PMT Group 7 Flashcards
List 2 properties of halogens
● Low melting and boiling points
● Exist as diatomic molecules
What is the trend in boiling
point down group 7? Why?
Increases down the group because:
- size of atom increases as more occupied electron shells
- therefore stronger London forces of attraction between molecules
- therefore take more energy to break
What is the trend in reactivity down group 7? Why?
Reactivity decreases because:
● Atomic radius increases
● Electron shielding increases
● Ability to gain an electron and form 1- ions
decreases due to lower attraction between that electron and the nucleus
Explain the trend of electronegativity down group 7
Down the group the electronegativity of the elements decreases.
This is because the atomic radii increases due to the increasing number of shells so
there is reduced nuclear attraction between the outermost electron and the nucleus.
What is the trend in oxidising
ability down group 7? Why?
Decreases down group (Cl strongest, I weakest)
This is because Cl has the fewest occupied electron shells,
greatest force of attraction between outer electrons and
nucleus and thus is the easiest to gain electrons and be
reduced → best oxidising agent
What is the trend in reducing
ability of the halides down the
group? Why?
Increases down the group (Cl- weakest, I- strongest)
I- has the most occupied electron shell so outer electrons are further from the nucleus, and so it has the weakest force of attraction between outer electrons and positive charge of nucleus.
Therefore it is the easiest to be oxidised and lose electrons → best reducing agent
Which species is oxidised in
this reaction:
Br2 (l) + 2Na (s) 2NaBr (s)
Na has been oxidised
Oxidation state of 0 to +1
When a more reactive halogen displaces a less reactive halide, what is the reaction called?
Displacement reaction
What is the colour of chlorine
in water?
Pale green
What is the colour of bromine
in water?
Orange
What is the colour of iodine in
water?
Brown
What is the colour of chlorine
in cyclohexane?
Pale green
What is the colour of bromine in cyclohexane?
Orange
What is the colour of iodine in
cyclohexane?
Violet
Out of the 3 halides Cl- , Br- & I- , which ones can be oxidised by chlorine?
Br- & I- ions
Write the equation for chlorine
oxidising bromide ions in water
and associated colour change
Cl₂ (aq) + 2Br- (aq) → 2Cl- (aq) + Br₂ (aq)
Yellow solution
Write the equation for Cl2
oxidising 2I- in cyclohexane
and associated colour change
Cl₂ (aq) + 2I- (aq) → 2Cl- (aq) + I₂ (aq)
Purple solution
Out of the 3 halides Cl- , Br- & I- , which one of these can be oxidised by bromine?
I- ions
Write the equation for bromine
oxidising iodide ions in water
and associated colour change
Br₂ (aq) + 2I- (aq) → 2Br- (aq) + I₂ (aq)
Brown solution
Out of the 3 halides Cl- , Br- &
I- , which one of these can be
oxidised by iodine?
Does not oxidise Cl-
or Br-
Define disproportionation
The oxidation and reduction of the same
element in a redox reaction
What is the equation for the
reaction of Cl2
with water?
Cl₂ (g) + H₂O (l) → HClO (aq) + HCl (aq)
What type of reaction is the
reaction of chlorine with
water?
Disproportionation; chlorine is both
oxidised and reduced
Why is chlorine added to
drinking water?
It kills the bacteria in the water and
makes it safer to drink
What are the two forms of the
chlorate ion?
ClO- is chlorate (I)
ClO₃- is chlorate (V)
What is the equation for
forming bleach and
conditions?
Cold dilute alkali
Cl₂ (aq) + 2NaOH (aq) → NaCl (aq) +
NaClO (aq) + H₂O (l)
NaClO is bleach
Show that the reaction of
chlorine with hot dilute NaOH
is a disproportionation reaction
3Cl2 (aq) + 6 NaOH(aq) 5 NaCl (aq) + NaClO3 (aq) + 3H2O (l) Chlorine is been reduced and oxidised. Oxidation state of chlorine has gone from: 0 to -1 in NaCl = reduction 0 to +1 in NaClO3 = oxidation
What do you use to test for
halide ions?
Acidified AgNO3
Why do you add HNO3
?
Why not HCl?
To remove CO₃2-
Adding HCl would add Cl- ions, giving a false
positive result
Result and equation for Cl- test
White precipitate
Ag+ + Cl- –> AgCl(s)
What is the result and
equation for the test for Br-?
Cream ppt
Ag+ + Br- → AgBr (s)
What is the result and
equation for the test for I-?
Yellow ppt
Ag+ + I- → AgI (s)
What happens (+ equations) to each of the silver halide precipitates when dilute/conc NH3 are added?
AgCl- dissolves in both dilute and conc
AgCl (s) + 2NH₃ (aq) → [Ag(NH₃)₂]+ (aq) + ClAg
Br- only dissolves in conc
AgBr (s) + 2NH₃ (aq) → [Ag(NH₃)₂]+ (aq) + Br
AgI- will not dissolve in either
What is the trend in
oxidising ability down the
group? Why?
Decreases down group (Cl best, I worst)
Because: Cl has fewest occupied electron shells, greatest
force of attraction between outer electrons and nucleus,
easiest to gain electrons and be reduced → best oxidising
agent
What is the trend in
reducing ability of the
halides down the group?
Why?
Increases down the group (Cl-
worst, I- best)
Because: I-
has the most occupied electron shells, so outer
electrons are further from the nucleus, weakest force of
attraction between outer electrons and positive charge of
nucleus → easiest to be oxidised and lose electrons → best
reducing agent
What products are formed when I- reduces H2SO4 ? Do equations for all 4.
H₂SO₄ + 2I- → SO₄2- + 2HI
H₂SO₄ + 2H+ + 2I- → SO₂ + I₂ + 2H₂O (SO2 is a choking gas with a pungent odour)
H₂SO₄ + 6H+ + 6I- → S + 3I₂ + 4H₂O (S is a yellow solid)
H₂SO₄ + 8H+ + 8I- → H₂S + 4I₂ + 4H₂O (H2S smells of bad/rotten eggs)
What are the products of Br-
+ H2SO4
?
HBr and SO₂
Does Cl-
reduce H2SO4
?
No, not a powerful enough reducing agent; only
HCl is formed
How can you test for
carbonate ions, CO3
2-?
Add strong acid to the sample
Collect the gas produced
Pass through lime water
What are the observations for
a positive test of carbonate
ions, CO3
2-?
Fizzing
Limewater turns cloudy
Write an equation for the
carbonate ion test
CO3
2- (aq) + 2H+
(aq) → H2O (aq) + CO2
(g)
How can you test for sulfate
ions, SO4
2-?
● Add dilute hydrochloric acid and
barium sulphate to the sample
What are the observations for
a positive test of sulfate ions,
SO4
2-?
White precipitate of barium sulfate is
produced
Write an equation for the
sulfate ion test
Ba2+ (aq) + SO4
2- (aq) → BaSO4
(s)
What do you use to test for
halide ions?
Acidified AgNO3
When testing for carbonate,
sulfate and halide ions, in
which order should the tests
be carried out and why?
- Carbonate test
- Sulfate test
- Halide test
Because barium ions forms insoluble precipitate of BaCO3
and silver ions form insolube precipitate of Ag2SO4
How can you test for
ammonium ions, NH4
+
?
Add sodium hydroxide to the sample and
warm it
Test the gas produced with red litmus paper
What are the observations for
positive ammonium ions test?
● Red litmus paper turns blue
● Ammonia has a pungent smell
Write the equation for
ammonium ions test
NH4+ (aq) + OH- (aq) → NH3 (aq) + H2O (aq)
