Module 3 Section 1 - Group 7 + Disproportionation

Question 1

What are group 7 elements also known as?

Answer 1

Halogens

Question 2

Colour and state of fluorine

Answer 2

Pale yellow
Gas

Question 3

Colour and state of chlorine

Answer 3

Green
Gas

Question 4

Colour and state of bromine

Answer 4

Red-brown
Liquid

Question 5

Colour and state of iodine

Answer 5

Grey
Solid

Question 6

What type of molecules do halogens exists as ?

Answer 6

Diatomic

Question 7

What is the trend in boiling points down group 7?

Answer 7

Boiling point increases because of the increasing strength of the London forces as the size and reactive masses of the atoms increase. This is shown by the changes in physical state down the group.

Question 8

What is the trend in reactivity down group 7?

Answer 8

It becomes less reactive

Question 9

Why are halogens less reactive down the group?

Answer 9

The atomic radii increase so the outer electrons are further from the nucleus. The outer electrons are also shielded more from the attraction of the positive nucleus, because there are more inner electrons. This makes it harder for larger atoms to attract the electron needed to form an ion.

Question 10

Are halogens reduced or oxidised?

Answer 10

Reduced

Question 11

What type of agents are halogens?

Answer 11

Oxidising agents

Question 12

How can you test halogens for their relative oxidising strengths?

Answer 12

Displacement reactions with halide ions.

Question 13

How to carry out displacement reactions?

Answer 13

Example
Add halogen solution so a solution containing another halogen’s ions.
When these displacement reactions occur, there will be a colour change
You can see the colour change more clearly by adding an organic solvent like hexane. The halogen that’s present will dissolve in the solvent above the aqueous layer.

Question 14

What ions will iodine solution displace ?

Answer 14

None

Question 15

What ions will bromine water displace?

Answer 15

Iodide ions.
Colour changes
In organic solution - purple
In aqueous solution - orange/brown

Question 16

What ions will chlorine water displace ?

Answer 16

Iodide ions and bromide ions

Colour change for iodine
In organic solution - purple
In aqueous solution - orange/brown

Colour change for bromine
In organic solvent - orange
In aqueous solvent - yellow

Question 17

Reaction of bromine with potassium iodide regular equation

Answer 17

Br2 (aq) + 2KI (aq) -> 2KBr (aq) + I2 (aq)

Question 18

Ionic equation for reaction of bromine with potassium iodide?

Answer 18

Br2 (aq) + 2I- (aq) -> 2Br- (aq) + I2 (aq)

Question 19

What type of reaction do halogens undergo with alkalis

Answer 19

Disproportionation

Question 20

What compounds are used to make bleach?

Answer 20

Chlorine and sodium hydroxide

Question 21

Reaction of chlorine ans sodium hydroxide to produce bleach?

Answer 21

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

Produces sodium chlorate (I) solution.

Question 22

Uses of sodium chlorate (I) solution

Answer 22

Water treatment
Bleach paper and textiles
Good for cleaning toilets

Question 23

What is chlorine used for in water?

Answer 23

To kill bacteria

Question 24

Reaction of chlorine and water

Answer 24

Cl2 (g) + H2O (l) <-> HCl (aq) + HClO (aq)

Produces hydrochloric acid and chloric (I) acid

Question 25

Reaction between chloric (I) acid and water

Answer 25

HClO (aq) + H2O (l) <-> ClO- (aq) + H3O+ (aq)

Produces chlorate (I) ions and these kill bacteria

Question 26

Benefits of using chlorine for water treatment

Answer 26

1) kills disease-causing microorganisms
2) some chlorine remains in the water and prevents reinfection further down the supply
3) it prevents the growth of algae, eliminates bad tastes and smells, and removes discolouration caused by organic compounds.

Question 27

Risks of using chlorine for water treatment

Answer 27

1) chlorine gas is very harmful if breathed in. It irritates the respiratory system. Liquid chlorine on the skin and eyes causes chemical burns. Accidents involving chlorine can be fatal.

2) chlorine reacts with compounds in water to form chlorinated hydrocarbons eg chloromethane. Many of these chlorinated hydrocarbons are carcinogenic.

Question 28

What are some alternatives to chlorine?

Answer 28

1) ozone (O3) - a strong oxidising agent, which makes it great at killing microorganism. However it’s expensive to produce and it’s short half life in water means treatment isn’t permanent.

2) ultraviolet light - it kills microorganism by damaging their DNA, but is ineffective in cloudy water and , like O3, it won’t stop water from being contaminated further down the line.