Module 3 Section 1 - Group 7 + Disproportionation Flashcards
What are group 7 elements also known as?
Halogens
Colour and state of fluorine
Pale yellow
Gas
Colour and state of chlorine
Green
Gas
Colour and state of bromine
Red-brown
Liquid
Colour and state of iodine
Grey
Solid
What type of molecules do halogens exists as ?
Diatomic
What is the trend in boiling points down group 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.
What is the trend in reactivity down group 7?
It becomes less reactive
Why are halogens less reactive down the group?
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.
Are halogens reduced or oxidised?
Reduced
What type of agents are halogens?
Oxidising agents
How can you test halogens for their relative oxidising strengths?
Displacement reactions with halide ions.
How to carry out displacement reactions?
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.
What ions will iodine solution displace ?
None
What ions will bromine water displace?
Iodide ions.
Colour changes
In organic solution - purple
In aqueous solution - orange/brown
What ions will chlorine water displace ?
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
Reaction of bromine with potassium iodide regular equation
Br2 (aq) + 2KI (aq) -> 2KBr (aq) + I2 (aq)
Ionic equation for reaction of bromine with potassium iodide?
Br2 (aq) + 2I- (aq) -> 2Br- (aq) + I2 (aq)
What type of reaction do halogens undergo with alkalis
Disproportionation
What compounds are used to make bleach?
Chlorine and sodium hydroxide
Reaction of chlorine ans sodium hydroxide to produce bleach?
2NaOH (aq) + Cl2 (g) -> NaClO (aq) + NaCl(aq) + H2O (l).
Produces sodium chlorate (I) solution.
Uses of sodium chlorate (I) solution
Water treatment
Bleach paper and textiles
Good for cleaning toilets
What is chlorine used for in water?
To kill bacteria
Reaction of chlorine and water
Cl2 (g) + H2O (l) <-> HCl (aq) + HClO (aq)
Produces hydrochloric acid and chloric (I) acid
Reaction between chloric (I) acid and water
HClO (aq) + H2O (l) <-> ClO- (aq) + H3O+ (aq)
Produces chlorate (I) ions and these kill bacteria
Benefits of using chlorine for water treatment
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.
Risks of using chlorine for water treatment
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.
What are some alternatives to chlorine?
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.
