The Halogens Group 7

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The HalogensGroup 7
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Group 7The Halogen Group

• All the elements in Group 7 are nonmetals except
  for astatine, which is a radioactive metalloid.

• These elements are called halogens, which means
  salt-former.

• All of the halogens form salts with sodium and
  with the other alkali metals.

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Halogen compounds
Silver halides (e.g. silver chloride, silver
bromide etc) These are used in photographic
paper. They are reduced by light and x-ray
radiation to leave a silver photographic image.
Hydrogen halides (e.g. hydrogen chloride,
hydrogen fluoride) When these dissolve in water
they make acids and will turn universal indicator
red.
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Recall

• What do they look like?
• What is their physical state?
• Therefore conclude the trend in their melting
  points and boiling points?
• How do we detect them?
• How do we test for the presence of the anions?
• How does their reactivity change as you descend
  the group?
• How does the electronegativity and ionisation
  energy change?

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• Halogens are highly reactive, and as such can be
  harmful or lethal to biological organisms in
  sufficient quantities. This high reactivity is
  due to the atoms being one electron short of a
  full outer shell of eight electrons. They can
  gain this electron by reacting with atoms of
  other elements.
• Fluorine is the most reactive element in
  existence, attacking otherwise inert materials
  such as glass, and forming compounds with the
  heavier noble gases.
• It is a corrosive and highly toxic gas. The
  reactivity of fluorine is such that if used or
  stored in laboratory glassware, it can react with
  glass in the presence of small amounts of water
  to form silicon tetrafluoride (SiF4). Thus
  fluorine must be handled with substances such as
  Teflon, extremely dry glass, or metals such as
  copper or steel which form a protective layer of
  fluoride on their surface.
• Both chlorine and bromine are used as
  disinfectants for drinking water, swimming pools,
  fresh wounds, dishes, and surfaces. They kill
  bacteria and other potentially harmful
  microorganisms through a process known as
  sterilization.
• Their reactivity is also put to use in bleaching.
  Sodium hypochlorite, which is produced from
  chlorine, is the active ingredient of most fabric
  bleaches and chlorine-derived bleaches are used
  in the production of some paper products.

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Group 7The Halogen Group

• Watch the reactions of Cl2, Br2 and I2 then state
  which you think is the most reactive and why.
  Write equations for each.

• The halogen fluorine is the most reactive of the
  halogens in combining with other elements.

• Chlorine is less reactive than fluorine,
  and bromine is less reactive than chlorine.

• Iodine is the least reactive of the four
  nonmetals.

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Some facts







1) Reactivity DECREASES as you go down the group
(This is because the electrons are further away
from the nucleus and so any extra electrons
arent attracted as much).
2) They exist as diatomic molecules (so that
they both have a full outer shell)
3) Because of this fluorine and chlorine are
liquid at room temperature and bromine is a gas
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The halogens some reactions
1) Halogen metal

2) Halogen non-metal

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Hydrogen Halides

• Name the hydrogen halides in the group
• Are these ionic or covalent in gas form?
• Do they dissolve in water?
• What forms when they do?
• H-Cl(g) H2O(l) Cl-(aq) H3O (aq)
• Which is the strongest acid?
• Why? Think electronegativity

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Ionic and covalent nature of their compounds

• What is the definition of ionic?
• What is the definition of covalent?
• Which would be the most ionic compounds?
• Which is more covalent? AlF3 AlCl3
• Mpt of AlF3 1000 oC and AlCl3 is 180 oC

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What is the test for a halide ion?

• Write an equation for the formation of this ion
• How can you tell the halides apart from this
  test?
• What about the fluoride ion? Find out
• If you are still unsure about which is which what
  can you do?
• Adding ammonia tells us which is whichhow? Find
  out
• Write an equation for the addition of two ammonia
  molecules to silver chloride
• AgCl (s) 2NH3(aq) Ag(NH3)2(aq) Cl-(aq)

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Halide ion Acidified Silver Nitrate Then
add
Ammonia
Nothing
Fluoride
Chloride
Bromide
Iodide
No ppt
Dissolves with dilute ammonia
White
Cream
Dissolves with conc ammonia
Yellow
Does not redissolve
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Reducing power of the halides ions

• What is the general formula of the hydrogen
  halides?
• HX
• Experiment
• Concentrated sulphuric acid is added to sodium
  compounds NaCl/NaBr/NaI an is heated
• What did you observe?

