Inorganic 3: Group 7

Question 1

Describe the physical properties of the halogens at room temperature.

Answer 1

Fluorine = pale yellow gas
Chlorine = green gas
Bromine = red/brown liquid
Iodine = black solid

Question 2

Why is the F-F bond so weak?

Answer 2

Fluorine atoms are very small
So the non-bonding electrons repel as they are so close together
Which weakens the bond

Question 3

What is electronegativity?

Answer 3

The power of an atom to attract the bonding pair of electrons in a covalent bond

Question 4

How do melting and boiling points change down group 7?

Answer 4

1) Melting and boiling points increase
2) Larger atoms have more electrons so van der Waal forces increase

Question 5

Which group 7 elements are the best oxidising agents?

Answer 5

1) Fluorine is the best oxidising agent
2) Because it is the most electronegative so attracts electrons more strongly
3) Oxidising ability decreases down the group

Question 6

How do halogens react with metal halides?

Answer 6

The halide in the compound will be displaced by a more reactive halogen

Question 7

How does reactivity change down group 7?

Answer 7

Fluorine is the most reactive
Iodine is the least reactive

Question 8

How would chloride ions react with chlorine, bromine, and iodine?

Answer 8

Chloride ions would not be displaced by any halogen

Question 9

How would bromide ions react with chlorine, bromine, and iodine?

Answer 9

Bromide ions would be displaced by chlorine
A yellow solution of aqueous bromine would form
Cl2 + 2Br- –> 2Cl- + Br2

Question 10

How would iodide ions react with chlorine, bromine, and iodine?

Answer 10

Iodide ions would be displaced by chlorine
A brown solution would form
Cl2 + 2I- –> I2 + 2Cl-

Iodide ions would be displaced by bromine
A brown solution would form
Br2 + 2I- –> 2Br- + I2

Question 11

Explain displacement reactions between metal halides and halogens.

Answer 11

Halogen atoms gain an electron when it oxidises the halide ion
The smaller the halogen, the easier it is to gain an electron
As there is less shielding
So a stronger force of attraction from the nucleus

Question 12

Which halide ions are the best reducing agents?

Answer 12

Strongest reducing agent = iodide
Weakest reducing agent = fluoride

Question 13

Why Is iodide the strongest reducing agent?

Answer 13

Iodide has the largest ionic radius, so the outer electron is further form the nucleus
So the attraction between the nucleus and outer electron is weaker

Question 14

Describe a reaction of chloride ions with concentrated sulfuric acid.

Answer 14

H+ + Cl- –> HCl
Observation: White fumes
Type of reaction: Acid-base

Question 15

Describe a reaction of bromide ions with concentrated sulfuric acid.

Answer 15

1) Acid-base
H+ + Br- -> HBR
Observation: White fumes

2) Redox
H2SO4 + 2H+ + 2Br- –> Br2 + SO2 + 2H2O
Observation: Orange/brown fumes (Br2) and colourless gas (SO2)

Question 16

Describe a reaction of iodide ions with concentrated sulfuric acid.

Answer 16

1) Acid base
H+ + I- –> HI
Observation: Steamy fumes

2) Redox
H2SO4 + 2H+ + 2I- –> I2 + SO2 + 2H2O
H2SO4 + 6H+ + 6I- –> 3I2 + S + 4H2O
H2SO4 + 8H+ + 8I- –> 4I2 + H2S + 4H2O
Observations:
Black solid (I2)
Colourless gas (SO2)
Yellow solid (S)
Colourless gas (H2S)

Question 17

Why doesn’t silver fluoride form a precipitate?

Answer 17

Because it is soluble in water

Question 18

Describe how you would test for metal halides.

Answer 18

1) Add dilute nitric acid to remove any carbonate or hydroxide impurities which would interfere with the test

2) Add silver nitrate (AgNO3) to form the silver halide precipitates
Silver chloride = white precipitate
Silver bromide = cream precipitate
Silver iodide = pale yellow precipitate

3) Further test: silver chloride dissolves in dilute ammonia, silver bromide dissolves in concentrated ammonia, silver iodide is insoluble in concentrated ammonia

Question 19

Describe how chlorine reacts with water (not in sunlight).

Answer 19

In a reversible reaction:
Cl2(g) + H2O(l) —> HClO (aq) + HCl (aq)
This is a disproportionation reaction as some atoms of the same element are oxidised while others are reduced

Question 20

What is HClO?

Answer 20

Chloric(I) Acid
Used as an oxidising agent to kill bacteria
Also a bleach

Question 21

What is chlorine used for?

Answer 21

Purifying water for drinking
In swimming baths

Question 22

Give an equation for how chlorine reacts with water in sunlight.

Answer 22

2Cl2(g) + 2H2O (l) —> 4HCl(aq) + O2(g)

Question 23

What is an alternative to direct chlorination of pools? (give an equation)

Answer 23

Add sodium chlorate (I) which dissolves in water to form chloric(I) acid (HClO) in a reversible reaction:

NaClO(s) + H2O(l) –> Na+ (aq) + OH-(aq) + HClO(aq)

Question 24

Why are swimming pools always kept slightly acidic?

Answer 24

Because in alkaline solution, the equilibrium of this reaction:
NaClO(s) + H2O(l) –> Na+ (aq) + OH-(aq) + HClO(aq)
Moves slightly to the left, so HClO is removed as ClO- ions (harmful to health)

Question 25

How does chlorine react with cold, dilute NaOH?

Answer 25

Cl2(g) + 2NaOH(aq) —> NaClO(aq) + NaCl(aq) + H2O (l)
Disproportionation reaction

Question 26

what is NaClO?

Answer 26

Sodium Chlorate (I)
Used in bleach
Oxidising agent