3.1.3 The Halogens

Question 1

How do halogens exist at RTP?

Answer 1

As diatomic molecules

Question 2

How does melting and boiling point change down the group?

Answer 2

• Melting and boiling points increase as you go down the group
• London forces increase due to the increasing size and relative mass of the atoms
• More energy is required to overcome the intermolecular force

Question 3

What are the physical states and colours of the halogens at RTP?

Answer 3

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

Question 4

What is the electron configuration of the outer shell of all halogens?

Answer 4

s2p5

Question 5

How do halogens form ions?

Answer 5

• Gain 1 electron
• Form 1- ions

Question 6

How can the reactivity of halogens be observed?

Answer 6

• Halogen-halide displacements
• Cyclohexane can be added to create distinguishable colours

Question 7

What are the results of the halogen-halide displacement reaction?

Answer 7

Cl2 and Br- = Orange colour from Br2 formation (Cl2 + 2Br- -> 2Cl- + Br2)
Cl2 and I- = Violet colour from I2 formation (Cl2 + 2I- -> 2Cl- + I2)
Br2 and Cl- = No reaction
Br2 and I- = Violet colour from I2 formation (Br2 + 2I- -> 2Br- + I2)
I2 and Cl- = No reaction
I2 and Br- = No reaction

Question 8

How does reactivity change down the group?

Answer 8

• Reactivity decreases as you go down the group
• Atomic radius increases and additional electron shells mean that there is more shielding
• This decreases the nuclear attraction so it’s harder to attract another electron

Question 9

What does displacement mean?

Answer 9

Oxidation and reduction of the same element

Question 10

What happens when chlorine reacts with cold water?

Answer 10

Cl2 + H2O -> ClO- + Cl- + 2H+
- Used in water treatment to kill bacteria
- Both oxidation (Cl2 -> ClO-) and reduction (Cl2 -> Cl-)

Question 11

What happens when chlorine reacts with cold, dilute sodium hydroxide?

Answer 11

Cl2 + 2NaOH -> NaClO + NaCl + 2H2O
- Used to form sodium chlorate (I) which is bleach
- Both oxidation (Cl2 -> NaClO) and reduction (Cl2 -> NaCl)

Question 12

What happens when chlorine reacts with warm, concentrated sodium hydroxide?

Answer 12

3Cl2 + 6NaOH -> NaClO3 + 5NaCl + 3H2O
- Disproportioned further to produce chlorate (V) ions

Question 13

What are the benefits of using chlorine in drinking water?

Answer 13

• Kills bacteria
• Reduces risk of cholera or typhoid

Question 14

What are the risks of using chlorine in drinking water?

Answer 14

• Chlorine is a toxic gas and a respiratory irritant
• Can react with organic hydrocarbons to form chlorinated hydrocarbons, which are suspected of causing cancer

Question 15

How do halide ions react with silver ions?

Answer 15

• Forms a silver halide precipitate
• Basis of the test for the presence of halides