Group 7 - Halogens

Question 1

Presentations of group 7 elements

Answer 1

Fluorine: very pale yellow gas
Chlorine: greenish gas
Bromine: red liquid
Iodine: shiny grey solid, purple gas

Question 2

Trend in melting and boiling point of group 7

Answer 2

Increase down the group

•As the molecules become larger they have more electrons and so have larger VDWs forces between the molecules
•As the intermolecular forces get larger more energy is needed to break the forces
•This increases the mp and bp

Question 3

Trend in Eneg in group 7

Answer 3

The Eneg decreases going down

•As you go down the atomic radius increases due to increasing number of shell
• the nucleus is therefore less able to attract to the outer electrons

Question 4

Oxidising strength of group 7

Answer 4

Decreases down the group

Oxidising agents are electron acceptors

Question 5

Are halogens good oxidising agents

Answer 5

Yes and one that is a strong one will displace a halogen that has a lower oxidising power from one of its compounds

Question 6

Halide ion test

Answer 6

Silver nitrate acidified with nitric acid added

Question 7

Results of halide ion test

Answer 7

Fluorides: no precipitate
Chlorides: produces white precipitate
Eg. Ag+ + Cl- -> AgCl(s)
Bromides: produce cream precipitate
Iodides: produce yellow precipitate

Question 8

The role of nitric acid in the halide ion test

Answer 8

To react with any carbonates present to prevent the formation of the precipitate Ag2CO3

Question 9

Why would you treat silver halide precipitates with ammonia

Answer 9

To help differentiate between them if the colours look similar

Question 10

Does silver chloride react with ammonia

Answer 10

It dissolves in dilute ammonia to form a colourless solution

Question 11

Does silver bromide react with ammonia

Answer 11

Dissolves in concentrated ammonia to form a colourless solution

Question 12

Does silver iodide react with ammonia

Answer 12

No it is too insoluble

Question 13

Explain the differing reducing powers of halides

Answer 13

• a reducing agent donates electrons
• the reducing power of the halides increases down the group
• they have a greater tendency to donate electrons
• this is because as the ions get bigger is it easier for the outer electrons to be given away as the pull from the nucleus in them becomes smaller

Question 14

Fluorides and chlorides salts with concentrated sulfuric acid

Answer 14

F- and Cl- ions aren’t strong enough reducing agents to reduce the S in H2SO4

No redox reactions occur only acid-base reactions

NaF(s) + H2SO4 -> NaHSO4 + HF
Observations: white steamy fumes of HF

NaCl (s) + H2SO4 -> NaHSO4 + HCl
Observations: white steam fumes of HCl

Question 15

Why is fluoride and chloride salts with sulfurix acid not a redox reaction

Answer 15

they are acid-base reaction as the H2SO4 plays the role of an acid (proton donor)

Question 16

Bromide with concentrated sulfúrico acid
(Give the acid base step, redox step, and overall equation)

Answer 16

Br- ions are stronger reducing agents than Cl- and F- and after the initial acid-base reaction, the Br- ions reduce the S in H2SO4 from +6 to +4 in SO2

Acid-base step:
NaBr + H2SO4 -> NaHSO4 + HBr (g)

• here the acid acts as an acid
Redox step:
2H+ + 2Br- + H2SO4 -> Br2 + SO2 + 2H2O

• here H2SO4 acts as an oxidising agent
Overall equation:
2NaBr + 3H2SO4 -> 2NaHSO4 + SO2 + Br2 + 2H2O

Question 17

Oxidation and reduction half equation for bromide ions with sulfúrico acid

Answer 17

Ox: 2Br- -> Br2 + 2e-
Re: H2SO4 + 2H+ + 2e- -> SO2 + 2H2O

Question 18

What is the reduction product of bromide ions with sulfúrico acid

Answer 18

Sulfur dioxide

Question 19

Observations when bromide ions react with sumfuric acid

Answer 19

White steamy fumes of HBr are formed

Orange fumes of bromine are also formed and colourless, acidic gas SO2

Question 20

Iodide ions and sulfuric acid

Answer 20

I- ions are the strongest halid reducing agents -> they can reduce sulfur from +6 in H2SO4 to +4 in SO2 to 0 in S and -2 in H2S

NaI + H2SO4 -> NaHSO4 + HI

Question 21

Oxidation and reduction half equations for iodide ions with sulfuric acid

Answer 21

Ox: 2I- -> I2 + 2e-
Red: H2SO4 + 2H+ + 2e- -> SO2 + 2H2O

Question 22

Observations of iodide ions with sulfuric acid

Answer 22

White steamy fumes of HI
Black solid and purple fumes of iodine are also produced

A colourless acidic gas SO2
A yellow solid of sulfur
H2S a gas with a bad egg smell

Question 23

Reduction products of iodide ions with sunfuric acid

Answer 23

Sulfur dioxide, sulfur and hydrogen sulfide

Question 24

What is a disproportionation reaction

Answer 24

A reaction where an element simultaneously oxidises and reduces

Question 25

Equation for chlorine with water

Answer 25

Cl2 + H2O ⇌ HClO (aq) + HCl (aq)

Question 26

What happens if universal indicator is added to the reaction of chlorine with water

Answer 26

The solution will first turn red due to the acidity of both reaction products -> it will then turn colourless as the HClO bleaches the colour

Question 27

Reaction of chlorine with water in **sunlight**

Answer 27

If the chlorine is bubbles through water in the presence of sunlight a different reaction occurs 2Cl2 + 2H2O -> 4H+ + 4Cl- + O2 The same reaction occurs to an eqm mixture of chlorine water when in sunlight -> the greenish colour of chlorine water fades as the Cl2 reacts and a colourless gas (O2) is produced -> the greenish colour of these solutions is due to Cl2

Question 28

Uses of chlorine

Answer 28

• in water treatment to kill bacteria • used to treat drinking water and swimming pools • the benefits to health of water treatment by chlorine outweigh its toxic effects

Question 29

Reaction of chlorine with cold dilute NaOH solution

Answer 29

Colour of halogen solution fades to colourless Cl2 + 2NaOH -> NaCl + NaClO + H2O The mixture of NaCl and NaClO is used as bleach and to disinfect/ kill bacteria

Question 30

Name of NaClO

Answer 30

Sodium chlorate (I)

Question 31

Name of NaClO3

Answer 31

Sodium chlorate (V)

Question 32

Name of K2SO4

Answer 32

Potassium sulfate (VI)

Question 33

Name of K2SO3

Answer 33

Potassium sulfate (IV)