Halogens
Elements in the Halogen
Group
Group 7 Elements
• Similar reactions to with other elements because
they all gain one electron.
• All react with metals to form compounds called
halides.
• Fluorine is the most reactive halogen, and the
reactivity decreases as you go down the group.
• They are diatomic (Cl2 Fl2, they always exist in
pairs)
Properties of Halogens
Need to learn!
Element
Symbol
State at
Colour
room
temperature
Fluorine
F
Gas
Pale Yellow
Chlorine
Cl
Gas
Bromine
Br
Liquid
YellowGreen
Red-Brown
Iodine
I
Solid
Grey
Equations for Reactions of
Halogens with Metals
You just saw iron reacting with chlorine, can you write a word
equation for this reaction?
What might the symbol equation for this reaction be?
Some More Examples
(Ide at the end when combined)
1. Potassium + Chlorine
Potassium Chloride
2. Calcium + Bromine
Calcium Chloride
Halogens and hydrogen (Used to
make acids)
• The halogens react with hydrogen gas to form halogen halides.
These form acids when they dissolve in water.
• For example:
hydrogen (g) +fluorine (g) → hydrogen fluoride (g)
H2
+
F2 →
2HF
When hydrogen fluoride dissolves in water it makes hydrofluoric
acid, HF(aq)
(aq) means aqueous – Latin for dissolved in water
Your turn
1. Write word and balanced symbol equations for the reactions
of hydrogen with
a) chlorine
b) bromine
2. Which acids are formed from a) and b)?
Answers
1. a)
hydrogen (g) + chlorine(g) → hydrogen chloride (g)
H2
+
Cl2 →
2HCl
b)
hydrogen (g) + bromine(g) → hydrogen bromide (g)
H2
+
Br2 →
2HBr
2. Hydrochloric acid, hydrobromic acid
Hydrofluoric acid
All the acids just named are extremely hazardous.
Hydrofluoric acid is used in
etching glass. It has the following
hazard symbols.
What do they mean?
When we say wear goggles, we
really mean it.
Example of burns from HF acid, from a home glass
polisher kit.
Toxic and
corrosive. It is
absorbed
through the
skin and bonds
to calcium in
your bones!
Displacement reactions
Some elements are more reactive than others. A reactive metal will
displace (take the place of) a less reactive one to form a compound.
For example:
Iron + copper sulphate → iron sulphate + copper
Which is the more reactive metal, iron or copper?
Iron!!
Displacement Reactions of
Halogens
• These can be used to work out how reactive the different halogens
are.
• A more reactive halogen will displace a less reactive halogen from
its compounds.
• In the example below chlorine is more reactive than bromine, so
chlorine displaces bromine from a bromide.
Potassium Iodide + Chlorine
Potassium Chloride + Iodine
The order of reactivity of the halogens is:
Reactivity decreases
Fluorine
Chlorine
Bromine
Iodine
Astatine
Balanced symbol Equations
NB, remember the halogens are diatomic molecules this
means that in their natural state they are so reactive that they
exist as two atoms joined together.
Chlorine + potassium bromide → potassium chloride + bromine (red-brown)
Cl2 + 2KBr → 2KCl + Br2
Chlorine + potassium iodide → potassium chloride + iodine (brown/purple)
Cl2 + 2KI → 2KCl + I2
Bromine + potassium iodide → potassium bromide + iodine
Br2 + 2KI → 2KBr + I2
Exam question
(c) When chlorine is added to a solution of potassium
bromide, a colour change is seen. When chlorine is
added to a solution of potassium fluoride, no colour
change is seen.
Explain how these observations provide evidence for
the order of reactivity of bromine, chlorine and
fluorine.
(6 marks)
*(c)
A explanation to include some of the following points
• colour change shows reaction occurs • chlorine reacts
with potassium bromide solution • bromine is formed
• colour is red brown • chlorine displaces bromide ions
• chlorine is more reactive than bromine • no colour
change shows no reaction / chlorine does not react with
potassium fluoride solution • chlorine does not displace
fluoride ions • chlorine is less reactive than fluorine • order
of reactivity from most reactive halogen is fluorine, chlorine,
bromine
Level
1
1-2
2
3-4
• a limited description of at least two relevant points either from one
reaction or from both reactions • the answer communicates ideas using
simple language and uses limited scientific terminology • spelling,
punctuation and grammar are used with limited accuracy
• a detailed description of one reaction (one minor omission may be
ignored) / a limited description of some aspects of both reactions • a
detailed description of one reaction (one minor omission may be ignored) /
a limited description of some aspects of both reactions • spelling,
punctuation and grammar are used with some accuracy
3
5-6
• a detailed description of both reactions (one minor ommision may be ignored) • the
answer communicates ideas clearly and coherently uses a range of scientific