Chemistry, Third edition

84 6 · REACTIONS OF IONS IN SOLUTION

During the reaction, the blue copper(II) sulfate solution changes to colourless zinc(II)

sulfate solution and the copper metal settles out.

The equation suggests that zinc atoms react directly with copper(II) sulfate.

However, this is misleading – copper(II) sulfate is a soluble salt and is therefore

fully dissociated into ions in solution. We can get a better picture of the reaction that

occurs if we write out the ions that are present as reactants and as products. In doing

so, we will be writing out an ionic equation.

How to assemble the ionic equation for the reaction

between copper(II) sulfate and zinc metal

1.Look at the table of valencies and symbols of ions (page 464). This shows that

copper(II) ions are symbolized Cu^2 and that sulfate ions are symbolized SO 42 .

In solution, copper(II) sulfate consists of separate copper(II) and sulfate ions. In

all ionic equations, solids (whether metals or insoluble compounds) are rep-

resented by their chemical formulae. Here, we represent the zinc metal as Zn(s).

The left-hand side of the chemical equation may now be written as

Cu^2 (aq)SO 42 (aq)Zn(s)

2.Use of the valence table allows us to write ZnSO 4 (aq) as Zn^2 (aq) and SO 42 (aq),

respectively. The copper metal is symbolized as Cu(s). The right-hand side of the

equation becomes:

Zn^2 (aq)SO 42 (aq)Cu(s)

3.The two halves are now written together:

Cu^2 (aq)SO 42 (aq)Zn(s)Zn^2 (aq)SO 42 (aq)Cu(s)

4.The sulfate ion appears unchanged on both sides of the equation. This shows that

the sulfate ion is not involved in the chemical reaction. The sulfate ion is termed a

spectator ionand may be eliminated in the same way as identical terms may be

cancelled on either side of a mathematical equation:

Cu^2 (aq)SO 42 (aq)Zn(s)Zn^2 (aq)SO 42 (aq)Cu(s)

4.giving

Cu^2 (aq)Zn(s)Zn^2 (aq)Cu(s)

The last equation shows that the reaction does not involve sulfate ions, but that

the products are made as a result of reaction between copper(II) ions and zinc

atoms. In Fig. 6.1(a) all the ions present in the reactants and products are shown. In

Fig. 6.1(b), the sulfate spectator ions have been removed and it is now easier to see

that only the copper ions and zinc atoms are involved in the reaction.

The reaction of Cu^2 (aq) and Zn(s) is also an example of a redox reaction(see

page 100).

Ionic equations

Assemble the ionic equation for the reaction of copper(II) nitrate with zinc metal:

Cu(NO 3 ) 2 (aq)Zn(s)Zn(NO 3 ) 2 (aq)Cu(s)

Exercise 6B