2.3 Group 7 Halogens

Question 1

What is the trend in bpt down group 7? Why?

Answer 1

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

Question 2

What is the trend in electronegativity down group 7?why?

Answer 2

Decreases down the group

Because: more occupied electron shells increasing the atomic radius so the outer electrons are further from the positive charge of the nucleus this causes the to be a lower force of attraction between the nucleus and electrons pair in the covalent bond

Question 3

What do you use to test for halide ions?

Answer 3

Acidified AgNO3 (silver nitrate)

Question 4

Why do you add HNO3?

Why not HCl?

Answer 4

Adding HNO3 removes 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 for Br-?

Answer 6

Cream ppt

Ag+ + Br- —> AgBr (s)

Question 7

What is the result and equation for the test for I-?

Answer 7

Yellow ppt

Ag+ + I- —> AgI (s)

Question 8

What happens (+equations) to each of the silver halide precipitates when dilute/conc NH3 are added?

Answer 8

AgCl- dissolves in both dilute and conc NH3

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

Question 9

What is the trend in oxidising ability down the group? Why?

Answer 9

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

Question 10

Write the 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 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

Question 12

What products are formed when I- reduces H2SO4? Do equations for all 4

Answer 12

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 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 (kill) all microorganisms in water
Once it has done it’s job little remains and the health benefits outweigh the risk of using it

Question 16

What are the 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(I) —> 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 two forms of the chlorite 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 desalinisation?

Answer 23

Converts saltwater into clean, potable water

Either by reverse osmosis (using a smart membrane) or by vacuum distillation at low pressure and low temperature

Question 24

What are the advantages and disadvantages of desalination

Answer 24

Advantages- safe drinkable water produces in places where it might not otherwise be available

Disadvantages- uses lots of energy, reverse osmosis has low efficiency, can disturb marine ecosystem