CH10 Group 7

Question 1

Trend in BP down group 7 - why

Answer 1

Increases - size of atoms increases so stronger van der Waals forces between molecules

Question 2

Trend in electronegativity down group 7 - why

Answer 2

Decreases - more occupied electron shells, further distance between electron and positive charge of nucleus so lower force of attraction

Question 3

What is used to test for halide ions

Answer 3

Acidified AgNO3

Question 4

Why is HNO3 added not HCl

Answer 4

To remove Co2 2-

Adding HCl would add Cl- ions and therefore give a false positive

Question 5

Result and equation for Cl- test

Answer 5

White ppt

Ag+ + Cl- –> AgCl (s)

Question 6

Result and equations for Br- test

Answer 6

Cream ppt

Ag+ + Br- –> AgBr (s)

Question 7

Result and equation for I- test

Answer 7

Yellow ppt

Ag+ + I- –> AgI (s)

Question 8

What happens when each of the silver halides is reacted with conc or diluted NH3

Answer 8

AgCl dissolves in both dilute and conc NH3
AgBr only dissolves in conc NH3
AgI does not dissolve in either

Question 9

Trend in oxidising ability down the group - why

Answer 9

Decreases - CL- has fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus

Question 10

Equation for Cl2 oxidising 2I-

Answer 10

Cl2 + 2I- –> 2Cl- + I2

Question 11

Trend in reducing ability down the group - why

Answer 11

Increases - I- has the most occupied electron shells, so outer electrons further from the nucleus and are easily lost

Question 12

Products formed when I- reduces H2SO4

Answer 12

H2SO4 + 2I- –> SO4 2- + 2HI
H2SO4 + 2H+ + 2I- –> SO2 + I2 + 2H2O
H2SO4 + 6H+ + 6I- –> S + 3I2 + 4H2O
H2SO4 + 8H+ + 8I- –> H2S + 4I2 + 4H2O

Question 13

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

Answer 15

Forms ClO- ions which oxidise all microorganisms

Once it has done its job, little remains and the health benefits outweigh the risks

Question 16

Potential risks of adding chlorine to water

Answer 16

Chlorine is toxic and damages the respiratory system. 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

Equation for Cl2 + water

Answer 18

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

Question 19

What type of reaction is this

Answer 19

Disproportionation; chlorine is both oxidised and reduced

Question 20

2 forms of chlorate ions

Answer 20

ClO- ==> chlorate (I)

ClO3- ==> chlorate (V)

Question 21

Equation for bleach

Answer 21

Cl2 + 2NaOH –> NaCl + NaClO + H2O

Question 22

Reaction of chlorine and water in presence of sunlight

Answer 22

2Cl2 + 2H2O –> 4HCl + O2

Question 23

What is desalination

Answer 23

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

Question 24

Advantages of desalination

Answer 24

Safe, clean, drinkable water produced in places where it may not be available