3.2.3 Group 7, Halogens

Question 1

Standard States - Fluorine

Answer 1

Pale yellow gas

Question 2

Standard States - Chlorine

Answer 2

Green gas

Question 3

Standard States - Bromine

Answer 3

Red-brown liquid

Question 4

Standard States - Iodine

Answer 4

Grey solid

Question 5

What is the trend of boiling points in group 7? (3)

Answer 5

Increase down the group
Size of molecules and relative mass increases
Stronger Van der Waals forces

Question 6

What is the trend in electronegativity in group 7? (3)

Answer 6

Decreases down the group
Shielding increases and outer electrons are further from nucleus
Larger atoms attract electrons less than smaller ones

Question 7

Why are halogens oxidising agents?

Answer 7

They gain an electron when they react

Question 8

How does the oxidising ability change down group 7? (3)

Answer 8

Become less oxidising down the group
Outer shell is further from nucleus
Less attraction to gain an electron

Question 9

When will a halogen displace a halide from solution?

Answer 9

If the halide is below it in the periodic table

Question 10

Displacement reactions - chlorine + potassium chloride

Answer 10

No reaction

Question 11

Displacement reactions - chlorine + potassium bromide

Answer 11

Orange solution (bromine) formed

Question 12

Displacement reactions - chlorine + potassium iodide

Answer 12

Brown solution (iodine) formed

Question 13

Displacement reactions - bromine + potassium chloride

Answer 13

No reaction

Question 14

Displacement reactions - bromine + potassium bromide

Answer 14

No reaction

Question 15

Displacement reactions - bromine + potassium iodide

Answer 15

Brown solution (iodine) formed

Question 16

Displacement reactions - iodine + potassium halide

Answer 16

No reaction with chloride, bromide or iodide

Question 17

How is bleach made? (2)

Answer 17

Mix chlorine gas with cold, dilute sodium hydroxide at room temperature
2NaOH (aq) + Cl2 (g) > NaClO (aq) + H2O (l)

Question 18

What is disproportionation?

Answer 18

When the same substance is both oxidised and reduced in the same reaction (e.g. chlorine in the process to make bleach)

Question 19

What happens when chlorine reacts with water? (2)

Answer 19

It undergoes disproportionation, forming chloride and chlorate(1) ions
Cl2 (g) + H2O (l) <> 2H+ (aq) + Cl- (aq) + ClO- (aq)

Question 20

What happens when chlorine and water react in sunlight? (2)

Answer 20

Water decomposes to form chloride ions and oxygen

Cl2 (aq) + H2O (l) <> 2H+ (aq) + 2Cl- (aq) + 1/2 O2 (g)

Question 21

Why is chlorine part of the water treatment process? (4)

Answer 21

Chlorate(1) ions kill bacteria
Some chlorine persists in water, preventing reinfection
Prevents the growth of algae, eliminating bad tastes and smells
Removes discolouration caused by organic compounds

Question 22

What are the disadvantages of adding chlorine to water? (4)

Answer 22

Toxic
The gas is an irritant for the respiratory system
Liquid chlorine causes severe chemical burns on skin and eyes
Can react with organic compounds in water to form chlorinated hydrocarbons which are carcinogenic

Question 23

Why is chlorine added to water, despite the disadvantages?

Answer 23

Risks are small compared to the risks from untreated water

Question 24

What is bleach used for? (3)

Answer 24

Water treatment
Bleaching paper and textiles
Cleaning toilets

Question 25

What is the trend of reducing ability of the halide ions? (3)

Answer 25

Increases down group
Shielding increases
Electrons are further from the nucleus so lost more easily

Question 26

Reactions with Sulphuric Acid - Sodium Chloride (3)

Answer 26

NaCl (s) + H2SO4 (l) > NaHSO4 (s) + HCl (g)
Forms misty fumes of HCl
Not strong enough to reduce sulphuric acid (not redox reaction)

Question 27

Reactions with Sulphuric Acid - Sodium Bromide (4)

Answer 27

NaBr (s) + H2SO4 (l) > NaHSO4 (s) + HBr (g)
Forms misty fumes of HBr
2HBr (aq) + H2SO4 (l) > Br2 (g) + SO2 (g) + 2H2O (l)
Forms choking fumes of SO2 and orange fumes of Br2

Question 28

Reactions with Sulphuric Acid - Sodium Iodide (5)

Answer 28

NaI (s) + H2SO4 (l) > NaHSO4 (s) + Hl (g)
Forms misty fumes of HI
2HI (aq) + H2SO4 (l) > I2 (g) + SO2 (g) + 2H2O (l)
6HI (g) + SO2 (g) > H2S (g) + 3I2 (s) + 2H2O (l)
Produces fumes of H2S and solid iodine

Question 29

How are halides tested for? (5)

Answer 29

Add dilute nitric acid to remove other ions and then silver nitrate solution
Fluoride - no precipitate (silver fluoride is soluble in water)
Chloride - white precipitate
Bromide - cream precipitate
Iodide - yellow precipitate

Question 30

What is the trend in solubility of the silver halides in ammonia

Answer 30

Going down the group 7 halides, solubility decreases

Question 31

How are group 2 ions identified? (4)

Answer 31

Flame tests - dip nichrome wire in HCl (cleans it) and then burn the unknown compound in a blue flame
Calcium - brick red
Strontium - red
Barium - pale green

Question 32

How are ammonium ions identified? (2)

Answer 32

Damp piece of red litmus paper (lets ammonia gas dissolve) - turns blue
Add hydroxide ions (dilute NaOH) to produce ammonia gas and gently heat mixture below a piece of litmus paper

Question 33

How are hydroxide ions identified?

Answer 33

Use a pH indicator and look for alkaline results

Question 34

How are carbonate ions identified? (3)

Answer 34

Add an acid
HCl causes carbonate ions to fizz due to the reaction with hydrogen ions to give carbon dioxide
Confirm presence of CO2 by using limewater (turns cloudy)