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3.2.5 Group VII
Energetics - Kinetics - Equilibria - Redox - Group VII - Group II - Metal Extraction - Haloalkanes - Alkenes - Alcohols - Analytical Tech... - Unit 1
Awesome Halogens
Halogen aka Group VII are highly reactive non metals. You will need to be familiar with their basic properties including colour, state and relative electronegativity.
Explaining the increase in boiling points down the group
This is due to the Van der Waals forces increasing in strength as the size and mass of the atom (and hence molecules) increases. This explains the physical states of the elements.
Explaining the electronegativity decreasing down the group
N.B. The definition of electronegativity is the tendency of an atom to attact a pair of electrons in a covalent bond.
All of the halogens are electonegative. The larger atoms (eg iodine) attract the electron pair less because there is shielding from the inner shells of electrons and the atomic radius has increased.
Displacement and reactivity of halogens
Halogens are all reactive but the more reactive (more electronegative) halogens will displace a a less reactive halide ion from a solution. (As the halogens become less electronegative their oxidising ability also decreases)
You can tell a displacement reaction has occurred because the less reactive halogen that is displace will now colourise the remaining solution. You have seen this in reverse with the bromine test when the bromine element is present in the solution it is orange/brown, when it has gone it is colourless.
Displacement reactions are useful because it can help us to identify which of the halides are present in a solution.
Fluorine will displace all other halides, chlorine will displace bromine and iodine and bromine will only displace iodine.
When bromine is displaced the solution turns orange, and when iodine is displaced the solution will turn brown.
Example equation
This is due to the Van der Waals forces increasing in strength as the size and mass of the atom (and hence molecules) increases. This explains the physical states of the elements.
Explaining the electronegativity decreasing down the group
N.B. The definition of electronegativity is the tendency of an atom to attact a pair of electrons in a covalent bond.
All of the halogens are electonegative. The larger atoms (eg iodine) attract the electron pair less because there is shielding from the inner shells of electrons and the atomic radius has increased.
Displacement and reactivity of halogens
Halogens are all reactive but the more reactive (more electronegative) halogens will displace a a less reactive halide ion from a solution. (As the halogens become less electronegative their oxidising ability also decreases)
You can tell a displacement reaction has occurred because the less reactive halogen that is displace will now colourise the remaining solution. You have seen this in reverse with the bromine test when the bromine element is present in the solution it is orange/brown, when it has gone it is colourless.
Displacement reactions are useful because it can help us to identify which of the halides are present in a solution.
Fluorine will displace all other halides, chlorine will displace bromine and iodine and bromine will only displace iodine.
When bromine is displaced the solution turns orange, and when iodine is displaced the solution will turn brown.
Example equation
Cl2(aq) + 2Br-(aq) --> 2Cl-(aq) + Br2(aq)
Uses of the Chlorine
Chlorine gas mixed with diluted NaOH (Sodium Hydroxide) will make sodium chlorate(I) solution NaClO.
Bleach is used to treat water (kill disease causing microorganisms and prevent algae growth), to bleach paper and textiles, and for cleaning.
Chlorine gas mixed with diluted NaOH (Sodium Hydroxide) will make sodium chlorate(I) solution NaClO.
Bleach is used to treat water (kill disease causing microorganisms and prevent algae growth), to bleach paper and textiles, and for cleaning.
2NaOH(aq) + Cl2(aq) --> NaClO(aq) + H2O(l)
When you mix chlorine with water;
You end up with a mixture of hydrochloric acid and chloric(I) acid (sometimes called hyperchlorous acid).
Cl2(g) + H2O(l) <--> (reversible) HCl(aq) +HClO(aq)
The aqeuous chloric(I) acid then ionises to make the chlorate(I) (sometimes called hypochlorate ions).
HClO(aq) + H2O(l) <--> ClO-(aq) + H3O+(aq)
Cl2(g) + H2O(l) <--> (reversible) HCl(aq) +HClO(aq)
The aqeuous chloric(I) acid then ionises to make the chlorate(I) (sometimes called hypochlorate ions).
HClO(aq) + H2O(l) <--> ClO-(aq) + H3O+(aq)
Chlorine gas is very harmful to people; (can be fatal)
If it is breathed in - this is because it irritates the respiratory system.
If the liquid is in contact with skin - it causes chemical burns.
You may get asked about whether it is ethical for all people's water to be chlorinated without consent.
