OCR A 3.1.3 Halogens

halogen elements colour

-grp 7

-F₂: pale yellow gas

-Cl₂: pale green gas

-Br₂: brown/orange liquid

-I₂: grey solid

grp 7 bp trend

-increases down grp

-london forces increase; due to increase in size and relative mass

grp 7 IE trend

-decreases down grp

-atom gets larger

-distance between +ve nucleus and bonding e increase

-more shielding

reactivity trend down grp 7

-decreases down grp

how to do grp 7 displacement reaction

-sol of each halogen added to aqueous sol oh another hallide

-eg: Cl₂ + 2Br^- →

-if halogen more reactive the halide present

-reaction will take place; sol changes colour

-to tell apart cyclohexane added and mixture is shook

why cyclohexane used

-halogen dissolve more readily in it

-helps to see colour change easily

-halogen present dissolves in organic layer which is above aqueous layer

Cl2 in water and cyclohexane

pale green

pale green

Br2 in water and cyclohexane

orange

orange

I2 in water and cyclohexane

brown

violet

Cl₂ + KBr

-aqueous layer; orange

-organic layer; orange

Cl2 + KI

-aq: brown

-org: violet

Br2 + KCl

aq: orange

org: orange

Br2 + KI

aq: brown

org: violet

I2 + KCl

aq: brown

org: violet

I2 + KBr

aq; brown

org: violet

how to make bleach

-disproportionation reaction

uses of bleach

-treat water

-bleach fabrics/paper

-cleaning agent

how does Cl treat water

-when added to water; kills bacteria

-useful in drinking water and pools

Cl in drinking water advantages

-destroys microorganism that cause disease

-long lasting; reduces bacteria build up further down supply

-reduce growth of algae; that discolours water and gives bad taste and smell

Cl in drinking water disadvantages

-Cl gas is toxic; irritates respiratory system

-Cl liquid causes sever chemical burns to skin

-Cl can react w/ organic compounds in water to chloroalkanes; cause cancer

ethical issue with chlorinating water

-no choice if our water is chlorinated

-forced medication on population

alternatives to chlorinating water

ozone;

-powerful oxidising agent that kills microorganisms

-short half life

-non perm and expensive

UV light

-damages DNA in microorganisms

-not work in cloudy water

disproportionation

redox reaction which same element is simultaneously oxidised and reduced