Group 7 Flashcards
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 → stronger van der Waals forces of attraction between molecules, take more energy to break
What is the trend in electronegativity down group 7? Why?
Decreases
Because: more occupied electron shells → greater atomic radius and outer electrons are further from the positive charge of the nucleus → lower force of attraction between the nucleus and electron pair in the covalent bond
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 ppt
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) + Cl-
AgBr- 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
Write the equation for Cl2 oxidising 2I-
Cl₂ + 2I- → 2Cl- + I₂
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
Why is chlorine added to drinking water? Why is it safe?
Forms ClO- ions which oxidise (kill) all
microorganisms in water.
Once it has done its job, little remains, and the health benefits outweigh the risks of using it
What are potential risks of adding chlorine to drinking water?
Chlorine is toxic and damages the respiratory system in large enough quantities; can form carcinogens with hydrocarbons
Why is ozone not used to purify water in the UK?
More expensive than chlorine, evaporates from water more quickly
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
What are the two forms of the chlorate ion?
ClO- is chlorate (I)
ClO₃- is chlorate (V)
What is the equation for making bleach?
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
NaClO is bleach
Give the equation for the reaction of chlorine and water in the presence of sunlight
2Cl₂ + 2H₂O → 4HCl + O₂ (g)
What is desalination?
Converts saltwater into clean, potable water Either by reverse osmosis (using a smart membrane) or by vacuum distillation at low pressure and low temperature
What are the advantages and disadvantages of desalination?
Advantages - safe, clean, drinkable water produced in places where it might not otherwise be available
Disadvantages - uses lots of energy, reverse osmosis has low efficiency, can disturb marine ecosystems
