Unit 1 Module 3 The Periodic Table - Pearson Schools

Water treatment
Module 3
The Periodic Table
Group 7 elements: uses and halide tests
Chlorine is a toxic gas. In the UK, small amounts of chlorine have been added to
drinking water to kill bacteria since the late 1890s. Some people claim that chlorination
of water has done more to improve public health than anything else. Certainly, chlorine
is a most effective bacteria killer in our water supplies.
Despite this, the addition of chlorine has not always been welcomed. Some
environmentalists are concerned that chlorine reacts with organic matter to form
traces of chlorinated hydrocarbons, suspected of causing cancer. In 1991, the
government in Peru thought that this was an unacceptable risk and stopped adding
chlorine to drinking water. Unfortunately, they had forgotten the very reason for adding
chlorine in the first place and an outbreak of cholera followed. This affected a million
people and caused 10 000 deaths. The Peruvian government swiftly reversed their
decision and resumed adding chlorine to their drinking water.
Testing for halide ions
The presence of halide ions can be detected with a simple test tube test.
• The unknown halide substance is first dissolved in water.
• An aqueous solution of silver nitrate, AgNO 3 (aq), is added.
• Silver ions, Ag + (aq), from the AgNO 3 (aq) react with any halide ions, X – (aq), present,
forming a silver halide precipitate, AgX(s).
• The silver halide precipitates are coloured – the colour tells us which halide is present.
Sometimes it is difficult to judge the exact colour.
• If you are unsure, then add aqueous ammonia, NH 3 (aq).
• Different halide precipitates have different solubilities in aqueous ammonia – this
confirms which halide is present.
Halide test results are:
chloride: Ag + (aq) + Cl – (aq) → AgCl(s) white precipitate, soluble in dilute NH 3 (aq)
bromide: Ag + (aq) + Br – (aq) → AgBr(s) cream precipitate, soluble in conc NH 3 (aq)
iodide: Ag + (aq) + I – (aq) → AgI(s) yellow precipitate, insoluble in conc NH 3 (aq)
This type of reaction is called a precipitation reaction. A precipitation reaction takes
place in aqueous solution when aqueous ions react together to form a solid precipitate
(see Figure 2).
Figure 2 Halide tests with aqueous
silver nitrate. Silver halides precipitated
by reacting aqueous silver nitrate with
aqueous halide solutions
Examiner tip
T
.
AgNO 3 (aq) pp Ag + (aq) ;
T pp Cl – (aq), Br – (aq) or I –
(aq) .
Then:
Ag + (aq) + Cl – (aq) ⎯→ AgCl()
Y j O 3– (aq) and
. T .
Key definition
A precipitation reaction
. p
x .
AgCl(s) AgBr(s) AgI(s)
Questions
1 Summarise the general properties of a Group 7 element.
2 What is the difference between a halogen and a halide?
3 How could you distinguish between NaCl, NaBr and NaI by using a simple test?
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