Halogens (Group 7) Flashcards
What is chlorine widely used for?
Treating water, making it fit for drinking.
What do the halogens exist as in nature?
Diatomic molecules
Describe the trend in boiling point down group 7.
As you go down group 7, the size of molecule increases.
London forces increase, as there are more electrons.
Need more energy to break
How can chlorine be used to make bleach?
Bleach is made by reacting chlorine with sodium hydroxide to make sodium chlorate (I) solution.
Cl2 + 2NaOH -> NaCl + NaOCl
This is a disproportionation reaction, as the same element (chlorine) is both oxidised (Cl2 -> NaOCl) and reduced (NaCl)
Describe the advantages and disadvantages of using chlorine in water treatment plants
+ - the chlorine kills bacteria in the water.
- chlorine is toxic, so care has to be taken in the plant.
- chlorine can react with organic matter to form chlorinated hydrocarbons which may be carcinogenic.
Write an equation for the equilibrium when a chlorine molecule reacts with a water molecule to form chloric (I) acid, HClO
Cl2 + H2O ⇌ HClO + HCl
What happens when the halogens form an ionic compound?
Halogen atoms each gain one electron when forming an ionic compound.
Describe the trend in reactivity as you go down group 7
Reactivity reduces down group 7.
Further down the group, less energy is released when the halide ion is formed.
Explain the trend in reactivity as you go down group 7
The reactivity reduces because, further down the group:
- the added electron is further from the nucleus
- the added electron is shielded more by electrons in inner shells
- less energy is released by forming the ion.
What is the rule of displacement reactions of halogens with halides?
A more reactive halogen will displace a less reactive halogen from its salt.
Result of Cl2 added to Cl- (aq)
No change
Result of Br2 added to Cl- (aq)
No reaction - chlorine more reactive
Result of I2 added to Cl- (aq)
No reaction - chlorine more reactive
Result of Cl2 added to Br- (aq)
Solution turns orange
Result of Br2 added to Br- (aq)
No change
Result of I2 added to Br- (aq)
No reaction - bromine more reactive
Result of Cl2 added to I- (aq)
Solution turns brown
Result of Br2 added to I- (aq)
Solution turns brown
Describe the displacement reactions of the halogens
Going down the group, oxidising ability…
decreases
Why do flouride ions and chloride ions not do redox reactions with H2SO4?
They are not strong enough reducing agents to reduce S in H2SO4.
Redox reaction between NaBr and H2SO4
2Br- + H2SO4 + 2H+ -> SO2 + 2H2O + Br2
In this reaction, the oxidation state of
sulfur is reduced from +6 to +4.
Which reactions take place between bromide ions and concentrated sulfuric acid, and what would we observe?
Acid-base reactions and redox reactions.
We would observe brown vapour.
When concentrated sulfuric acid is added to sodium iodide salt, we can make several observations.
State the chemical formula of the substances which is observed as:
- steamy fumes
- black solid
- smell of rotten eggs
steamy fumes = HI (g)
black solid = I2 (s)
rotten eggs = H2S (G)
When concentrated sulfuric acid is added to sodium iodide salt, what type of reactions take place?
- Acid-base reaction
- Redox reaction
What is the name given to a halogen that has gained an electron?
Halide
What happens to reducing power down group 7?
Down the group of halides, reducing power increases.
This trend is explained by the increasing number of electron shells.
This causes the distance between the outer electrons and the nucleus to increase, so that attraction decreases.
Therefore, the electron is more easily lost and donated to another atom which reduces that atom.
Why do we acidify the aqueous solution of silver nitrate?
Soluble carbonates and hydroxides would react with silver nitrate to form insoluble precipitates. This would invalidate using silver nitrate as a test to identify halide ions.
In order to prevent these precipitates forming, we acidify the aqueous solution using dilute
nitric acid.
This reacts with hydroxides to form
water.
It also reacts with carbonates to form
water and carbon dioxide.
