2.3 Group 7

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 de 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
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

Question 3

What do you use to test for halide 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 creating a false positive

Question 5

Result and equation for Cl- test?

Answer 5

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 ammonia are added ?

Answer 8

AgCl (s) - dissolves in both conc and dilute ammonia
AgCl (s) + 2NH3 —> [Ag(NH3)2]+ (aq) + Cl-
AgBr (s) - dissolves in conc ammonia
AgBr (s) + 2NH3 —> [Ag(NH3)2]+ (aq) + Br-
AgI (s) will not dissolve in either

Question 9

What is the trend in oxidising ability down Group 7? Why?

Answer 9

Decreases down the 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 down Group 7?

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 positive charge of nucleus and outer electrons —> easiest to be oxidised and lose electrons —> best reducing agent

Question 12

What products are formed when I reacts with H2SO4? [4]

Answer 12

H2SO4 + 2I- —> SO4 2- + 2HI
H2SO4 + 2H+ + 2I- —> SO2 + I2 + 2H2O (SO2 is a choking gas with a pungent smell)
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 kill all microorganisms in the water.
Once it has done it’s job, little remains, and the health benefits outweigh the risks of using it

Question 16

What are the potential risks of adding chlorine to 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 reaction of Cl2 with water?

Answer 18

Cl2 (g) + H2O (l) —> HClO (aq) + HCl (aq)

Question 19

What type of reaction is 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 —> NaCl + NaClO + H2O

Question 22

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

Answer 22

2Cl2 + 2H2O —> 4HCl + O2

Question 23

What is desalination?

Answer 23

Converts saltwater into clean, potable water
Either by reverse osmosis (using a smart membrane) or by vacuum distillation at low temperature and low pressure

Question 24

What are the advantages and disadvantages of desalination?

Answer 24

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