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Results
Halide Observations Products Oxidation states
NaCl
NaBr


NaI





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Halide Observations Products Oxidation states
NaCl Steamy fumes HCl Cl 1-
NaBr Steamy fumes HBr Br 1-
Brown fumes Br2 Br (0)
Colourless gas SO2 S (IV)
NaI Steamy fumes HI I (1-)
Purple fumes I2 I (0)
Yellow solid S S (0)
Smell of bad eggs H2S S (2-)
Colourless gas SO2 S (IV)

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Equations

• NaCl(s) H2SO4 (aq) HCl(g) NaHSO4(aq)
• NaBr(s) H2SO4 (aq) HBr(g) NaHSO4(aq)
• 2HBr(g) H2SO4 (aq) 2H2O(l) Br2 SO2
• NaI(s) H2SO4 (aq) HI(g) NaHSO4(aq)
• 2HI(g) H2SO4 (aq) 2H2O(l) I2(g) SO2(g)
• H2SO4(l) 8HI(g) H2S(g) 4I2(g) 4H2O (l)

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Explain your observations

• Recall
• What is the definition of oxidation and
  reduction?
• Iodide and bromide are reductants
• Concentrated sulphuric acid is a powerful oxidant
• Which is the most powerful reductant?
• What is the trend going down the group?
• Is concentrated sulphuric acid useful to make HBr
  or HI?
• We can use a less powerful oxidant like
  phosphoric acid
• Work out the oxidation numbers of the sulphur
  from start to finish in each
• Which halogen will never act as a reducing agent?

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Write the equations when halogens displace each
other

• What happens when bromine is bubbled through
  sodium chloride
• What happens when bromine is bubbled through
  sodium Iodide
• 2NaI(aq) Br2 2NaBr(aq) I2 (l)
• Which is being reduced and oxidised?
• Write ionic equation for this reaction

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Displacement reactions
To put it simply, a MORE reactive halogen will
displace a LESS reactive halogen from a solution
of its salt.
Potassium chloride KCl(aq) Potassium bromide KBr(aq) Potassium iodide KI (aq)
Chlorine Cl2
Bromine Br2
Iodine I2
F
Cl
Br
I
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The different oxidation states of the group 7
non-metals

• What is the usual oxidation state of the ions?
• Do you know of any different compounds which have
  these substances in different oxidation states?
• Chlorine has many oxidation states when bonded to
  oxygen (chlorates)
• ClO-
• ClO2-
• ClO3-
• ClO4-
• Work out the oxidation state of chlorine in each
• Why is oxygen always the same in these compounds?

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What happens when chlorine dissolves in water?

• Recall
• When you tested chlorine at the electrode last
  year what did you observe?
• Did you notice the litmus paper changing colour
  before it was bleached?
• If so, what colour did it go?
• The disproportionation of chlorine in water
• Cl2(aq) H2O(l) HCl(aq) HClO(aq)
• What are the oxidation states of the various
  atoms in this equation?
• Does this explain the observation in the previous
  question?
• This is called hypochlorous acid HClO

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What is bleach, and why do we associate this with
chlorine?

• Cl2(aq) H2O(l) HCl(aq) HClO(aq)
• The species present in most bleaches is sodium
  hypochlorite NaClO
• How is it made commercially?
• Cl2(g) 2NaOH(aq) NaCl(aq) NaClO(aq) H2O
  (aq)
• Answer Electrolysis of brine
• Why is chlorine not used on its own?
• What about chlorine makes it ideal for bleach?
• Why was it recommended not to mix acid with
  bleaches?
• Why are all bleach bottles opaque?
• Answer Reacts with UV light.

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The formation of Potassium iodate (V)

• Predict what will happen if NaOH is reacted with
  I2

• You have just seen that Halogens react with
  sodium hydroxide. What happens when the alkali is
  hot or cold does this change what is made?
• Write down your observations
• How can you tell you have something different?

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You can use titration to see how much I2 has been
converted

• Think how you could do this and what would be a
  suitable indicator for the reaction
• What concentration of sodium thiosulphate would
  you use? How would you prepare this?