TASK: Learn these equations;
write down the equations in sentence form
eg chlorine gas raects with water to....
Then turn your sentences into symbol equations (dont forget state symbols).
Trend in reducing power
As you go down the group the reducing power of the halides (halogen ions) increases. This is because to reduce something a species needs to lose an electron from it's outer shell.
The larger the atom;
the further away the outer shell is from the nucleus and the more shielding from inner shells
and so the attraction between the outer shell and the nucleus is weaker
and so the electron is lost easier
making a better reducing agent.
Testing for halide ions
To test for halide ions we use the Silver nitrate test;
1. Add dilute nitric acid - to remove any ions that might interfere wit the test.
2. Add silver nitrate solution
3. Look at the colour of the precipitate
Fluoride - no precipitate
Chloride - white
Bromide - cream
Iodide - yellow
4. Add ammonia solution
Chloride precipitate will dissolve in dilute ammonia
Bromide precipitate will dissolve in concentrated ammonia
Iodide precipitate is insoluble in concentrated ammonia
Reactions with sulfuric acid
If it is breathed in - this is because it irritates the respiratory system.
If the liquid is in contact with skin - it causes chemical burns.
You may get asked about whether it is ethical for all people's water to be chlorinated without consent.
TASK: Learn these equations;
write down the equations in sentence form
eg chlorine gas raects with water to....
Then turn your sentences into symbol equations (dont forget state symbols).
Trend in reducing power
As you go down the group the reducing power of the halides (halogen ions) increases. This is because to reduce something a species needs to lose an electron from it's outer shell.
The larger the atom;
the further away the outer shell is from the nucleus and the more shielding from inner shells
and so the attraction between the outer shell and the nucleus is weaker
and so the electron is lost easier
making a better reducing agent.
Testing for halide ions
To test for halide ions we use the Silver nitrate test;
1. Add dilute nitric acid - to remove any ions that might interfere wit the test.
2. Add silver nitrate solution
3. Look at the colour of the precipitate
Fluoride - no precipitate
Chloride - white
Bromide - cream
Iodide - yellow
4. Add ammonia solution
Chloride precipitate will dissolve in dilute ammonia
Bromide precipitate will dissolve in concentrated ammonia
Iodide precipitate is insoluble in concentrated ammonia
Reactions with sulfuric acid
NaF or NaCl
NaF(s) + H2SO4(aq) --> NaHSO4(s) + HF(g)
HF is a gas so you would see misty fumes.
HF and HCl arent strong enough reducing agents to reduce the sulfuric acid so the reaction stops.
NaF(s) + H2SO4(aq) --> NaHSO4(s) + HF(g)
HF is a gas so you would see misty fumes.
HF and HCl arent strong enough reducing agents to reduce the sulfuric acid so the reaction stops.
NaBr
NaBr(s) + H2SO4(aq) --> NaHSO4(s) + HBr(g)
HBr is a gas so you would see misty fumes.
HBr is a strong enough reducing agent to reduce the sulfuric acid so there is a redox reaction.
2HBr(aq) + H2SO4(aq) --> Br2(g) + SO2 +2H2O(l)
SO2 and Br2 fumes are made these are choking.
NaI
NaI(s) + H2SO4(aq) --> NaHSO4(s) + HI(g)
HI is a gas so you would see misty fumes.
HI is a strong enough reducing agent to reduce the sulfuric acid so there is a redox reaction.
2HI(aq) + H2SO4(aq) --> I2(g) + SO2 +2H2O(l)
HI is a really strong reducing agent so will even reduce SO2
6HI(g) + SO2(g) --> H2S(g) + 3I2(s) + 2H2O(l)
H2S gas is toxic and smells of rotten eggs.
General Formula
NaX(s) + H2SO4(aq) --> NaHSO4(s) + HX(g)
HX is a gas so you would see misty fumes.
IF X=Br or I then;
HX is a strong enough reducing agent to reduce the sulfuric acid so there is a redox reaction.
2HX(aq) + H2SO4(aq) --> X2(g) + SO2 +2H2O(l)
If X=I; HI is a really strong reducing agent so will even reduce SO2
6HI(g) + SO2(g) --> H2S(g) + 3I2(s) + 2H2O(l)
H2S gas is toxic and smells of rotten eggs.
Practice Exam questions
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Energetics - Kinetics - Equilibria - Redox - Group VII - Group II - Metal Extraction - Haloalkanes - Alkenes - Alcohols - Analytical Tech... - Unit 1