CH10 Group 7 Flashcards
(24 cards)
Trend in BP down group 7 - why
Increases - size of atoms increases so stronger van der Waals forces between molecules
Trend in electronegativity down group 7 - why
Decreases - more occupied electron shells, further distance between electron and positive charge of nucleus so lower force of attraction
What is used to test for halide ions
Acidified AgNO3
Why is HNO3 added not HCl
To remove Co2 2-
Adding HCl would add Cl- ions and therefore give a false positive
Result and equation for Cl- test
White ppt
Ag+ + Cl- –> AgCl (s)
Result and equations for Br- test
Cream ppt
Ag+ + Br- –> AgBr (s)
Result and equation for I- test
Yellow ppt
Ag+ + I- –> AgI (s)
What happens when each of the silver halides is reacted with conc or diluted NH3
AgCl dissolves in both dilute and conc NH3
AgBr only dissolves in conc NH3
AgI does not dissolve in either
Trend in oxidising ability down the group - why
Decreases - CL- has fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus
Equation for Cl2 oxidising 2I-
Cl2 + 2I- –> 2Cl- + I2
Trend in reducing ability down the group - why
Increases - I- has the most occupied electron shells, so outer electrons further from the nucleus and are easily lost
Products formed when I- reduces H2SO4
H2SO4 + 2I- –> SO4 2- + 2HI
H2SO4 + 2H+ + 2I- –> SO2 + I2 + 2H2O
H2SO4 + 6H+ + 6I- –> S + 3I2 + 4H2O
H2SO4 + 8H+ + 8I- –> H2S + 4I2 + 4H2O
Products of Br- and 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
Forms ClO- ions which oxidise all microorganisms
Once it has done its job, little remains and the health benefits outweigh the risks
Potential risks of adding chlorine to water
Chlorine is toxic and damages the respiratory system. 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
Equation for Cl2 + water
Cl2 (g) + H2O (l) –> HClO (aq) + HCl (aq)
What type of reaction is this
Disproportionation; chlorine is both oxidised and reduced
2 forms of chlorate ions
ClO- ==> chlorate (I)
ClO3- ==> chlorate (V)
Equation for bleach
Cl2 + 2NaOH –> NaCl + NaClO + H2O
Reaction of chlorine and water in presence of sunlight
2Cl2 + 2H2O –> 4HCl + O2
What is desalination
Converts saltwater into clean, potable water by either reverse osmosis or by vacuum distillation at low pressure and low temperature
Advantages of desalination
Safe, clean, drinkable water produced in places where it may not be available
