Group 7 Flashcards
Explain the trend in boiling point of group 7:
- There are Van der Waals forces between molecules
- The molecules get larger as you go down the group as there’s more electrons
- Therefore there’s stronger Van Der Waals forces between molecules
- So there’s more energy required to overcome the intermolecular forces
- Therefore the boiling point increases
Explain the trend in atomic/ionic radius in group 7:
- Number of filled shells increases so the shielding increases
- Forces of attraction between the nucleus and outer electrons decreases
- Therefore atomic radius increases
Explain the trend in electronegativity in group 7:
- Shielding increases
- Nuclear charge increases
- Atomic radius increases
- Forces of attraction between nucleus and outer electrons decreases
- Therefore, electronegativity decreases
Explain the trend in reactivity in group 7:
- Shielding, atomic radius increases
- Forces of attraction between the nucleus and outer electrons decreases
- It’s harder to gain an electron
- Reactivity decreases
What does oxidising power mean?
How good it is at accepting electrons.
(An oxidising agent is the one that’s reduced).
Predict and explain the trend in oxidising power of the halogens:
(Strongest) F2 > Cl2 > Br2 > I2 (Weakest)
It’s essentially the same as reactivity.
- Shielding increases
- Atomic radius increases
- Forces of attraction between nucleus and outer electrons decreases
- Harder to accept electrons so their oxidising power decreases
Describe the appearance of chlorine gas and chlorine water
Pale yellow green gas
Very pale green solution, often colourless
Describe the appearance of bromine water
Orange-brown liquid
A gas can form brown fumes
Describe the appearance of Iodine:
Brown solution/ Black solid
Describe the appearance of flourine:
Pale yellow gas
Describe and explain the trend of reducing powers of halide ions:
- Increases
- More shells, more shielding, atomic radius increases
- Weaker forces of attraction between nucleus and outer electrons
- So it’s easier to lose those electrons
Why does F- not act as a reducing agent?
- Fewer shells/ little shielding
- Small ionic radius
- Very strong forces of attraction between nucleus and outer electrons
- Requires a lot of energy to lose those electrons
Concentrated H2SO4 can be further reduced by which halide ions?
Bromide ions
Iodide ions
Why can’t chlorine reduce sulfuric acid?
It’s not a strong enough reducing agent
What can bromide ions reduce sulfuric acid into?
SO2
What can iodide ions reduce sulfuric acid into?
SO2
S
H2S
What are the observations when bromide ions reduce sulfuric acid into SO2?
- Brown fumes of Br2
- SO2 is colourless (There’s no observation)
What are the observations when iodide ions reduce sulfuric acid into S?
- Black solid Iodine
- S is a yellow solid
What are the observations when iodide ions reduce sulfuric acid into sulfur?
S = yellow solid
I2 = black solid
What are the observations when iodide ions reduce sulfuric acid into H2S?
Bad egg smell (H2S)
Black solid = I2
When answering exam questions on testing for halides, what should you be careful of?
In exam questions, they often start with solids. So make sure the first step in a method is to add water to make a solution.
How do we test for halides?
- Place a solution of the halide in a test tube, 5cm3
- Add 1cm3 dilute nitric acid. This removes any other ions that could give a precipitate with silver nitrate.
- Add silver nitrate solution
- White, cream or yellow precipitate formed
- treat any precipitate with dilute ammonia solution
- if precipitate still exists, add concentrated ammonia
Which halide makes a white precipitate?
Chloride
Which halide gives a cream precipitate?
Bromide
Which halide gives a yellow precipitate?
Iodide
Why do we use nitric acid and not something like HCL?
HCL would react with the silver and precipitate.
What do you observe when you add:
- dilute NH3
- Concentrated NH3
to Agcl?
AgCl dissolves in both
What do you observe when you add:
- dilute NH3
- Concentrated NH3
to AgBr?
Dilute NH3;
- does not dissolve
Conc:
- dissolves
What do you observe when you add:
- dilute NH3
- Concentrated NH3
to AgI?
Dilute NH3:
Does not dissolve
Conc.
Does not dissolve
(Think of it as the halogens become less reactive as you go down the group).
Why have we not mentioned flouride ions in the test for halides?
AgF is soluble in water. You cannot test for F- using silver nitrate.
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