We use chemical equations to help us describe chemical reactions.For a reaction involving ions, we have a choice of chemical equations,depending on the kind of info we want to convey.We can represent such a reaction by a molecular equation,a complete ionic equation or a net ionic equation.
Precipitate : Sometimes when whater solutions of two different ionic compounds are mixed,an insoluble solid seperates out of solution.The precipitate that forms is itself ionic; the cation comes from one solution, anion from other.To predict the occurence of reactions of this type, you must know which ionic substances are insoluble in water.Find this solubility rule table for ionic compounds.
Molecular equation: molecular equation which is a chemical equation in which the reactant and products are written as if they were molecular sunstances, even though they may actually exist in solutions as ions.
Ca(OH)2 (aq) + Na2CO3 (aq) --------> CaCO3 (s) + 2NaOH (aq)
complete ionic equation: A molecular equation is useful in describing actual reactant and product substances, it does not tell you what is happening at the level of ions.Ca(OH)2 and Na2CO3 both are soluble ionic substances and therefore strong electrolytes;when they dissolve in water, they go into solution as ions.Each foumula unit of Ca(OH)2 forms one Ca2+ ion and two OH- ions in solutions.Similary, Na2CO3 forms two Na+ ion and one CO3 2- ION IN solution.
Now let us look at the product side of the equation.One product is the precipitate CaCO3 (s).This is an ionic compound,but it is in the precipiate form.We will leave the formula as CaCO3 (s) to show that it is a precipitate.On the other hand,NaOH is a soluble ionic substance and therefore a strong electrolyte;it dissolves in aqueous solution to give ions,which we denote by writing Na+(aq) + OH- (aq).The complete equation is:
Ca2+ (aq) + 2OH- (aq) + 2Na+ (aq) + CO3 2- (aq) ---------> CaCO3 (s) + 2Na+ (aq)+ 2OH- (aq)
Net ionic equations: A spectator ion is an ion in an ionic equation that does not take part in the reaction.You can cancel such ions from both sides to express the essential reaction that occurs.
Ca2+ (aq) + CO3 2- (aq) ---------> CaCO3 (s) +
This is the net ionic equation , an ionic equation from which spectator ions have been cancelled.I shows that reaction actually occurs at ionic level is between calcium ions and carbonate ions to form solid calcium carbonate.
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)