Group 7 - inorganic I

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
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

Why do you add HNO3? Why not HCl?

Answer 3

To remove CO32-
Adding HCl would add Cl- ions, giving a false positive result

Question 3

What do you use to test for halide ions?

Answer 3

Acidified AgNO3

Question 4

Result and equation for Cl- test?

Answer 4

white ppt
Ag+ + Cl- → AgCl (s)

Question 5

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

Answer 5

Cream ppt
Ag+ + Br- → AgBr (s)

Question 6

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

Answer 6

Yellow ppt
Ag+ + I- → AgI (s)

Question 7

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

Answer 7

AgCl- dissolves 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- will not dissolve in either

Question 8

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

Answer 8

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 9

What is the trend in reducing ability of the halides down the group? Why?

Answer 9

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 10

Write the equation for Cl2 oxidising 2I-

Answer 10

Cl2 + 2I- → 2Cl- + I2

Question 11

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

Answer 11

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 bad/rotten eggs)

Question 12

What are the products of Br- + H2SO4?

Answer 12

HBr and SO2

Question 13

Does Cl- reduce H2SO4?

Answer 13

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

Question 14

Why is chlorine added to drinking water? Why is it safe?

Answer 14

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

Question 15

What are potential risks of adding chlorine to drinking water?

Answer 15

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

Question 16

Why is ozone not used to purify water in the UK?

Answer 16

More expensive than chlorine, evaporates from water more quickly

Question 17

What is the equation for the reaction of Cl2 with water?

Answer 17

Cl2 (g) + H2O (l) → HClO (aq) + HCl (aq)

Question 18

What type of reaction is the reaction of chlorine with water?

Answer 18

Disproportionation; chlorine is both oxidised and reduced

Question 19

What are the two forms of the chlorate ion?

Answer 19

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

Question 20

What is the equation for making bleach?

Answer 20

Cl2 + 2NaOH → NaCl + NaClO + H2O NaClO is bleach

Question 21

What is desalination?

Answer 21

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 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 are the advantages and disadvantages of desalination?

Answer 23

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