substance and a solid ionic substance ( a precipitate ) forms.
Prediction precipitation reactions: Now let us see how you would go about predicting whether a precipitation reactio will occur.Suppose you mix togather solutions of nickel(II) chloride,NiCl2 and sodium phophate Na3PO4.You can write the potential reactant as follows:
NiCl2 + Na3PO4 ----------->
How can you tell if a reaction will occur and if it does,what product to expect?
When you write a precipitation reaction as a molecular equation the reaction has a form of an exchange reaction.An exchange reaction is a reaction between compounds that ,when written as a molecular equation,appears to involve the exchange of parts between the two reactants.
Lets see reaction between nickel(II) chloride,NiCl2 and sodium phophate Na3PO4.If you exchange the anions you will get nickel (II) phosphate and sodium chloride.(recall the how to write the formula of ionic compound while writing the equation).The balanced equation is
3NiCl2 + 2Na3PO4 ---------> Ni3(PO4)2 + 6NaCl
Lets verify that NiCl2 & Ni3(PO4)2 are soluble and then check the solubilities of the products.From the solubility rules we can say that chlorides are soluble with some exception which do not include nickel(II) chloride.Also,sodium phophate is soluble.The potential product nickel (II) phosphate is insoluble as solubility rules says phosphates are soluble with the exception of alkali metal phosphates and ammonium phosphate.And sodium chloride is soluble.
Now we can lable the equation:
3NiCl2 (aq) + 2Na3PO4(aq) ---------> Ni3(PO4)2(s) + 6NaCl(aq)
write the complete ionic equation:
3Ni2+(aq) + 6Cl-(aq) + 6Na+(aq) + 2PO43-(aq) ----------> Ni3(PO4)2 (s) + 6Na+(aq) + 6Cl-(aq)
net ionic equation after cancelling spectator ions:
3Ni2+(aq) + 2PO43-(aq) ------------> Ni3(PO4)2 (s)