3.1.3 The Halogens

Question 1

Physical properties

Answer 1

Exist as diatomic molecules at RTP

Form simple molecular lattice structure in solid state

Question 2

Trend in boiling point

Answer 2

Boiling point increases down the group

• More electrons
• Stronger London forces
• More energy is required to overcome the intermolecular forces

State at RTP changes from gas -> liquid -> solid
F2 (g)- pale yellow
Cl2 (g) - pale green
Br2 (l) - red/brown
I2 (s) - shiny grey/black
At2 - ?

Question 3

Oxidising agent

Answer 3

7 outer shell electrons - 2 in the outer s-subshell and 5 in the outer p-subshell
1 electron on the outer p-subshell is gained in each halogen atom and reduced to form a 1- ion
Another species will lose 2 electrons to be oxidised
Cl2 + 2e- -> 2Cl-

Question 4

Halogen-halide displacement reaction

Answer 4

A more reactive halogen can displace a less reactive halide ion from its solution - causes a colour change
Cl2 reacts with Br- and I-
Br2 reacts with I
I2 does not react

Cl2 = pale green
Br2 = orange
I2 = brown (water) violet (cyclohexane)

E.g. Cl2 + 2Br- -> 2Cl- + Br2

• 2Br is oxidised -1 -> 0 (+2)
• 2Cl is reduced 0 -> -1 (-2)

Question 5

Trend in reactivity

Answer 5

Reactivity decreases down group 7

• Atomic radius increases
• Electron shielding increases
• Less attraction between the nucleus and outer shell electrons
• The tendency to gain electrons decreases as it is harder to attract an electro; from another species

Weaker oxidising agents down the group

Question 6

Reaction of Cl2 with water

Answer 6

Disproportionation reaction
Cl2 (aq) + H2O (l) -> HClO(aq) + HCl (aq)
- 1Cl is oxidised 0 in Cl2 -> +1 in HClO
- 1Cl is reduced 0 in Cl2 -> -1 in HCl
Forms solution with chloric acid (HClO) and chlorate ions (ClO-) - kills bacteria in water treatment
Indicator turns red then bleaches in Cl2 + Water

Question 7

Reaction of Cl2 with cold dilute NaOH (aq)

Answer 7

Disproportionation reaction
Cl2 (aq) + 2NaOH(aq) -> NaClO(aq) + NaCl(aq) + H2O(l)
- 1Cl is oxidised 0 in Cl2 -> +1 in NaClO
- 1Cl is reduced 0 in Cl2 -> -1 in NaCl
Forms solution with large conc. of chlorate ions (ClO-) - bleach

Question 8

Cl2 use benefits and risks

Answer 8

Uses
- water purification
Kills bacteria - protects against diseases e.g. typhoid, cholera
Makes water potable

Risks
- Can react with organic hydrocarbons in drinking water to produce chlorinated hydrocarbons
May cause cancer
- extremely toxic gas
- respiratory irritant in small concentrations - can be fatal in large concentrations

Question 9

Reaction with Silver ions (aq)

Answer 9

Precipitation reaction of aqueous halide ions with silver ions (aq) to form silver halides
Ag+(aq) + X-(aq) -> AgX(s)