2.3 Group 7 the halogens

Question 1

What is the trend in bpt down group[ 7, why?

Answer 1

Increases down group
Size of atom increases as more occupied electron shells –> Stronger van der waals forces of attraction between molecules, take more energy to break

Question 2

What is the trend in electronegativity down group 7, why

Answer 2

Decreases
More occupied electron shells –> Greater atomic radius and outer electrons are further from positive charge of nucleus –> lower FOA between the nucleus and electron pair in the covalent bond

Question 3

What do you use to test for halide ions

Answer 3

Acidified AgNO3

Question 4

Why do you add HNO3 not HCl

Answer 4

To remove CO3 2-
Adding HCl would add Cl- ions giving a false positive result

Question 5

Result and equation for cl- test

Answer 5

White ppt
Ag+ + Cl- –> AgCl (s)

Question 6

What is the result and equation for the test of Br-

Answer 6

Cream ppt
Ag+ + Br- –> AgBr (s)

Question 7

What is the result and equation for the test I-

Answer 7

Yellow ppt
Ag+ + I- –> AgI (s)

Question 8

What happens to each silver halide precipitates when dilute/conc NH3 are added

Answer 8

AgCl- dissolves in both dilute and conc
AgBr- only dissolves in conc
AgI will not dissolve in either

Question 9

What is the trend in oxidising ability down the group why

Answer 9

Decreases down
Cl has fewest occupied electron shells, greatest FOA between outer electrons and nucleus, easiest to gain electrons and be reduced –> best oxidising agent

Question 10

Write equation for Cl2 oxidising 2I-

Answer 10

Cl2 + 2I- –> 2Cl-+ I2

Question 11

What is the trend in reducing ability of the halides down the group, why

Answer 11

Increases
I- has most occupied electron shells, so outer electrons are further from the nucleus, weakest FOA between outer electrons and positive charge of nucleus –> easiest to be oxidised and lose electrons –> best reducing agent

Question 12

What products are formed when I- reduces H2SO4 do equations
H2SO4 + 2I- –> SO42- + 2HI
H2SO4 + 2H+ + 2I- –> SO2 + I2 + 2H2O (SO2 is choking gas)

Answer 12

H2SO4 + 2I- –> SO42- + 2HI
H2SO4 + 2H+ + 2I- –> SO2 + I2 + 2H2O (SO2 is choking gas)
H2SO4 + 6H+ + 6I- –> S + 3I2 + 4H2O (s is a yellow solid)
H2SO4 + 8H+ + 8I- –> H2S + 4I2 + 4H2O (H2S smells of rotten eggs)

Question 13

What are the products of Br- + H2So4

Answer 13

HBr and SO2

Question 14

Does Cl- reduce H2SO4

Answer 14

No, not a powerful enough reducing agent, only HCl is formed

Question 15

Why is chlorine added to drinking water, why is it safe

Answer 15

Forms ClO- ions which oxidise all microorganisms in water
Once it has done its job, little remains and the health benefits outweighs the risks of using it

Question 16

What are potential risks of adding chlorine to drinking water

Answer 16

Chlorine is toxic and damages the respiratory system in large enough quantities; can form carcinogens with hydrocarbons

Question 17

Why is ozone not used to purify water in the UK

Answer 17

More expensive than chlorine, evaporates from water more quickly

Question 18

What is the equation for the reaction of Cl2 with water

Answer 18

Cl2(g) + H2O(l) –> HClO(aq) + HCl(aq)

Question 19

What type of reaction is the reaction of chlorine with water

Answer 19

Disproportionation; chlorine is both oxidised and reduced

Question 20

What are the 2 forms of the chlorate ion

Answer 20

ClO- is chlorate (I)
ClO3- is chlorate (V)

Question 21

What is the equation for making bleach

Answer 21

Cl2 + 2NaOH –> NaCl + NaClO + H2O
NaClO is bleach

Question 22

Give the equation for the reaction of chlorine and water in the presence of sunlight

Answer 22

2Cl2 + 2H2O –> 4HCl + O2 (g)

Question 23

What is desalination

Answer 23

Converts saltwater into clean, potable water
Either by reverse osmosis or by vacuum distillation at low pressure and low temperature

Question 24

What are the advantages and disadvantages of desalination

Answer 24

A: Safe, clean, dirnkable water produced in places where it might not otherwise be available

Question 25

What are the advantages and disadvantages of desalination

Answer 25

A: Safe, clean, drinkable water produced in places where it might not otherwise be available
D: Uses lots of energy, reverse osmosis has low efficiency, can disturb marine ecosystems