The Foundations of Chemistry

Similarly, when bromine is added to a solution containing iodide ions, the iodide ions are

displaced by bromine to form iodine and bromide ions:

Br 2 (
)
2[Na
(aq)
I(aq)] 88n2[Na
(aq)
Br(aq)]
I 2 (s)

bromine sodium iodide sodium bromide iodine

Each halogen will displace less active (heavier) halogens from their binary salts;that is, the

order of decreasing activities is

F 2 Cl 2 Br 2 I 2

Conversely, a halogen will notdisplace more active (lighter) members from their salts:

I 2 (s)
2F88nno reaction

EXAMPLE 4-8 Displacement Reactions

Which of the following combinations would result in a displacement reaction? Write balanced

formula unit, total ionic, and net ionic equations for reactions that occur.

(a) I 2 (s) 
NaBr(aq)88n

(b) Cl 2 (g)
NaI(aq) 88n

(c) Br 2 (
)
NaCl(aq)88n

Plan

The activity of the halogens decreases from top to bottom in the periodic table. We see (a)

that Br is above I and (c) that Cl is above Br in the periodic table; therefore, neither combi-

nation (a) nor combination (c) could result in reaction. Cl is above I in the periodic table, and

so combination (b) results in a displacement reaction.

(a) Bromine, Br 2 , in water (pale
orange) is poured into an aqueous
solution of NaI, the top layer in the
cylinder. (b) Br 2 displaces Ifrom
solution and forms solid iodine, I 2.
The I 2 dissolves in water to give a
brown solution but is more soluble
in many organic liquids (purple
bottom layer).

152 CHAPTER 4: Some Types of Chemical Reactions

(a) (b)

Activity of the halogens decreases
going down the group in the
periodic table.