8.2 - The Halogens

Question 1

State the physical properties of the halogens.

Answer 1

• G7 - most reactive non-metal group of periodic table

- Halogens exist as stable halide ions dissolved in sea water or bonded to Na, K as solid salts

Question 2

State the colours and states of the halogens.

Answer 2

F2 - Yellow, Gas
Cl2 - Green, Gas
Br2 - Red-brown, Liquid
I2 - Grey/Black, Solid

Question 3

State the reasons for increased BP down G7.

Answer 3

• More electrons
• Stronger London Forces - more energy to break int. forces
• Increases BP

Question 4

What are the features of redox reactions of G7 elements?

Answer 4

• 7 outer shell electrons (-s2p5 in each halogen at end)
• Halogen always reduced, forms X- with electron config of nearest noble gas
• Oxidising agent

Question 5

Give the practical steps for halogen displacement reactions.

Answer 5

Demonstrates decrease in reactivity down G7

1. Add halogen to aq solution of another halide
2. More reactive halogen displaces less reactive one, the colour of the displaced/less reactive halogen will be in the remaining solution

Cl2 - pale green
Br2 - orange
I2 - brown

Question 6

What is added at the end in a halogen displacement reaction and why?

Answer 6

Organic non-polar cyclohexane added as non-polar halogens dissolve easier and the colours are more distinct.

Cl2 - Pale green
Br2 - Orange
I2 - Violet

Question 7

Why aren’t flourine and astatine included in halogen displacement reactions?

Answer 7

Flourine (pale yellow gas), reacts with everything it comes into contact with

Astatine - is rare and radioactive and rapidly decays (least reactive halogen)

Question 8

Describe and explain the trend in reactivirty down G7.

Answer 8

In redox, halogens react by gaining an electron
Down G7, reactivity decreases.
- Atomic radius increases
- More inner shells, more shielding
- Lower nucleic attraction to attract another e-
- Decrease in reactivity

Question 9

Define disproportionation reaction.

Answer 9

A reaction where the same element is both oxidised and reduced

Question 10

How is chlorine used in water purification?

Answer 10

Small amounts of chlorine and water create weak bleach

Cl2(aq) + H2O(l) > HClO(aq) + Hcl(aq)
(Disproportionation as chlorine is oxidised and reduced)
- Bacteria killed by HClO (chloric I acid) and ClO- (chlorate ions)
- HClO is a weak bleach - add indicator to Cl2 + H2O, first red, then colour disappears showing bleach)

Question 11

What are the limitations of just using chlorine in water for purification, and how can these be overcome?

Answer 11

First reaction limited by the solubility of chlorine in water, NaOH(aq) is used, it is more soluble

Cl2 + 2NaOH(aq) > NaClO(aq) + NaCl(aq) + H2O(l)

• Larger conc. of ClO- ions from sodium chlorate (I)
• Found in household bleach

Question 12

What are the pros and cons of using chlorine in water purification?

Answer 12

PROS
- Adding chlorine to water, more benefits thant risks, stops cholera, typhoid (water borne diseases)

CONS
- Cl2(g) is toxic, respiratory irritant in small amounts, fatal in large conc.
- Cl2 in drinking water reacts with organic hydrocarbons
Forming chlorinated hydrocarbons, can cause cancer