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Halogens as oxidising agents

• Fluorine is the strongest oxidant of all the
  elements, explain what is mean by this and what
  causes it to be so powerful.

• Put an arrow on the diagram to indicate how the
  oxidising power increases

• How does this relate to the reactivity of the
  group/ or the ability to steal electrons

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Electronegativity

• The more electronegative the element the
• Finish off the statement relating to an elements
  oxidising power
• Oxidising power is the ability to pull electron
  off another atom

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The manufacture of bromine from seawater

• How much bromide is there is the sea?
• Think of a way that you could free the bromine
  from the bromide ion without using electrolysis.
• Is chlorine freely available?
• Where does it come from?
• Outline the procedure to make bromine from
  bromide found in seawater
• Write an equation for the redox reaction.

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Silver halide react with light, hence why they
are used for photographic film.

• Light reduces the silver
• Write a half equation for this reaction.

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• Silver chloride - no graduation pictures without
  it
• Photography is just one of the amazing gifts of
  science and technology that add enjoyment to our
  lives.  Chlorine chemistry plays a role in
  traditional film photography in the form of the
  light-sensitive compound, silver chloride, AgCl.
• Photochemistry
• The light sensitivity of the silver halides is
  key to the photographic process.  Tiny crystals
  of all three of these compounds are used in
  making photographic film.  When exposed to light,
  a chemical reaction darkens the film to produce
  an image.
• AgCl, for example, consists of crystals of
  tightly packed ions of silver and chlorine,
  denoted Ag and Cl-.  When film containing Ag
  and Cl- is exposed to light energy, the chlorine
  ion's extra electron is ejected and then captured
  by a silver ion.
• Electron ejected from chlorine (Oxidation)
• Ag Cl- light energy ? Ag   Cl 1
  electron   
• Electron captured by silver (Reduction)
• Ag 1 electron ? Ag (metal)
• When silver metal is produced as a result of the
  electron capture, it forms a dark image on film. 
  Ag has been reduced to Ag (metal), and Cl- has
  been oxidised.

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Hydrogen halides react with ammonia
NH3 HCl ? NH4Cl

• This is a white crystalline solid you have
  probably seen this reaction in y10 when
  discussing diffusion
• HCl and NH3 are placed on cotton wool and put at
  either end of a tube
• When they meet in the middle a white solid is
  formed
• Find the properties of this solid
• Answer it is mildly acidic in water
• - it has a low melting point

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Predictions about F2 and At2

• What are their physical states?
• What colour are they?
• What will they react with?
• How will they react?
• Draw a dot and cross diagram for F2
• What do you notice about this diagram link with
  shapes and molecules

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Astatine

• the elements get darker in color with increasing
  molecular weight and atomic number.
• Thus, following the trend, astatine would be
  expected to be a nearly black solid, which, when
  heated, sublimes into a dark, purplish vapor
  (darker than iodine).
• Astatine is expected to form ionic bonds with
  metals such as sodium, like the other halogens,
  but it can be displaced from the salts by
  lighter, more reactive halogens.
• Astatine can also react with hydrogen to form
  hydrogen astitide, which when dissolved in water,
  forms hydroastatic acid. Astatine is the least
  reactive of the halogens, being less reactive
  than iodine.
• It is difficult to study as it is very rare,
  radioactive only last hours when you do isolate
  it.

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Fluorine

• It has an oxidation number -1, except when bonded
  to another fluorine in F2 which gives it an
  oxidation number of 0.
• Fluorine even combines with argon, krypton,
  xenon, and radon.
• Even in dark, cool conditions, fluorine reacts
  explosively with hydrogen. The reaction with
  hydrogen occurs even at extremely low
  temperatures, using liquid hydrogen and solid
  fluorine.
• It is so reactive that metals, and even water, as
  well as other substances, burn with a bright
  flame in a jet of fluorine gas. It is far too
  reactive to be found in elemental form. In moist
  air it reacts with water to form also-dangerous
  hydrofluoric acid.
• Fluorides are compounds that combine fluorine
  with some positively charged counterpart. They
  often consist of crystalline ionic salts.
  Fluorine compounds with metals are among the most
  stable of salts.
• Hydrogen fluoride is a weak acid when dissolved
  in water. Consequently, fluorides of alkali
  metals produce basic solutions.

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