3.2.3 Group 7, the halogens

Question 1

what is the trend in boiling point down group 7? why?

Answer 1

increases
because size of atom increases as more occupied shells - stronger VdW forces between molecules
take more energy to overcome

Question 2

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

Answer 2

decreases
because more occupied electron shells
greater atomic radius
outer electrons are further from the positive charge of the nucleus
lower force of attraction between nucleus and electron pair in the covalent bond

Question 3

what do we use to test for halide ions?

Answer 3

acidified silver nitrate

Question 4

why do we add HNO3 when testing for halide ions? why not HCl?

Answer 4

to remove CO3(2-)
adding HCl would add Cl- ions, giving a false positive result

Question 5

what is the result of the Cl- test? what chemical do we use?

Answer 5

white precipitate
silver nitrate

Question 6

what is the result of the Br- test? what chemical do we use?

Answer 6

cream precipitate
silver nitrate

Question 7

what is the result and equation for the test for I-? what chemical solution do we use?

Answer 7

yellow precipitate
silver nitrate

Question 8

what happens to each of the silver halide precipitates when dilute/conc NH3 are added?

Answer 8

AgCl dissolves in both dilute and conc
AgBr only dissolves in conc
AgI will not dissolve in either.

Question 9

what is the trend in oxidising ability down group 7? why?

Answer 9

decreases
Cl has fewest occupied electron shells
greatest attraction between outer electrons and the nucleus
easiest to gain electrons and be reduced
best oxidising agent

Question 10

write the equation for Cl2 oxidising 2I-

Answer 10

2I- + Cl2 —> I2 + 2Cl-

Question 11

what is the trend in reducing ability down group 7? why?

Answer 11

increases
I- has the most occupied electron shells
outer electrons further from the nucleus, weak attraction
easiest to be oxidised and lose electrons
best reducing agent

Question 12

what products are formed when I- reduces H2SO4?

Answer 12

2HI or
SO2 + I2 + 2H2O or
S + 3I2 + 4H2O or
H2S + 4I2 + 4H2O

Question 13

what are the products of Br- + H2SO4?

Answer 13

HBr and SO2

Question 14

does Cl- reduce H2SO4?

Answer 14

no
not 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 its job, little remains and the 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 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 then chlorine, evaporates from water more quickly

Question 18

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

Answer 18

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

Question 19

what type of reaction is the reaction between chlorine and water?

Answer 19

disproportionation - chlorine both oxidised and reduced

Question 20

what are the two forms of the chlorate ion?

Answer 20

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

Question 21

what is the chemical bleach?

Answer 21

NaClO

Question 22

What chemicals react to form bleach?

Answer 22

Cl2
2NaOH

Question 23

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

Answer 23

2Cl2 + 2H2O —> 4HCl + O2

Question 24

What is desalination?

Answer 24

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 produced in places where it might not otherwise be available
Disadvantages
Uses lots of energy, reverse osmosis has low efficiency, can disturb marine ecosystems