Group 7- Halogens

Question 1

What is the trend in boiling point going down group 7? Why?

Answer 1

Increases down the group
Because the size of the atom increases as more occupied electron shells, stronger van Der waals forces between molecules. Requires more energy to break.

Question 2

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

Answer 2

Decreases
More occupied electron shells so it has a greater atomic radius and outer electrons are further from the nucleus. There’s a lower force of attraction between nucleus and electron pair in the covalent bond.

Question 3

What do you use to test for haloed ions?

Answer 3

Acidified AgNO3

Question 4

Why do you add HNO3? Why 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 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 to each of the silver halide ppts when dilute/conc NH3 are added? (Include equations)

Answer 8

AgCl: dissolved 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: doesn’t dissolve in either

Question 9

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

Answer 9

Decreases down the group
Cl has fewest occupied electron shells,greater 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

Increase down the group.
I- has the most occupied electron shells, so outer electrons are further from nucleus. Weaker force of attraction between outer electrons and nucleus. Easiest to be oxidised and lose electrons. Best reducing agent

Question 12

What products are formed when I- reduces to H2SO4?

Answer 12

H2SO4 + 2I- —-> SO4 2- + 2HI

H2SO4 + 2H+ + 2I- —> SO2 + I2 + 2H2O ( SO2 is a choking gas with a pungent colour)

H2SO4 + 6H+ + 6I- —> S + 3I2 + 4H2O
( S is a yellow solid)

H2SO4 + 8H+ + 8I- —> H2S + 4I2 + 4H2O
(H2S smells of rotten egg)

Question 13

What are the products of Br and 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 and why is it safe?

Answer 15

Forms ClO- ions which oxidise/kill all micro organisms in water. Once it has done it’s job, little remains, and health benefits outweigh the risks of using it

Question 16

What are potential risks of adding chlorine to drinking water?

Answer 16

Chlorine is toxic and damages respiratory system in large 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 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 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 —-> NaCk + 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 salt water 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,clean,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 ecosystems